CA1122504A - End point drift correction in automatic titration - Google Patents

Abstract

Abstract of the Disclosure An apparatus and method are described for automatically correcting drift in automatic titrations, such as coulometric titrations of water. An end-point detector provides a signal indicative of the state of the titration mixture and the detector signal is monitored by two comparators, responsive to titration mixture states on opposite sides of the endpoint. The comparators are connected to current sources for controlling forward and back titration. At the end of a titration, the titration time and a signal representing the amount of the titrant used are stored in memory elements, and circuitry is provided including an array of gates for monitoring the signals from the comparators during two post titration time periods to deter-mine both the direction and rate of drift, and to correct the titration results for the amount of drift detected.

Description

ENDPOINT DRIFT CORREC`~ION FOR AUTOMATIC TITRATIONS

This invention resides in a method and an apparatus for coulometric titration. More parkicularly, the invention resides in a method and an apparatus for accurate and precise determination of the results of a coulometric titration and compensation or drift.

In a particularly preferred embodiment, the invention resides in an improved method and an apparatus for the coulometric titration of water by the Karl Fischer reaction. The amount of elec-tric charge required by the electrolysis electrodes (in order to reach the endpoint state~ is measured to give a measurement of water titrated. By using a constant electrolysis current, the coulometric measurement can utilize conventional timers.

In such an apparatus, the signal from the detector is inherently a rather noisy signal.
With conventional iltering methods the signal-to--noise ratio is improved by using a long time--constant RC ~ilter system, but the titration con-trol is delayed, which can allow the titration to 27,741-F

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progress beyond the endpoint. Back titration must then be employed. In addition baseline drift frequently occurs after titration of sample water is complete, necessitating both positive and negative stabilization.

As described in U.S. Patent No. 3,726,778, titrations are subject to positive and negative base-line drift, that is, drift away from the endpoint con-dition not resulting from sample addition. Such drift can be caused by a variety of factors, including side reactions in the titration mixture which either generate or consume the substance being titrated. In coulometric titration of water, aldehydes or ketones in samples will react to produce water. While U.S. Patent No. 3,726,778 provides one approach to compensation for drift, it would be desirable to provide an apparatus and method for deter-mining accurately the actual amount of drift in each titration and for automatically correcting the titration results.

The invention resides in an automatic coulometric titrator comprising titration means for introducing a titrant into a titration mixture, detection means for detecting the endpoint of the titration, means responsive to the endpoint detection means for controlling the titra-tion means to introduce titrant when the titration mixture is not at the endpoint, and titrant measuring means for measuring the amount of titrant introduced, characterized by (a) timer means for measuring the titration time until the titration mixture first reaches an initial endpoint;
(b) means responsive to one of the endpoint detection means and the titration means 27,741-F

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for detecting the introduction of titrant during a predetermined post-titration time period after the initial endpoint;
(c) means for measuring the amount of titrant introduced during a second post-titration time period after the initial endpoint, said second time period beginning with the detection of titrant introduction by means (b), and ending with a predetermined subsequent introduction of titrant;
(d) means including a timer for measuring the duration of said second post-titration period and the amount of titrant introduced during said period, said time and amount measurements by means (c) being indicative of the drift rate during the titration, and the drift rate and titration time measured by means (a) being indicative of the amount of drift.

The invention further resides in a coulometric titrator having electrolysis electrodes for electrolytically generating titrant in a titration mixture, a constant current source for providing electrolysis current to said electrodes, and an endpoint detector for providing an end-point signal indicative of the status of the titration mixture relative to the endpoint or the titration, char-acterized by a clock for generating a time signal T during and after a titration;
a titration counter for generating a char~e signal Q corresponding to the charge passed through the titration mixture by the electrodes and constant current source;

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-2b-a comparator connected to the endpoint detector and adapted to provide a signal Sl having an Sl=ON state when the endpoint signal is above a pre-determined endpoint level and an Sl=OFF state ~hen the endpoint signal is at or below the endpoint level;
means including a switch responsive to the comparator signal Sl for activating the current source and the titration counter when Sl=ON and inactivating said source and counter when Sl=OFF;
first gate means responsive to the time sig~
nal T and comparator signal Sl for detecting a recurrence of the Sl=ON state during a predetermined tim~ period after Sl first attains the Sl=OFF state at the end of a titration;
memory means responsive to ~ignals Sl, T and Q including a titration time memory and a titration charge memory for storing T and Q signals, Ttitr and Qtitr~ when Sl first attains the Sl=OFF state at the end of a titration;
display means for displaying the signal Q as a titration result Qtitr in response to the first gate means signal corresponding to the lapse of the first gate time interval without a recurrence f ~1 attaining the Sl=ON state;
first drift correction memory means responsive to signals T and Q and the first gate means, for storing the T and Q signals as Qco and TCo in response to the first gate signal indicative of occurrence of the Sl=O~
state during the first gate time interval;
drift correction means including second and third gates, the second gate being responsive to the first gate and the time signal T to activ te the third gate at a predetermined time after the first gate 27,741-F

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-2c-signal indicative of the occurrence of the Sl=ON state during the first gate time interval, and the third gate being responsive to the Sl signal and the second gate;
second drift correction memor~ means respon-sive to the Q and T signals and to the third gate means for storing the Q and T signals as QC and TC at the time the third gate indicates occurrence of the Sl=ON
state of signal Sl.

The invention also resides in a method for correcting positive drift in an automatic coulometric titrator, characterized by the steps of generating a signal Q corresponding to the amount of titrant introduced into the titration mi~ture;
generating a time signal T corresponding to time elapsed from beginning of the titration;
introducing titrant continuously until a pre-determined endpoint condition is detected, and thereafter reintroducing titrant periodically as needed to maintain the reaction mixture àt the endpoint condition;
storing the Q and T signals as Qtitr and Ttitr at the time the endpoint is first reached;
determining the titration result from (Qtitr) when no additional titrant is introduced during a pre-determined time interval after the endpoint is irst reached; and generating a drift rate signal DR when addi-tional titrant is introduced during said interval, said drift signal DR corresponding to the difference between (i) the Q signal at the time (TCo) of an additional titrant introduction (Qco~ and the Q signal at the time (TC ) of a predetermined later additional titrant 27,741-F
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Z~5C~4 -2d-introduction (QC ) divided by (ii) the difference in time (TC -TCo) between said introductions;

generating a correction signal (Qcorr) corres-ponding to the product of the drift rate signal DR and the stored (Ttitr) signal, said correction signal thus corresponding to the amount of titrant consumed by drift during the initial titration; and determining the;titration result from the difference between the stored (Ttitr) signal and the (TCorr) signal. c As with coulometric titrations generally, the method and apparatus of the invention includes a titra-tion compartment, with means for introducing a titration reagent and sample, an electrolysis anode and cathode in the compartment to pass current through the titration mixture and generate a titrant from the reagent; a detector for determining the state of the titration mix-ture relative to a predetermined endpoint;. and a source of electrolysis current for the electrolysis electrodes, 27,7~1-F

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and means for measuring the amount of electric charge passed through the -titration mixture.
The appara-tus can also include additional com-ponents, such as an auxiliary electrolysis current source of opposite polarity to the main source for back titration, appropriate filters, buffers, amplifiers and control circuitry to allow the detector to control the titration, and display elements for displaying the results in various convenient forms, such as coulombs, titration time, or concentration units~

In coulometric titrations, and particu-larly in coulometric titration of water by the Karl Fischer reaction, the endpoint can be subject to drift, due to a variety of factors such as side reactions, or leakage of atmospheric contaminants into the titration compartment. Depending on the chemistry of different samples, drift due to side reactions can vary in both direction and amount, adversely affecting the results of the titration.

The invention provides a method and apparatus for compensating for the actual drift occurring during the titration, so that the effects of drift elements in each titration can be accounted for.

In describing the invention, it is con-venient to refer to various states of the titration mixture in terms of a graphical plot of the state of the mixture (on the vertical axis) against time (on the hori~ontal axis). It will also be convenient to refer to state of the titration mixture as starting 27,741-F

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;25C~4 "above the endpoint", proceeding "downward" to the endpoint during the titration, and possibly going "below the endpoint" in cases of over titration beyond the endpoint or negative drift. In this frame of reference, positive drift is "upward"
drift, and negative dri~t is "downward" drift.
It is understood that the terminology of upward and downward movement is merely a convention adopted to simplify the description. Depending on the parameters used in various embodiments to monitor the state of the titration mixture relative to the endpoint, the titration may result in either a decrease in the selected parameter or an increase.

In the process of the invention, an elec-trolysis current is passed through a titration mixture of reagent and sample to generate titrant therein. ~hen the titration mixture first reaches a predetermined endpoint, as indicated by an end-point signal provided by a detector, the elec-trolysis current is turned off. After the endpointis reached, electrolysis current is again passed through the mixture (whenever the detector signal exceeds or rises above the endpoint level), until the mixture is titrated back to the endpoint. The amount of electric charge (Q) used for electrolysis is measured, and the titration time (T) is also measured. After the endpoint condition is reached for the first time, the condition of the mixture is monitored and the mixture titrated back to the endpoint as needed, but the mixture is otherwise held without measurements of charge being taken for a first predetermined short time period to allow the mixture to stabilize. Monitoring of the 27,741-F

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condition of the mixture is then continued for a second, longer time period immediately following the first time period, without interruption. If -the detector signal does not depart from the endpoin-t level during the second time period, the amount of the sample is determined from the amount of electrolysis current passed through the mixture until the initial endpoint was reached. If, during the second time period, the titration mixture departs from the end-point, monitoring is continued over a third timeinterval (during which drift is measured), beginning with a return to the endpoint continuing for a pre-det~rmined time period and ending with another departure from and subsequent retitration of the endpoint. Mixing of the titration mixture is continued throughout the process so that any heterogeneity due to localized dif-fering conditions at the titration electrodes or detector electrodes will be eliminated.

The first post-titration time interval is relatively short, e.g. 5-10 seconds, being only long enough to ensure that localized differences in the titration mixture are eliminated. The second post-titration time interval is generally longer;
e.g., 30-60 seconds, when there is no forward or back titration of drift. The pxedetermined time duration of ~he second time interval is selected in relation to system time response of the titration and titration rate to correspond to, or to exceed the ordinary duration of the initial titration so that the absence of forward or back titration during this period will indicate that drift was also absent during the initial titra-tion. The first time period and the predetermined duration of the 27,741-F

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second time period can be preselected and con-trolled by a timer. When titratable drift occurs, the end of the second time period, and the beginning and end of the third time period are determined in response to the sta-te of the titration mixture.
The third time period must begin and end with departures from, or retitrations returning to, -the predeterminded endpoint in order to obtain an accurate determination of the drift rate.

Since the first time period is provided to allow stabilization of the titration mixture, the second and third time periods are the two time intervals during which drift observation measure-ments and corrections are conducted. The first time period can be referred to as a "stabilization period". The second time period can be referred to as a "drift monitoring period" during which the titration mixture is monitored for drift away from the endpoint. The third time period can be referred to as a "drift measuring period" during which measure-ments of drift rate are made (when drift has occurred during the drift monitoring period). In titrations for which no post-endpoint stabilization is required, for example, with rapid mixing and a slow titration rate, the stabilization period can be eliminated.

Electrolysis current is passed through the mixture during the third time period, and the total duration of the third time period are measured.
The average drift rate is determined from the amount of electric charge used during the third drift measurement period and the duration of the third time period. The amount of sample titrated is 27,741-F

determined from the amount of charge introduced by the electrolysis current until the end o~ the titration procedure, adjusted by the drift rate and measured titration time.

AS mentioned above, coulometric titrations can be subject to either positive or negative drift.
Preferably the detector signal is monitored and reverse polarity back titration current is passed through the mixture when the signal reaches a pre-determined level below th~ endpoint, and the reverse current is also measured as stated above, and used to measure the negative drift rate and adjust the titration results for negative drift.

The apparatus of the invention includes conventional elements of a coulometric titrator in-cluding electrolysis electrodes, current source, titration compartment, and a detector ~or detecting the condition of the titration mixture relative to a predetermined endpoint. The detector output signal is connected through a variable low pass filter to a comparator for comparing -the detector output to the predetermined endpoint level. The filter is adapted to vary in response to the out-put signal from the comparator between a short time constant condition above the endpoint and a long time constant, high damping condition at or below the endpoint. The comparator is also con-nected to means for controlling the titration cur-rent.

27,741-F

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The apparatus also includes means for measuring the titration electric charge, and clock means for measuring titra-tion time and for generating a time signal, first gate means responsive to the time signal and to the comparator for controlling the first post-titration period; a second gate means responsive to the first gate means, the time signal and the comparator for transmitting the titration charge measure-ment to an output means such as a display when the second post-titration time period (the drift observation period) has elapsed without a depar-ture from the endpoint, and for transmitting signals indicative of the measurements of titration time (Ttitr) and titration charge (Qtitr) at the end of the initial titration and signals indicative of the time and charge measurements ~Tco and Qco) at the beginning of the third post-titration time period (the drift measuring period) to first and second memory means; a third gate means responsive to the second gate means, the time signal and the comparator for transmitting signals indicative of the measurements of titration electric charge (Qcl) and time ~TCl) from the charge measuring means and the clock means when the selected retitration occurs; at the end of the third time period. Subtractor/divide.r means is provided which is responsive to the third gate means and which is connected to the second memory means for generating a signal corresponding to Qcl ~ Qco which thus corresponds to the average _ T 1 ~ T
drift rate (D.R.) during the third time interval;
a multiplier means connected to the subtractor/-divider means and the first memory means for 27,741-F

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. g generating a signal corresponding to the amount of drift taking place during the titration, (D. R.
X Ttitr = Qcorr)i and adder and subtractor means connected to the first memory means and the multi-plier means for correcting the titration result Qtitr by subtracting or adding the amount of electric charge Qcorr attributable to titration of positive or negative drift.

The titration time, Ttitr measured in lG the process and apparatus of the invention is the total of (a) a relatively short term sample intro-duction period (during which the sample to be titrated is introduced into the titration reagent), (b) the time between the end of the sample intro-duction period and the beginning of titrant intro-duction by switching on the main electrolysis current source; and (c) the time during which titration current is passed through the mixture until the endpoint condition is first reached. Period (a) is a relatively short period, e.g. 5-lO seconds to allow time for the introduction of the sample.
Although, period (b) can be eliminated, it is preferred in the case of certain samples to allow a delay of 10 seconds to as long as one hour, to permit complete mixing, or extraction of materials from solid samples into the titration mixture.
This can be provided by including a conventional timer to delay operation of the current source for a preselected time period. Since drift due to side reactions or atmospheric contamination, for example, can occur during periods (a) and (b), it is necessary to include such periods in the ti-tration time, Ttitr, in addition to period ~c~
during which the titration itself actually occurs.

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Figure 1 illustrates a filtered detector output signal in a titration exhibiting relatively slow positive drift;

Figure 2 is a block diagram illustrating the operating and control circuits of a titrator of the invention;

~ igure 3 illustrates details of the measurement and drift correction circuits of a titrator of the invention in an embodiment adapted for coulometric Karl Fischer titrations.

As illustrated in Figures 2 and 3, the apparatus comprises a pair of electrolysis elec-trodes 34, 36 connected to a main electrolysis cur-rent source 35; an endpoint detector 39, which includes a pair of electrometric sensing elec-trodes 40, 41, and an indicator current source 42.
A negative drift auxiliary current source 56 is connected to electrodes 34, 36 but with opposi~e polarity to that of the main current source 35.
These basic elements are conventional and described, for example, in U.S. Patent 3,726,778~

Endpoint detector 39 is connected through a conventional buffer/amplifier and voltage limiter 43 to a variable low pass filter circuit 10. The output of filter 10 (the filtered detector signal) is connected to an endpoint comparator 24 and a negative drift comparator 52. A voltage source 28 is connected to endpoint comparator 24 to provide a pre-set reference voltage corresponding to the ti-tration endpoint. The output signal of comparator 24(indicated as Sl in Fig. 3) is connected to a light 27,741-F

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~.t2251~4 actuated switch, LAS 33, which controls the operation of the main current source 35 while electrically iso-lating the comparator 24 from current source 35, a titration counter 64, clocks 60, 62, and drift correction and readout circuits 65. In a coulometric Karl Fischer titrator embodiment with an amperometric sensing electrode as th~ endpoint detector 39, the detector signal is a voltage signal which decreases during the titration as water is consumed to an endpoint value lQ of, for example, about 70 millivolts. Comparator 24 and LAS 33 are connected to activate electrolysis current source 35 when the signal from endpoint detector 39 indicates the presence of any untitrated sample (by a detector signal above the endpoint reference level) and to switch off current source 35 when the detector signal reaches the endpoint reference level provided by reference voltage source 28. The current source 35 remains switched off by LAS 33 until the detector signal at comparator 24 rises above the end-point level (due to drift, untitrated sample, or newsample addition,). The elect.rolysis current source 35 will be activated periodically after the endpoint is first reached.

An unfiltered output signal from detector 39 is an extremely noisy signal, whereas a filtered output signal is more ameanable to measurements in typical titrations. As best shown in Figure 1, a titration continues for an initial titration time Ttitr, beginning with the start of a sample introduction period, con-tinuing through a period during which the constantcurrent source operates, and ending when the titration reaches it initial endpoint at T-Ttitr=O in Figure 1.
(The signal illustrated is a voltage limited to a maxi-mum shown at (a)). Although the electrolysis current 27,741-F

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is swi-tched off at the endpoin-t, a-t T-Ttitr=0, the detector signal drops below the endpoint to (f) and then begins to rise during a five-second initial post-titration stabilization period from T-Ttitr=0 to T-Ttitr=5. The second, drift observation time period begins at T-Ttitr=5 seconds. The second post-titration period ends and the third period begins at T-Ttitr=10 seconds, when the titration current is switched on (and later off), giving rise to a retitration l'spike'l ~b).

In the vicinity of the endpoint, ~he voltage signal from the endpoint detector 39 is extremely sensitive to very small amounts of water in the titration mixture which results in very large changes in the output signal. During a spike (b), (c), (d) or (e), the titration current is switGhed on; titrant is generated at the electrodes 3~, 36;
titrant is mixed with the titration mixture, ti-trating the water; the detector 39 detects the return to the endpoint; and the titration current is switched off again, all substantially instantaneously.
The sensitivity effects, as well as the response time of the recorder pen, contribute to the height and shape o~ the spikes. It is apparent from the spikes of Figure 1 that it would be difficult, if not impossible, to measure drift direc-tly from the spikes in the de-tector output signal.

In the spike at (b), the signal again returns substantially instantaneously, down to the endpoint and below, then gradually drifts upward at (g) during the third time period. The upward drift ts followed by another retitration spike (c), an "overshoot" (below the endpoint) and a gradual 27,741-F

2Z!5~4 upward drift (h) to another retitration spike (d).
With the third time period having a minimum time duration of, for example, 30 to 50 seconds, the third time period would be 55 seconds long, from the beginning of spike (b) at T-Ttitr=10 seconds to the beginning of spike (c) at T=65 seconds.
With a minimum third period duration of 60 seconds, th~ third time period would continue until the beginning of spike (d) at T-Ttitr=102 seconds, making the third period 92 seconds long (from T-l'titr=10 to T Ttitr As best shown in Figure 1, the drift cycles, (f), (g), (h), and ~i~ are not uniform in duration. It i5 necessary for the third time period to begin and end with post-titration spikes, preferably with the beginnings of spikes. The third time period in different titrations will not neces-sarily be of the same duration.

To provide a quick shut-off of titration current at the endpoint, together with adequate filtering o~ the detector si~nal so that return to the endpoint can be reliably detected, the filter circuit 10 is connected between the buffer/-amplifier/voltage limiter circuit 43 and the com-parators 24, 52. The ~ilter circuit 10 comprisesa capacitor 12, connected in series through resistors 15 and 17 to the buffer/amplifier and voltage limiter 43 to receive the detector input. Capacitor 12 is also connected to an amplifier 22. Two additional resi~tors 14 and 16 are connected in series with each other and each resistor is also connected in parallel with one of the res~stors 15 and 17, respectively, via switches 18 and 20. Switches 18 and 20 ~both shown in the 27,7~1-F

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open position in Figure ~) are controlled by swltch actuator 26 (connected to the output of comparator 24) so that both the switches 18 and 20 are closed or bokh switches are opened substantially simultaneously.
Resistors 14 and 16 are selected to provide different resistances in the RC filter circuit and thus pro~ide different cut off frequencies and time constants depending on whether resistors 14 and 16 are switched into or out of the filter circuit.
Switch actuator 26 is connected to comparator 24 to provide short-time-constant/high-pass filtering during th titration and long-time-constant/low--pass filter characteristics when the endpoint is met or exceeded.

The input signal, e.g. the detector elec-trode signal from the titrator, is connected through the buffer/amplifier and voltage limiter 43 to the filter 10 which can be connected as an RC filter circuit.
In one mode the RC filter circuit includes capacitor 12 and resistors 15, 17 or, in another mode, when switches 18 and 20 are closed, resistor 14 in parallel with resistor 15 and resistor 16 in parallel with resistor 17. The filter signal is supplied to the input amplifer 22.

The junction of resistors 15 and 17 is connected through a capacitor 19 to the negative input terminal and the output terminal of amplifier 22.
The filter circuit 10, including amplifier 22, can be characterized as a second order VCVS (voltage-con-trolled-voltage-source) low pass active filter with variable frequency characteristics, depending on whether switches 18 and 20 are open or closed.

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The output amplifier 22 is connected to the input of the endpoint comparator-ampli~ier 24, the other input of which is connected to the ref-erence voltage source 28. The output of comparator--amplifier 24 is connected to the switch actuator 26.
The output of the comparator-amplifier 24 is also con-nected, through a resistor 30, to the light activated switch LAS 33, preferably using a light emitting diode (not sho~n) which is connected to the com-parator-amplifier 24 output and to ground, and a phototransistor (not shown~.

In a coulometric titrator, the input signal supplied to filter 10 is the output from the endpoint detector (39), typically the amplified signal from a potentiometric s~nsing electrode.
The reference voltage supplied by voltage source 28 is a predetermined voltage corresponding to the titration endpoint conditions. The LAS 33 is con-nected to the electrolysis circuit, so that the electrolysis current can be switched on or off by the LAS 33 in response to the output ~rom the comparator-amplifier 24.

A representative endpoint voltage is 70 millivolts and the voltage-limited input signal is preferabLy amplified by the amplifier/Yoltage limiter 43 so that a reference voltage of 7.0 volts can be used at the comparator-amplifier 24 to identify the endpoint with amplifier 22 having a gain of unity. In a typical coulometric titrator, the input signal can be the detector electrode signal amplified by a gain of 10, limited by amplifier~voltage limiter 43, and amplified again 27,741-F

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by a gain of 10 by -the amplifier/voltage limiter 43. The amplifier/voltage limiter 43 preferably limits the maximum signal voltage to, for example, 0.9 or 1.0 volt, to limi-~ the voltage range at the filter 10. The maximum limited voltage is selected to be sufficiently above the endpoint voltage so as not to interfere with detection.

During the titration, switches 18 and 20 are closed, so that resistors 14 and 16 are in ~he filter circuit. In this mode, the filter 10 has as fast time response, with a time constant of about 0.1 second and a cut-off frequency of about 10 Hertz. As the detector voltage approaches the endpoint level, the filtered signal supplied to comparator 24 approaches the reference voltage.
~hen the endpoint condition is reached, the short time constant of the filter provides a fast re-sponse at the comparator 24. The resulting end-point output from the comparator 24 shuts off the electrolysis current via LAS 33; and, through switch actuator 26, switches 18 and 20 are both opened. With the values given, the resulting state of filter 10 now has a slow time response, with a time constant of about lQ seconds and the input signal is heavily filtered, with a cut-off frequency of about 0.1 Hertz.

At this time the input signal continues to drop below the predetermined endpoint level, thus "overshooting" the endpoint for a brief period.
Some "overshooting" is an inherent result of locally different conditions which may exist in the ~iquid titration mixture at the titration electrodes and at the detector electrodes. The input si~nal then 27,741-F

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gradually increases, as the ti-tration mixture be-comes more uniform with continued mixing, and the reaction conditions at the endpoint detector approach a uniform condition of the entire titration mixture.
During this "catch-up" period, the input signal increases slowly back to the endpoint level.

During the "catch-up" cycles, the input signal is not only changing at a slower rate than during the titration, but the input signal is much closer to the endpoint than in the early titration phase. The increased filtering provided by filter 10 during this phase allows an accurate and reliable detection of the return to the endpoint at the end of a "catch-up" phase.

When the input signal rises to the end-point level, the filtered input to the comparator 24 indicates the presence of untitrated sample.
In response to the comparator 24 the electrol~sis current is again switched on via LAS 33, to ti-trate the residual untitrated sample. Simul-taneously, actuator 26 opens switches 18 and 20 returning the filter 10 to its short-time-constant state.

As shown in Figure 2, the filtered de-tector signal is also fed to the negative driftcomparator 52, which is connected to a reference voltage source 55. The reference voltage from the source 55 is maintained at a predetermined level corresponding to the negative dri~t of the detector signal below the endpoint level. For example, in a Karl Fischer titrator with an endpoint reference 27,741-F

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voltage of 70 millivolts, a suitable negative drift reference voltage level can be 55 millivolts. Com-parator 52 is connected to a LAS 54 which switches the negative drift auxiliary current source 56 on or off, in response to an output signal S2 (Fig. 3 of comparator 52.

The outputs of comparators 24 and 52 are connected through the respective LAS 33 and 54 to the resettable titration counter 64 which measures the electric charge used to titrate or to correct drift, and to the drift correction/readout circuit 65.
When both current sources 35 and 56 are constant current sources, for example, the main electrolysis current source 35 preferably provides ~100.35 milliamperes when switched on, and the negative drift auxiliary current source 56 provides -25.09 milliamperes. With constant current electrolysis, the titration counter 64 is preferably a clock or timer which records the time during which the current sources operate. Clock 60 is connected to both titration counter 64 and real time clock 62 to provide uniform timing measurement.

As shown in Figure 3, endpoint comparator 24 through LAS 33 provides a signal Sl, which corresponds to the endpoint status of the detector signal and the main electrolysis current from main current source 35.
Similarly, a signal S2 from comparator 52 and its LAS 54 indicates the negative drift status of the detector signal and negative drift auxiliary current source 56.
Titration counter 64 provides a signal Q which corresponds to the electric charge used in the titration, the signal Q being a measurement of the time that either current source 35 or 56 has been switched on. Real time clock 62 provides a signal T, 27,741-F

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~%~;04 which corresponds to the actual duration of the ti-tration procedure independently of switching on and off of sources 35 and 56 before and after the endpoi~t is first reached.

For simplicity, signals Sl, S2, T and Q
are shown repeatedly as inputs or connections to various elements in Figure 3. It is understood that the apparatus includes appropriate connections bet~7een the comparators 24 and 52, LAS 33 and LAS 54;
counter 6~, and clock 62 to supply these signals.

The drift correction and readout circuits 65 include a series of gate circuits (gates 70 to 74);
reader/memory elements 75 to 78 for signals T and Q, which are at various times controlled by real time clock 62 and comparators 24 and 52 through the gates 70, 71, 72 and 73; and signal function manipulators 80, 81, 82, 83A, 83B and 84 some of which are con-nected to certain of the reader/memory elements 75 to 78 and gate 74; and a readout circuit. The readout ~0 circuit includes conventional display counter 85A and display 85B for displaying the results. A conven-tional converter/multiplier 94 and unit factor memory 98 is provided ~or converting the results into desired numerical units, such as coulombs, ~5 moles, millimoles, or micrograms and into desired numerical base, such as binary, octal, or decimal units.

A reset element 86 is connected to real time clock 62 and counter 64 to reset the real time clock 62 and counter 64 and clear the memories 75-78 when a start signal from a start switch 90 27,741-F

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is produced. A conventional clear display circuit ~37 is connected to start switch 90 to clear display counter 85A and display 85B.

The gates 70-74, reader/memory elements 75-78, and signal function manipulators 80-84, follow the titration and any post-titration drift, correct the result for drift (if any) and transmit the final result to the display circuits 85A and 85B.
When the display circuits receive the final result, reset element 86 resets the titration counter 64 and real time clock 62. In a further embodiment, the device can include additional circuits to monitor drift periodically in the apparatus between titrations, and -to apply the most recent drift correction to the readout during subseguent -ti-trations until the first endpoint.

The apparatus is controlled at several points by signals S1 and S2 from comparators 24 and 52. Each comparator signal corresponds to one of two states of the detector signal and current sources 35 and 56. For brevity, the two signal states are hereinafter referred to as OFF and ON, corres-ponding to the following:

S1 = OFF, detector signal belo~ endpoint, main titration current source 35 off;
S = ON, detector signal at or above end-1 point, main titration current source on;
S2 = OFF, detector signal above predeter-mined negative drift limit (which is below the endpoint level); auxiliary current source off; and 27,741-F

.

S2 = ON, detector signal below negative drift limit; au~iliary current source on.

Operation A titration is started, e.g., by actuating the "start" switch 90 just prior to adding a sample to the titration reagent, clearing the display cir-cuits 85A and 85B and actuating reset element 86.
Reset element 86 resets and starts the real time clock 62, and resets counter 64 to zero and briefly inhibits operation of the current source 35 for a predetermined sample introduction period. If the sample contains water detected by endpoint detector 39, Sl goes to the ON state, and starts counter 64.
When the titration first reaches the endpoint, the signal Sl from the comparator 24 goes to OFF, stopping the titration counter 64 but not the clock 62; and T and Q are transferred to memory elements 75 and 76. These signals are Ttit~ (titration time~
and Qtitr (titration charge). A 5-second stabili-zation period, and a 3Q-second maximum drift obser-vation period are suitable for use. The irst stabili-zation period "5-second" gate 7Q follows the signal T from real time clock 62 for 5 seconds after Sl irst goes to OFF, then activates the drift obser-vation period gate, "3Q-second" gate 71. The 3Q
second gate 71 monitors the clock signal T, and Sl and S2. If both S1 and S2 remain OFF for the entire 3Q-second observation period~ gate 71 directs 3Q the transmission of the signal Qtitr from memor~ 75 through subtractor 82 and converter/multiplier 94 to the display 85A. S1 and S2 both remain OFF only if there is no detectable drift during this period, therefore Qcorr in subtractor 82 is zero. Gate 70 27,741-F

, .

controls the post-titration stabilization period and gate 71 controls the drift observation period (the second post-titration time period).

In the usual case, some drift will occur and either Sl or S2 will switch ON during the period controlled by gate 71, as the device either titrates positive drift back to the endpoint or back titrates negative drift back to the pre-set limit. When either Sl or S2 goes to ON during this period, the 30-second gate 71 directs the txansmission of the signals T and Q from real time clock 62 and counter 64 to the memory element 77. T and Q at this time can be designated as TCo and Qco the subscript "Co" denoting the beginning of the drift correction measurements. Gate 71 also activates gate 72 in response to which comparator signal (Sl or S2) is ON. Gate 71 also activates correction selector gate 74 to connect function elements 81, 82 for Sl = ON or function elements 83, 84 for S2 = ON.

A useful minimum duration or the drift measurement period is 45 seconds. Once activated by Sl or S2 going to ON, the gate 72 monitors the time signal T from real time clock 62 and then activates the Gate 73 after 45 seconds. The subscript "Cl"
for gate 73 denotes the end of the correction measure-ment period. The activated Gate 73 monitors T, Sl and S2 and when either Sl or S2 goes to ON again, or if neither Sl or S2 goes ON for 30 seconds gate 73 trans-fers Q and T at that time to QC ~ TC reader/memory element 78.

27,741-F

2~ 4 The apparatus has thus de-termined the titration char~e Qtitr and time Ttitr un first reached OFF; delayed 5 seconds; then moni-tored Sl and S2 during a second time interval, and de-termined T and Q at the beginning (Tco, Qco) andend (TC ~ QC ) of a third time interval which started with S1 or S2 switching ON after the 5 second delay and ended with the next ON state of Sl or S2 which occurxed after the expiration of 45 seconds from the start of the third time interval or, if there is no subsequent ON state within the 30 seconds of ~ate 73, which time period ended after the 45 seconds of gate 72 plus the 30 seconds of gate 73.

Gate 73 also activates the drift rate calculator function element 80, which uses Qco and T from memorY 77 and QC1 and Tc1 memory element 78 to generate a drift rate signal corresponding to:

QC ~ QC
D.R. =
TC1 Tco Element 80 thus comprises subtractors and a divider circuit element.

I~ S1 was ON durin~ the post-titration period, correction selector gate 74 transmits the drift rate signal from calculator 80 to correction multiplier 81 which multiplies the drift rate sig-nal D-R- by Ttitr stored in memory element 76 to produce a drift correction value signal Qcorr 27,741~F

o~

corresponding to the amount of titration charge Qcorr which was consumed during -the titration by drift compensation ra-ther than by titrating sample water. Qcorr is transmitted from multiplier 81 to sub~ractor 82, which receives Qtitr from memory 75. Subtractor 82 subtracts Qcorr from Qtitr and transmits the resulting signal, now corres-ponding to the titration result corrected for t (~titr Qcorr) to display 85A via converter/-multiplier 94.

If negative drift occurred, S2 was ONduring the post-titration period. Gate 74 trans-mits the drift rate ~.R. from drift rate cal-culator 80 to correction multipliers 83A and 83B, which divides the drift rate D.R. (in 83A) by a factor Kl corresponding to the ratio of main ti-tration current to negative drift auxiliary current to provide a drift rate signal DR' in units equivalent to the main titration measurement Qtitr- Correction multiplier 83B multiplies the thus adjusted drift rate signal D~' by Ttitr. For example, when the main current is 100.35ma and the auxiliary current is -25.Q9ma, the drift rate from calculator 80 must be reduced by a factor of 4~ since QCl ~ Qco is actually only a meaSurement of time, not current. The resulting correction value Qcorr from 83B corresponds to the amount of additional charge which would have been used in the titration if negative drift had not occurred.
Multiplier 83B transmits Qcorr to adder 84 which dds Qcorr to Qtitr (transfered from memory 75) and transmits the resulting signal to the display circuit 85A through converter/multiplier 94.

27,741-F

s~

It will be apparent from the foregoin~
that the correc-ted result signal from either sub~
tractor 82 or adder 84 (or from counter 64 as directed by gate 71 when there is no drift) is a measurement of titration charge in terms of ti-tration time at constant titration current. The display circuits 85A and 85B preferably include a conventional converter/multiplier 94 and unit factor memory 98 to convert the corrected Q signal to coulombs, or to moles, or micrograms of water or some other desired units.

In an alternative embodiment of the inven-tion the display elements 85A and 85B can be supplied with a partially corrected Q signal during the titration and before the drift correction process is com-pleted. This allows the operator to observe approx-imate results as the titration progresses so that unusual conditions such as sample addition errors can be signalled early. This can be done, for example, by providing an additional memory element (not shown) to store the most recently determined drift rate DR (or DR/Kl in case of negative drift) and correcting the Q display using the most recent drift rate correction.

The interaction of the drift correction circuits, drift monitor circuits and display cir-cuits 85A and 85B is illustrated in Figure 3.

The display circuit comprises a digital display panel 85B controlled by a display counter 85A.
The result signals Qtitr + Qcorr from the adder 84r or Qtitr ~ Qcorr from the subtractor 82 are, as noted 27,741-F

, above, in units corresponding to time a-t constant current. The converter/multiplier 9~ is provided to multiply the Q signals by the appropriate con-version factor (from the unit factor memory 98~ so that the display 85B is in desired units such as micrograms of water. In construction and operation the titration counter may be counting time at a much higher rate than is desired for the display counter.
(e.g., if clocks 60 and 62 and titration counter 64 are counting at 600 Hertz, it may be desirable to have the display counter operate at 1 Hertz or less, partic~larly when it is desired to display a Q signal during the titration.) Converter/multiplier 94 thus can use two conversion factors, a display factor to convert titration counter time units to display counter time units, and a second conversion factor proportional to the main electrolysis current (coulombs/time~ to provide a display in units such as coulombs, moles, or micrograms of water.

When it is desired to display a corrected Q value continuously during the titration, the apparatus can be modified to provide an additional memory element (not shown) to store the drift rate (DR, or DR' for negati~e drift) from the most re-cent titration, and multiplier elements to continuously apply that drift rate factor to the Q signal to be displayed. If desired, a drift correction factor used for continuous Q displays can be revised periodically between titrations by resetting clock 62 and counter 64, and clearing memory elements 75-78 at the end of a titration correction se~uence, and using the gates 70-74, memories 77 and 78 and element 80 to monitor drift conditions between titrations.

27,741-F

Elements 70-84, 94 and 98 can be assembled using separate integrated circuit chips, such as, gates, memories, adders, subtractors, or multi-pliers, or preferably using a conventional arith-metic logic unit for the arithmetic operations.
Preferably, these elements as well as clocks 60, 62 and counter 64 are memory locations in a micro-processor. Com~lercially available units such as an Intel microprocessor No. 8085, with two program-mable read-only memory units (Intel No. 2716) and a random access memory ~Intel No. 8156), Intel Corp., Santa Clara, California are used in a preferred apparatus. It will be apparent, also that the memory elements 75 to 78 can be constructed using separate display elements, or an appropriate printer, to provide the necessary values of Qtitr Ttitr, Qco Tco, QC and TC I so that the correct computations can be carried out separatel~, either manually or on a separate, appropriately programmed digital com-puter, for example. Also the time periods applied by the various gates 70, 71, 72 and 73 can be varied.

The invention has been described with 2S respect to coulometric Karl Fischer titration of water, with an endpoint condition sensed as volt-age and approached during the titration from a higher endpoint detector voltage so that positive drift corresponds to increasing voltage and nega-tive drift corresponds to negative voltage. Itwill be apparent, however, that the method and apparatus can be readily adapted to other titrations and other endpoint detection systems, such as, 27,741-F

~ . .

~2~ 4 for example, the system described in U.S. Patent No. 3,723,062.

An apparatus of the type described was constructed using a 100.35 milliampere titration current a -25.09 ma negative drift titration cur-rent, and 70 millivolt endpoint, with the above--mentioned Intel microprocessor and memory compo-nents using a 5 second stabilization period at gate 70, a 3~ second period for gate 71, a 45 second period at gate 72 and a 30 second period for gate 73. In representative titrations of a sample con-taining 450 micrograms of water, with no drift, the mean result was 450 micrograms with a standard deviation of ~5. With a 450 microgram sample and positive drift (due to acetone in the sample) of about 300 micrograms/minute, the mean result cor-rected by the invention was ~68 micrograms with a standard deviation of +13 micrograms. The un-corrected mean result was 745 micrograms ~18 micro-grams. In a similar operation with moderate drift,the uncorrected mean Qtitr of replicate samples containing 608 micrograms i8.9 micrograms water was 2200 micrograms, while the apparatus and method of the invention gave a mean corrected result of 575 micrograms with a standard deviation of ~25.6 micro-grams. It will be apparent that the invention not only provides improved results in coulometric titrations, but allows coulometric titrations under drift conditions in which coulometric titration was previously impractical.

27,741-F

Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An automatic coulometric titrator comprising titration means for introducing a titrant into a titration mixture, detection means for detecting the endpoint of the titration, means responsive to the endpoint detection means for controlling the titration means to introduce ti-trant when the titration mixture is not at the endpoint, and titrant measuring means for measuring the amount of titrant introduced, characterized by (a) timer means for measuring the titration time until the titration mixture first reaches an initial endpoint;
(b) means responsive to one of the endpoint detection means and the -titration means for detecting the introduction of titrant during a predetermined post-titration time period after the initial endpoint;
(c) means for measuring the amount of titrant introduced during a second post-titration time period after the initial endpoint, said second time period beginning with the detection of titrant introduction by means (b), and ending with a pre-determined subsequent introduction of titrant;

27,741-F

(d) means including a timer for measuring the duration of said second post -titration period and the amount of titrant introduced during said period, said time and amount measurements by means (c) being indicative of the drift rate during the titration, and the drift rate and titration time measured by means (a) being indicative of the amount of drift.

2. The titrator of Claim 1 wherein the titration means includes electrolysis electrodes and a constant current source for electrolytic generation of titrant in the titration mixture, and the titrant measuring means includes a timer responsive to the current source for measuring the duration of electrolytic generation of titrant, and generating a titration measurement signal Q, and the timer means (a) includes a clock means for generating a time signal T, characterized by (e) gate means responsive to one of the end-point detection means and the current source and to the time signal T for de-tecting operation of the current source during a predetermined post-titration period;
(f) display means responsive to gate means (e) for displaying the titration measure-ment signal Q when the gate means detects expiration of the time period without operation of the current source;

27,741-F

(g) gate means responsive to gate means (e) and to the time signal T for determining the minimum predetermined duration of -the second post-titration period, beginning with detection of current source opera-tion, being also responsive to the end-point detection means for determining the end of the second post-titration period;
(h) memory means responsive to the endpoint detection means for storing the titration measurement signal Qtitr and the time signal Ttitr at the initial endpoint;
(i) second and third memory means for de-termining the time and titration measure-ment signals, TCo and Qco at the beginning, and TC1 and QC1 at the end, of the second post-titration period.

3. A coulometric titrator having electro-lysis electrodes for electrolytically generating titrant in a titration mixture, a constant current source for providing electrolysis current to said electrodes, and an endpoint detector for providing an endpoint signal indicative of the status of the titration mixture relative to the endpoint of the titration, characterized by a clock for generating a time signal T
during and after a titration;
a titration counter for generating a charge signal Q corresponding to the charge passed through the titration mixture by the electrodes and constant current source;

27,741-F

a comparator connected to the endpoint detector and adapted to provide a signal S1 having an S1=ON state when the endpoint signal is above a predetermined endpoint level and an S1=OFF state when the endpoint signal is at or below the end-point level;
means including a switch responsive to the comparator signal S1 for activating the current source and the titration counter when S1=ON and inactivating said source and counter when S1=OFF;
first gate means responsive to the time signal T and comparator signal S1 for detecting a recurrence of the S1=ON state during a predeter-mined time period after S1 first attains the S1=OFF
state at the end of a titration;
memory means responsive to signals S1, T and Q including a titration time memory and a titration charge memory for storing T and Q sig-nals, Ttitr and Qtitr, when S1 first attains the S1=OFF state at the end of a titration;
display means for displaying the signal Q as a titration result Qtitr in response to the first gate means signal corresponding to the lapse of the first gate time interval without a recur-rence of S1 attaining the S1=ON state;
first drift correction memory means res-ponsive to signals T and Q and the first gate means, for storing the T and Q signals as QCo and TCo in response to the first gate signal indicative of occurrence of the S1=ON state during the first gate time interval;
drift correction means including second and third gates, the second gate being responsive to the first gate and the time signal T to activate 27,741-F

the third gate at a predetermined time after the first gate signal indicative of the occurrence of the S1=ON state during the first gate time interval, and the third gate being responsive to the S1 signal and the second gate;
second drift correction memory means responsive to the Q and T signals and to the third gate means for storing the Q and T signals as QC1 and TC1 at the time the third gate indicates occurrence of the S1=ON state of signal S1.

4. The titrator of Claim 3 characterized by drift rate calculator means responsive to the third gate for generating a drift rate signal DR
from the Q and T signals in the first and second drift correction memory means, the signal DR car-responding to

5. The titrator of Claim 4, characterized by multiplier means for providing a result correction signal Qcorr from the titration time signal Ttitr in the first memory means and the drift rate signal DR, Qcorr being the product of Ttitr x DR;
a subtractor for subtracting the Qcorr signal from the Qtitr signal from the first memory means; and means for transmitting the difference from said subtractor to the display means to display the corrected result.

27,741-F

6. The titrator of Claim 5, characterized by a negative drift constant current source of opposite polarity to the electrolysis current source;
a second comparator connected to the end-point detector and adapted to provide a signal S2 having an S2=ON state when the endpoint signal is below the endpoint by a predetermined amount and a S2=OFF state when the endpoint signal is above said predetermined amount;
means for switching the negative drift current source ON and OFF in response to the second comparator signal S2 being ON or OFF; and wherein the first and third gate means and the first and second drift correction memory means are adapted to operate the same when either S1 or S2 is in the ON state; and means including a correction selector gate responsive to the first gate means for indi-cating which of S1 and S2 attains the ON state during the first gate time interval.

7. The device according to Claim 3, including a VCVS low pass active filter having a signal input and signal output;
an RC filter circuit comprising a resis-tance connected to the input and a grounded capacitor;
an operational amplifier with an input connected to the output of the RC circuit, char-acterized by a) a second resistance switchably con-nected in parallel with the resistance of the RC
circuit; and 27,741-F

b) switching means responsive to the output of the operational amplifier for selectively switching the second resistance in response to a predetermined output signal.

8. The device of Claim 7, characterized by the feature that the switching means comprises a comparator having an input connected to the operational amplifier output, and another input connected to a reference voltage source to pro-vide a comparator output signal responsive to the level of the operational amplifier output relative to the reference voltage, said reference voltage being indicative of endpoint of the titration.

9. A method for correcting positive drift in an automatic coulometric titrator, char-acterized by the steps of generating a signal Q corresponding to the amount of titrant introduced into the titration mixture;
generating a time signal T corresponding to time elapsed from beginning of the titration;
introducing titrant continuously until a predetermined endpoint condition is detected, and thereafter reintroducing titrant periodically as needed to maintain the reaction mixture at the endpoint condition;
storing the Q and T signals as Qtitr and Ttitr at the time the endpoint is first reached;
determining the titration result from (Qtitr) when no additional titrant is introduced during a predetermined time interval after the endpoint is first reached; and 27,741-F

generating a drift rate signal DR when additional titrant is introduced during said in-terval, said drift signal DR corresponding to the difference between (i) the Q signal at the time (TCo) of an additional titrant introduction (Qco) and the Q signal at the time (TC ) of a predeter-mined later additional titrant introduction (QC1) divided by (ii) the difference in time (TC1 -TCo ) between said introductions;
generating a correction signal (Qcorr) corresponding to the product of the drift rate signal DR and the stored (Ttitr) signal, said correction signal thus corresponding to the amount of titrant consumed by drift during the initial titration; and determining the titration result from the difference between the stored (Ttitr) signal and the (Tcorr) signal-

10. The method of Claim 9, characterized in that the titrant is introduced by passing a con-stant current through the titration mixture to gen-erate titrant electrolytically, and wherein the introduction of titrant is controlled by providing an endpoint detection signal, S1, having a first state S1=ON when the titration mixture is above or at a predetermined endpoint, and a second state S1=OFF when the titration mixture is below the end-point, and switching the constant current source ON when S1=ON and OFF when S1=OFF.

27,741-F

11. The method of Claim 10, characterized in that the signal Q is generated by generating a time signal while the S1 signal is in the S1=ON
state, whereby the signal Q corresponds to the time during which the constant current source is ON.

27,741-F