CA1132660A - Method of measuring with redox- or ion-sensitive electrodes - Google Patents
Method of measuring with redox- or ion-sensitive electrodesInfo
- Publication number
- CA1132660A CA1132660A CA351,040A CA351040A CA1132660A CA 1132660 A CA1132660 A CA 1132660A CA 351040 A CA351040 A CA 351040A CA 1132660 A CA1132660 A CA 1132660A
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- Prior art keywords
- measuring
- potential
- reagent
- concentration
- component
- Prior art date
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- 238000000034 method Methods 0.000 title claims description 42
- 239000003153 chemical reaction reagent Substances 0.000 claims abstract description 32
- 238000005259 measurement Methods 0.000 claims abstract description 30
- 238000006243 chemical reaction Methods 0.000 claims abstract description 25
- 150000002500 ions Chemical class 0.000 claims abstract description 17
- 238000013208 measuring procedure Methods 0.000 claims abstract description 3
- MHAJPDPJQMAIIY-UHFFFAOYSA-N Hydrogen peroxide Chemical compound OO MHAJPDPJQMAIIY-UHFFFAOYSA-N 0.000 claims description 47
- 239000000243 solution Substances 0.000 claims description 27
- XFXPMWWXUTWYJX-UHFFFAOYSA-N Cyanide Chemical compound N#[C-] XFXPMWWXUTWYJX-UHFFFAOYSA-N 0.000 claims description 26
- 239000000306 component Substances 0.000 claims description 25
- 238000001311 chemical methods and process Methods 0.000 claims description 8
- 239000007864 aqueous solution Substances 0.000 claims description 3
- 230000006872 improvement Effects 0.000 claims description 2
- 230000001276 controlling effect Effects 0.000 claims 1
- 229960002163 hydrogen peroxide Drugs 0.000 description 19
- 238000012360 testing method Methods 0.000 description 7
- 230000002452 interceptive effect Effects 0.000 description 6
- 150000001875 compounds Chemical class 0.000 description 5
- 238000011437 continuous method Methods 0.000 description 4
- 239000007788 liquid Substances 0.000 description 4
- 238000012544 monitoring process Methods 0.000 description 4
- 239000002253 acid Substances 0.000 description 3
- 239000003054 catalyst Substances 0.000 description 3
- XLJMAIOERFSOGZ-UHFFFAOYSA-M cyanate Chemical compound [O-]C#N XLJMAIOERFSOGZ-UHFFFAOYSA-M 0.000 description 3
- 230000003334 potential effect Effects 0.000 description 3
- 150000007513 acids Chemical class 0.000 description 2
- 230000003111 delayed effect Effects 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 238000006073 displacement reaction Methods 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- FOIXSVOLVBLSDH-UHFFFAOYSA-N Silver ion Chemical compound [Ag+] FOIXSVOLVBLSDH-UHFFFAOYSA-N 0.000 description 1
- 239000012190 activator Substances 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 239000000337 buffer salt Substances 0.000 description 1
- 238000004364 calculation method Methods 0.000 description 1
- 238000011088 calibration curve Methods 0.000 description 1
- 230000002844 continuous effect Effects 0.000 description 1
- 238000010924 continuous production Methods 0.000 description 1
- 238000000354 decomposition reaction Methods 0.000 description 1
- 230000002939 deleterious effect Effects 0.000 description 1
- 230000001627 detrimental effect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 238000004880 explosion Methods 0.000 description 1
- 239000012467 final product Substances 0.000 description 1
- 239000001257 hydrogen Substances 0.000 description 1
- 229910052739 hydrogen Inorganic materials 0.000 description 1
- -1 hydrogen ions Chemical class 0.000 description 1
- 230000001788 irregular Effects 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 238000002156 mixing Methods 0.000 description 1
- 150000002825 nitriles Chemical class 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 238000000746 purification Methods 0.000 description 1
- 239000000376 reactant Substances 0.000 description 1
- 230000036647 reaction Effects 0.000 description 1
- 239000012429 reaction media Substances 0.000 description 1
- 229910052709 silver Inorganic materials 0.000 description 1
- 239000004332 silver Substances 0.000 description 1
- 230000002269 spontaneous effect Effects 0.000 description 1
- 238000003756 stirring Methods 0.000 description 1
- 238000003860 storage Methods 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/416—Systems
- G01N27/4163—Systems checking the operation of, or calibrating, the measuring apparatus
- G01N27/4165—Systems checking the operation of, or calibrating, the measuring apparatus for pH meters
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- Chemical & Material Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Health & Medical Sciences (AREA)
- Physics & Mathematics (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Molecular Biology (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analyzing Non-Biological Materials By The Use Of Chemical Means (AREA)
- Investigating Or Analysing Biological Materials (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
To compensate for the zero-point changes in continuous measuring procedures with redox- or ion-sensitive electrodes a partial flow is branched off from the system to be measured and the potentiometric measurement is so carried out therein that on measuring potentiometrically the instantaneous concentration of the ions to be determined the zero point is newly determined by complete reaction of this ion type with a suitable reagent.
The two measurements are suitably carried out by inserting a cycle-control device.
To compensate for the zero-point changes in continuous measuring procedures with redox- or ion-sensitive electrodes a partial flow is branched off from the system to be measured and the potentiometric measurement is so carried out therein that on measuring potentiometrically the instantaneous concentration of the ions to be determined the zero point is newly determined by complete reaction of this ion type with a suitable reagent.
The two measurements are suitably carried out by inserting a cycle-control device.
Description
~3Z66(~
The present invention relates to a method of measuring with redox- or ion-sensitive electrodes.
It is known that in measuremen-ts, primarily those with redox- or ion-sensitive electrodes, changes occur on the elec-trodes, or example, due to interfering ions, and cause displace-ments in potential.
A phenomenon which is very undesirable when continuously carrying out chemical processes with potentiometric monitoring is for example, the drift of the potential measuring chain which normally is not important in the potentiometric monitoring of discontinuous processes because of the short duration of the single measurement in each case. However, particularly the pos-sibility of being able to carry out potential measurements on continuous processes is very important for chemical processes which are monitored, controlled and adjusted by these potential measurements primarily since in the systems involved in many industrial processes interfering ions or other detrimental . factors such as electrode encrustations are encountered, as for example, in the detoxication of effluents. Monitoring these systems by taking samples and tes-ting them by chemical analysls : in laboratories usually is extremely expensive. Furthermore, this monitoring method as a control for safely carrying out chemical processes is not without danger since unforeseen operat-., .
ing fluctuations usually are recognized too late.
Therefore, the present invention provides a potentio-metric procedure by means of which chemical processes in which unknown interfering influences on the measuring chain are to be expected can be controlled and monitored.
~ It has now been found that chemical processes, primar-:~ 30 ily those in which changes in the potentiometric measuring chain ;~ due to interfering influences are to be expected, can be carried ' out continuously and monitored and/or controlled and/or adjusted Z~6~
by using redox- or ion-sensitive electrodes when the measurements of the concentration of the component concerned is carried out on a partial flow of the system in,such a way that the potential corresponding to the instantaneous concentration of the component concerned is determined first, whereupon a reagent is added, i.e.
a reagent which completely reacts with the component, whose con-centration is to be determined, to form a compound which does not influence the potential of the measuring chain or influences it only to a negligibly small extent so that the electrode shows the potential corresponding to the concentration zero of the component concerned, the difference between the two potentials indicating the actual concentration of the component concerned.
The reagent is to be added in an amount which is at least equiva-lent to the amount required for the reaction of the component to be measured. It is added preferably in excess.
According to the present invention there is provided in,a method for at least one of measuring, controlling and adjusting chemical processes to determine the concentration of a specific dissolved component by means of redox- or ion-sensitive electrodes in which the zero point of the potential is subject to changes during the measuring procedure, the improvement in which the measurement of the concentration of said component by a measuring chain is carried out on a partial flow of the system such that the potential corresponding to the instantaneous con-centration of said component is determined first, followed by the addition of a reagent which either does not influence the potential of the measuring chain or influences it only to a negligibly small extent, so that the electrode shows the poten-tial corresponding to the concentration zero of said component whereupon the difference of the two potentials indicates the actual concentration of said component.
Of course, the method is also applicable to pH elec-Z6~
trodes as a special case of ion-sensitive electrodes.
The sequence of the method according to the invention is not necessarily restricted to the sequence "measurement of the potential of the instantaneous concentration - measurement of the zero potential".
Of course, the sequence may also be reversed, i.e. by starting with the measurement of the zero potential and after a predetermined interval the instantaneous concentration of the compound concerned is measured.
The sequences preferably are repeated cyclically, but the sequences must not necessarily be carried on to their end, i.e., for example, the zero potential is measured first and after ~-completion of one or several sequences the zero potential is again measured.
The reagent, which completely reacts the components to be measured to form a specific compound so that this component is no longer present in the reaction medium at the time of measuring the zero point, must be so chosen that neither the reagent nor the resulting compound with the component concerned changes the potential of the measuring chain.
r~hile for a number of components to be measured the reagent suitable for this purpose can be selected solely by the knowledge of their generally known properties as well as of those of the reagent to be added and of those of the resulting compound, r,; a small-scale test with the corresponding reagent is recommended in other cases. For example, for the measurement of hydrogen ions in acid and alkaline ranges the usual buffer salt solutions ' from strong bases/weak acids or weak acids/strong bases have beenfound to be suitable reagents. Even for the determination of very small amounts of deleterious substances in the important , .
field of effluent purification the selection of the corresponding reagent usually causes no difficulties.
,, ''' ~`~
~3~66~
Primarily the determination of the smallest amounts of cyanide ions can thus be satisfactorily determined with the aid of hydrogen peroxide at a drifting zero point or other inter-fering influences on the potential.
In order to explain the process in greater detail, this latter process is used as an example.
The fact that the zero point of a measuring chain is displaced parallel to the original calibration curve during the measurement, i.e., that the zero point is drifting, is a known phenomenon, which has a decisive effect primarily when carrying out the process continuously (see M. Hofton - Continuous Deter-mination of Free Cyanide in Effluents Using Silver Ion Selective Electrode - Environmental 10 (1976) 3,277/280).
In many cases, the interfering factors which cause this phenomenon are not known~ Satisfactory potentiometric measurements of concentrations are no longer possible in these systems. According to the process of the German Patent, 2,352,856 cyanide- or nitrile-containing effluents can be satisfactorily reduced with hydrogen peroxide and a special catalyst to amounts lower than 0.1 mg of cyanide ions per litre. The final concen-tration of cyanide ions is determined potentiometrically. It has been found that when continuously carrying out the detoxica-tion of these effluents the 2ero point of the redox potential is slowly displaced. By applylng the method according to the invention, in which hydrogen peroxide is used as the reagent in this case, the displacement of the zero potential in the continu-ous operation could be compensated for and the measurement of the cyanide ion concentration in the continuous detoxication of effluents could be carried out satisfactorily. The redox poten-tial was not affected by the hydrogen peroxide itself or by the cyanate formed.
Although the method according to the invention can ~L~2~
also be carried out in the actuai reaction vessels, the measure-ments on partial flows are preferred. The procedure is such that the effluents partial flow to be tested is passed, without adding the reagent (hydrogen peroxide in the present case), through a reaction zone while the corresponding potential of the measuring chain is determined, whereupon after a certain interval the reagent is continuously fed into the reaction zone during a second interval and the resulting zero potential of the measur-ing chain is measured. The addition of hydrogen peroxide is then interrupted. Thereupon the instantaneous concentration of the kind of ions to be measured (in the present case cyanide ions) is then attained again and the cycle withthe addition of the reagent (in the present case hydrogen peroxide) starts again. The reagent can be added in an amount which is stoichiometric for the reac-tion, but it is preferably added in excess. The calculation of the stoichiometric amount of reagent should be based preferably on the maximum of the expected amount of the kind of ion to be measured. In the case of unexpectly occurring variations of the concentration in the solution to be measured no incorrect results can thus be obtained. This amount can be easily deter-; mined by a small-scale test.
; The method according to the invention is independent of temperature insofar as no specific temperature range is required for carrying it out. The measuring temperature depends on the ~eactions concerned. However, at elevated temperature the rate of reaction usually is faster. The electrometric force of the measuring chain is a function of temperature in a conventional manner and as is customary for redox- and ion-sensitive electrodes this force can becompensated for, e.g. automatically by means of resistance thermometers.
The interval between 'che measurement of the zero potential and that of the cyanide ion concentration, which ..3Z66~
readjusts itself after some time, can preferably be the same for control engineering reasons. However, the result of the method per se is not affected either by irregular intervals between the various concentration measurements. The suitable intervals are determined by a preliminary test, for example, they can be from 1 to 2 minutes.
The advantage of using regular intervals lies in the possibility of inserting a cycle-control device.
The length of the interval between the determination of the instantaneous concentration and the addition of the hydro-gen peroxide in excess should be kept as short as possible since the reaction of the hydrogen peroxide with the cyanide ions ; present also requires a certain time.
The present invention will be further illustrated by way of the accompanying drawings in which:
Fig. 1 is a plot of the potential of the measuring chain with time for the addition of hydrogen peroxide to cyanide con-taining aqueous solutions, Fig. 2 is a diagram of a system using the continuous method according to one embodiment of the present invention;
Fig. 3 is a diagram of a system using the discontinuous method according to another embodiment of the present invention;
and Fig. 4 is a graph of the change in electrode potential with CN concentration in a series of effluentshaving differentCN
concentrations.
When using a cycle control device the intervals up to the measurement of the instantaneous concentration of the com-ponent of the zero potential should best be identical. The time slope of the measuring chain voltage with identical intervals for measuring the component concentration (in this case the concentration of the cyanide ions) and for measuring the zero '~
~ - 6 -, .
1~3~
potential (in this case by adding hydrogen peroxide in excess) is shown in Figure 1. The potential of the measuring chain has been plotted on the ordinate and the time on the abscissa. The intervals in which the hydrogen peroxide is added are shown as dashes 2. In this case the procedure starts with the setting of the zero potential, i.e. with the addition of the hydrogen peroxide in excess over the equivalent amount for the cyanide ions present. The intervals are measured in minutes.
As is evident from Figure 1 the potential of the measuring chain always returns to the set zero point as soon as the corresponding amount of hydrogen peroxide is fed to the system. The difference between the potentials for the points of measuring time 3 and 4 is a criterion of the cyanide ion concentration. It must be once more emphasized that the poten-tial of the measuring chain is practically un-influenced by the excess hydrogen peroxide. This was established prior to carry-ing out the measurements.
The concentration of the reagent (in general aqueous solutions are used) depends on the concentration of the type of ion to be measured. If this concentration is high, then more highly concentrated reagent solutions may be used. If this con-centration is lower, then more weakly concentrated solutions are preferable for indicating the final product.
The process according to the invention can be carried out continuously and also discontinuously. This is illustrated by means of the example of cyanide detoxication of effluent with hydrogen peroxide.
The system shown in Figure 2 operates in the following manner:
A small partial flow of the cyanide-ions-containing effluent flows via the pipe 1 and la through a reaction zone 2.
At the outlet of the reaction zone 2 the redox potential is ~L~3~Z6~
measured, for example, with a silver/thalamide measuring chain at point 3. At the inlet of the reaction zone, i.e. at the inlet of the pipe 5a into the pipe la dilute hydrogen peroxide solution from the tank 5 is fed in at lb alternately via the pipe 5a and via a valve 4, which is controlled by a time clock.
The potentiai corresponding to the cyanide-ion concentration of zero ismeasured at 3. From the difference of the two measure-ments the momentary cyanide-ion concentration is obtained. The second measurement, i.e., that of the zero potential is carried ` 10 out at 3 with the same measuring chain. The effluent is con-tinuously removed via the pipe 6 and rejected. However, when carrying out chemical processes the measured partial flow is preferably returned to the main flow.
The system is ventilated via the pipe lc.
Although the results of the measurements when continu-ously carrying out the method according to the invention are satisfactory, technical difficulties were encountered in certain cases when carrying out the measurements in practice for example by the hold-up in the reaction zone. Therefore, effective mixing of the solution to be tested with the reagent added could not always be accomplished.
For this reason it is preferable to carry out the method according to the invention discontinuously.
- The procedure is as follows: As compared with the continuous method the measurement is not carried out in a vessel through which the solution to be measured flows continu-- ously, but in a reaction vessel 3 provided with a stirrer 3 (see Fig. 3). This reaction vessel is combined with two measur-ing vessels, one vessel being intended for the solution to be tested and the other for the reagent to be added. In a first step a measured amount of the solution concerned from the measur-ing vessel 2, which is determined by the valves 2a and 2b, is .~
.
~3;26~V
fed into the reaction vessel 3. For reasons of safety against explosion, the v~lves 2a and 2b are preferably controlled by compressed air. The electrode which measures the ion concentra-tion that is momentarily of interest, for example, the cyanide-ion concentration is at 5. This electrode at 5 should prefer-ably be able to determine this concentration immediately upon adding the solution from the measuring vessel 2.
From the storage tank 6, the reagent i.e. the hydrogen peroxide in the case of measuring the cyanide ions is passed via the pipe 7 into the measuring vessel 8 for the reagent.
Like the measuring vessel 2, the measuring vessel 8 is provided with corresponding valves 8a and 8b, which also are preferably -controlled by compressed air.
In the step following the measurement of the ion con-centration the hydrogen peroxide is passed into the vessel 3.
The reaction is watched over a certain interval o~ time, where-upon the zero potential obtained is measured with the electrode at 5. The valve 9 is then opened and the vessel 3 is emptied.
The sequence is repeated correspondingly. The valves 2a and 2b as well as 8a and 8b are so controlled that a fric-tionless pro-gress of the method described above is assured.
The discontinuous manner of carrying out the method according to the invention is remarkable in that the above-mentioned difficulties encountered when carrying out the process , continuously are avoided and that the discontinuous operation is also satisfactory in industrial plants.
Furthermore, the time required for carrying out the process discontinuously can be reduced as compared with the continuous measurement. Thus, for example, in the determination of the cyanide ions the time required for a measuring cycle is approximately 2 to 3 minutes as compared with 10 minutes when carrying out the method continuously. The reason lies in the 66~
spontaneous reaction, for example of the cyanide ions with hydro-gen peroxide, in the reaction vessel which, as mentioned herein-before, is provided with a stirrer so that the reactants are mixed instantaneously. Even despite the additional time required for emptying in the discontinuous process time is still gained as compared with the continuous method.
~ oreover, the apparatus per se is substantially simpler and less prone to trouble since no streams of liquids but volumes are mixed and therefore, no flowmeters are required.
The measuring method according to the invention is also applicable to potentiostatic systems.
The present invention will be further illustrated by the following Examples:
Exarnple 1: Continuous Method A solution having a CN concentration of 12 mg per litre and a proportion of catalyst of 0.02% by volume (see German Patent 2,352,856) is used as a partial flow of liquid to be tested. The solution has a temperature of 95C and a pH value of 11.5.
A flow of approximately 100 litres of the starting solution per hour is fed continuously through the pipes 1, la and lb into the reaction zone 2. After the starting solution has passed through the reaction zone 2 its redox potential is rneasured with the electrode 3 in the first interval. For the subsequent second interval the valve 4 opens for a period of approximately 5 minutes and 0.5 litre of a 3.5% by weight H2O2 solution flows per time interval from the vessel 5 through the pipes 5a and lb into the reaction zone 2. Oxygen liberated by the decomposition of H2O2 is removed from the measuring apparatus via the air vent pipe lc. During the second interval the vari~
able redox potential which corresponds to the CN zero concentra-tion because of the complete conversion of the cyanide into ~3~6t~
cyanate, is measured. For the starting solution a potential difference of 336mV is obtained from the measurement of the two redox potentials. Because of the fixed cycle-controlled valve 4, which takes into account the hold-up in the reaction zone, the time required by the measuring process is approximately 10 minutes.
Example 2: Discontinuous ~ethod i The discontinuous testing apparatus with the fixed-cycle-controlled valves is shown in E`igure 3.
A solution, having a CN concentration of 120 mg per ~itre and a proportion of catalyst of 0.02% by volume (see German Patent 2,352,856) is used as a partial flow of liquid to be tested. The solution has a temperature of 90C and a pH
value of 11.3.
With the valve 2a closed and with the opening of the valve 2b delayed the 1000 ml measuring volume 2 is filled with the solution. After closing the valve 2b and delaying the open-ing of the valve 2c, the solution flows into the stirring vessel 3 in the first cycle. The redox potential of the starting solu-tion is measured with the electrode 5. From the measuring volume 8 of 30 ml, which has been filled up from the vessel 6 via the ` pipe 7 in the meantime, a 3.5~ by weight H2O2 solution is added ~ ~y closing the valve 8b and by delayed opening of the valve 8a.
-~ In the second cycle the stirrer 4 causes the two liquids to be ~; mixed thoroughly. The H2O2, which is present in excess over the stoichiometric amount, comple~ely reacts the amounts of cyanide present to cyanate and a redox potential corresponding to the CN zero concentration is measured. For the starting solution a potential difference of 458 mV is obtained. After the measure-ment the vessel 3 is emptied in the third cycle. A time of approximately 2.5 minutes is required for the measurement. For the equialization of pressure the vessels 3 and 6 are ventilated ., : ~
'Z~6~
via the pipes 10 and 11 respectively.
Example 3 The measuring method was tested in different test series with effluents having different CN concentrations. The non-detoxicated starting solution had a CN concentration of 120 mg per litre. This solution was diluted in stages to 1.2 mg per litre and the potentials were recorded. Furthermore, solutions having cyanide ion concentrations of 1 mg per litre, 0.1 mg per litre and 0.01 mg per litre were produced in a laboratory. The pH value was adjusted to 12. The amount of activator of 0.02%, as prescribed in the German Patent 2,352,856, was added to the solution. Figure 4 shows the relationship between change in potential and CN concentration in the range from 0.01 to 100 mg per litre. In this singly logarithmic representation an approximately linear relationship is obtained over a wide range of concentrations.
In order to determine whether the change in potential ; of 10 mV measured at 0.01 mg per litre is due to the CN concen- -tration, a test with effluent without CN was carried out. A
change in potential was observecl, its amount is certainly smaller. This value cannot be plotted on this curve in Figure 4 because of the logarithmic representation of the CN concentra-; tion. When defining this 5 mV measured at CN concentrations - of zero as zero point instability thenCN concentrations to ; approximately 0.01 mg per litre can be detected therewith with .. :
~ adequate certainty.
.''' .
. ' . ., : ' ..
The present invention relates to a method of measuring with redox- or ion-sensitive electrodes.
It is known that in measuremen-ts, primarily those with redox- or ion-sensitive electrodes, changes occur on the elec-trodes, or example, due to interfering ions, and cause displace-ments in potential.
A phenomenon which is very undesirable when continuously carrying out chemical processes with potentiometric monitoring is for example, the drift of the potential measuring chain which normally is not important in the potentiometric monitoring of discontinuous processes because of the short duration of the single measurement in each case. However, particularly the pos-sibility of being able to carry out potential measurements on continuous processes is very important for chemical processes which are monitored, controlled and adjusted by these potential measurements primarily since in the systems involved in many industrial processes interfering ions or other detrimental . factors such as electrode encrustations are encountered, as for example, in the detoxication of effluents. Monitoring these systems by taking samples and tes-ting them by chemical analysls : in laboratories usually is extremely expensive. Furthermore, this monitoring method as a control for safely carrying out chemical processes is not without danger since unforeseen operat-., .
ing fluctuations usually are recognized too late.
Therefore, the present invention provides a potentio-metric procedure by means of which chemical processes in which unknown interfering influences on the measuring chain are to be expected can be controlled and monitored.
~ It has now been found that chemical processes, primar-:~ 30 ily those in which changes in the potentiometric measuring chain ;~ due to interfering influences are to be expected, can be carried ' out continuously and monitored and/or controlled and/or adjusted Z~6~
by using redox- or ion-sensitive electrodes when the measurements of the concentration of the component concerned is carried out on a partial flow of the system in,such a way that the potential corresponding to the instantaneous concentration of the component concerned is determined first, whereupon a reagent is added, i.e.
a reagent which completely reacts with the component, whose con-centration is to be determined, to form a compound which does not influence the potential of the measuring chain or influences it only to a negligibly small extent so that the electrode shows the potential corresponding to the concentration zero of the component concerned, the difference between the two potentials indicating the actual concentration of the component concerned.
The reagent is to be added in an amount which is at least equiva-lent to the amount required for the reaction of the component to be measured. It is added preferably in excess.
According to the present invention there is provided in,a method for at least one of measuring, controlling and adjusting chemical processes to determine the concentration of a specific dissolved component by means of redox- or ion-sensitive electrodes in which the zero point of the potential is subject to changes during the measuring procedure, the improvement in which the measurement of the concentration of said component by a measuring chain is carried out on a partial flow of the system such that the potential corresponding to the instantaneous con-centration of said component is determined first, followed by the addition of a reagent which either does not influence the potential of the measuring chain or influences it only to a negligibly small extent, so that the electrode shows the poten-tial corresponding to the concentration zero of said component whereupon the difference of the two potentials indicates the actual concentration of said component.
Of course, the method is also applicable to pH elec-Z6~
trodes as a special case of ion-sensitive electrodes.
The sequence of the method according to the invention is not necessarily restricted to the sequence "measurement of the potential of the instantaneous concentration - measurement of the zero potential".
Of course, the sequence may also be reversed, i.e. by starting with the measurement of the zero potential and after a predetermined interval the instantaneous concentration of the compound concerned is measured.
The sequences preferably are repeated cyclically, but the sequences must not necessarily be carried on to their end, i.e., for example, the zero potential is measured first and after ~-completion of one or several sequences the zero potential is again measured.
The reagent, which completely reacts the components to be measured to form a specific compound so that this component is no longer present in the reaction medium at the time of measuring the zero point, must be so chosen that neither the reagent nor the resulting compound with the component concerned changes the potential of the measuring chain.
r~hile for a number of components to be measured the reagent suitable for this purpose can be selected solely by the knowledge of their generally known properties as well as of those of the reagent to be added and of those of the resulting compound, r,; a small-scale test with the corresponding reagent is recommended in other cases. For example, for the measurement of hydrogen ions in acid and alkaline ranges the usual buffer salt solutions ' from strong bases/weak acids or weak acids/strong bases have beenfound to be suitable reagents. Even for the determination of very small amounts of deleterious substances in the important , .
field of effluent purification the selection of the corresponding reagent usually causes no difficulties.
,, ''' ~`~
~3~66~
Primarily the determination of the smallest amounts of cyanide ions can thus be satisfactorily determined with the aid of hydrogen peroxide at a drifting zero point or other inter-fering influences on the potential.
In order to explain the process in greater detail, this latter process is used as an example.
The fact that the zero point of a measuring chain is displaced parallel to the original calibration curve during the measurement, i.e., that the zero point is drifting, is a known phenomenon, which has a decisive effect primarily when carrying out the process continuously (see M. Hofton - Continuous Deter-mination of Free Cyanide in Effluents Using Silver Ion Selective Electrode - Environmental 10 (1976) 3,277/280).
In many cases, the interfering factors which cause this phenomenon are not known~ Satisfactory potentiometric measurements of concentrations are no longer possible in these systems. According to the process of the German Patent, 2,352,856 cyanide- or nitrile-containing effluents can be satisfactorily reduced with hydrogen peroxide and a special catalyst to amounts lower than 0.1 mg of cyanide ions per litre. The final concen-tration of cyanide ions is determined potentiometrically. It has been found that when continuously carrying out the detoxica-tion of these effluents the 2ero point of the redox potential is slowly displaced. By applylng the method according to the invention, in which hydrogen peroxide is used as the reagent in this case, the displacement of the zero potential in the continu-ous operation could be compensated for and the measurement of the cyanide ion concentration in the continuous detoxication of effluents could be carried out satisfactorily. The redox poten-tial was not affected by the hydrogen peroxide itself or by the cyanate formed.
Although the method according to the invention can ~L~2~
also be carried out in the actuai reaction vessels, the measure-ments on partial flows are preferred. The procedure is such that the effluents partial flow to be tested is passed, without adding the reagent (hydrogen peroxide in the present case), through a reaction zone while the corresponding potential of the measuring chain is determined, whereupon after a certain interval the reagent is continuously fed into the reaction zone during a second interval and the resulting zero potential of the measur-ing chain is measured. The addition of hydrogen peroxide is then interrupted. Thereupon the instantaneous concentration of the kind of ions to be measured (in the present case cyanide ions) is then attained again and the cycle withthe addition of the reagent (in the present case hydrogen peroxide) starts again. The reagent can be added in an amount which is stoichiometric for the reac-tion, but it is preferably added in excess. The calculation of the stoichiometric amount of reagent should be based preferably on the maximum of the expected amount of the kind of ion to be measured. In the case of unexpectly occurring variations of the concentration in the solution to be measured no incorrect results can thus be obtained. This amount can be easily deter-; mined by a small-scale test.
; The method according to the invention is independent of temperature insofar as no specific temperature range is required for carrying it out. The measuring temperature depends on the ~eactions concerned. However, at elevated temperature the rate of reaction usually is faster. The electrometric force of the measuring chain is a function of temperature in a conventional manner and as is customary for redox- and ion-sensitive electrodes this force can becompensated for, e.g. automatically by means of resistance thermometers.
The interval between 'che measurement of the zero potential and that of the cyanide ion concentration, which ..3Z66~
readjusts itself after some time, can preferably be the same for control engineering reasons. However, the result of the method per se is not affected either by irregular intervals between the various concentration measurements. The suitable intervals are determined by a preliminary test, for example, they can be from 1 to 2 minutes.
The advantage of using regular intervals lies in the possibility of inserting a cycle-control device.
The length of the interval between the determination of the instantaneous concentration and the addition of the hydro-gen peroxide in excess should be kept as short as possible since the reaction of the hydrogen peroxide with the cyanide ions ; present also requires a certain time.
The present invention will be further illustrated by way of the accompanying drawings in which:
Fig. 1 is a plot of the potential of the measuring chain with time for the addition of hydrogen peroxide to cyanide con-taining aqueous solutions, Fig. 2 is a diagram of a system using the continuous method according to one embodiment of the present invention;
Fig. 3 is a diagram of a system using the discontinuous method according to another embodiment of the present invention;
and Fig. 4 is a graph of the change in electrode potential with CN concentration in a series of effluentshaving differentCN
concentrations.
When using a cycle control device the intervals up to the measurement of the instantaneous concentration of the com-ponent of the zero potential should best be identical. The time slope of the measuring chain voltage with identical intervals for measuring the component concentration (in this case the concentration of the cyanide ions) and for measuring the zero '~
~ - 6 -, .
1~3~
potential (in this case by adding hydrogen peroxide in excess) is shown in Figure 1. The potential of the measuring chain has been plotted on the ordinate and the time on the abscissa. The intervals in which the hydrogen peroxide is added are shown as dashes 2. In this case the procedure starts with the setting of the zero potential, i.e. with the addition of the hydrogen peroxide in excess over the equivalent amount for the cyanide ions present. The intervals are measured in minutes.
As is evident from Figure 1 the potential of the measuring chain always returns to the set zero point as soon as the corresponding amount of hydrogen peroxide is fed to the system. The difference between the potentials for the points of measuring time 3 and 4 is a criterion of the cyanide ion concentration. It must be once more emphasized that the poten-tial of the measuring chain is practically un-influenced by the excess hydrogen peroxide. This was established prior to carry-ing out the measurements.
The concentration of the reagent (in general aqueous solutions are used) depends on the concentration of the type of ion to be measured. If this concentration is high, then more highly concentrated reagent solutions may be used. If this con-centration is lower, then more weakly concentrated solutions are preferable for indicating the final product.
The process according to the invention can be carried out continuously and also discontinuously. This is illustrated by means of the example of cyanide detoxication of effluent with hydrogen peroxide.
The system shown in Figure 2 operates in the following manner:
A small partial flow of the cyanide-ions-containing effluent flows via the pipe 1 and la through a reaction zone 2.
At the outlet of the reaction zone 2 the redox potential is ~L~3~Z6~
measured, for example, with a silver/thalamide measuring chain at point 3. At the inlet of the reaction zone, i.e. at the inlet of the pipe 5a into the pipe la dilute hydrogen peroxide solution from the tank 5 is fed in at lb alternately via the pipe 5a and via a valve 4, which is controlled by a time clock.
The potentiai corresponding to the cyanide-ion concentration of zero ismeasured at 3. From the difference of the two measure-ments the momentary cyanide-ion concentration is obtained. The second measurement, i.e., that of the zero potential is carried ` 10 out at 3 with the same measuring chain. The effluent is con-tinuously removed via the pipe 6 and rejected. However, when carrying out chemical processes the measured partial flow is preferably returned to the main flow.
The system is ventilated via the pipe lc.
Although the results of the measurements when continu-ously carrying out the method according to the invention are satisfactory, technical difficulties were encountered in certain cases when carrying out the measurements in practice for example by the hold-up in the reaction zone. Therefore, effective mixing of the solution to be tested with the reagent added could not always be accomplished.
For this reason it is preferable to carry out the method according to the invention discontinuously.
- The procedure is as follows: As compared with the continuous method the measurement is not carried out in a vessel through which the solution to be measured flows continu-- ously, but in a reaction vessel 3 provided with a stirrer 3 (see Fig. 3). This reaction vessel is combined with two measur-ing vessels, one vessel being intended for the solution to be tested and the other for the reagent to be added. In a first step a measured amount of the solution concerned from the measur-ing vessel 2, which is determined by the valves 2a and 2b, is .~
.
~3;26~V
fed into the reaction vessel 3. For reasons of safety against explosion, the v~lves 2a and 2b are preferably controlled by compressed air. The electrode which measures the ion concentra-tion that is momentarily of interest, for example, the cyanide-ion concentration is at 5. This electrode at 5 should prefer-ably be able to determine this concentration immediately upon adding the solution from the measuring vessel 2.
From the storage tank 6, the reagent i.e. the hydrogen peroxide in the case of measuring the cyanide ions is passed via the pipe 7 into the measuring vessel 8 for the reagent.
Like the measuring vessel 2, the measuring vessel 8 is provided with corresponding valves 8a and 8b, which also are preferably -controlled by compressed air.
In the step following the measurement of the ion con-centration the hydrogen peroxide is passed into the vessel 3.
The reaction is watched over a certain interval o~ time, where-upon the zero potential obtained is measured with the electrode at 5. The valve 9 is then opened and the vessel 3 is emptied.
The sequence is repeated correspondingly. The valves 2a and 2b as well as 8a and 8b are so controlled that a fric-tionless pro-gress of the method described above is assured.
The discontinuous manner of carrying out the method according to the invention is remarkable in that the above-mentioned difficulties encountered when carrying out the process , continuously are avoided and that the discontinuous operation is also satisfactory in industrial plants.
Furthermore, the time required for carrying out the process discontinuously can be reduced as compared with the continuous measurement. Thus, for example, in the determination of the cyanide ions the time required for a measuring cycle is approximately 2 to 3 minutes as compared with 10 minutes when carrying out the method continuously. The reason lies in the 66~
spontaneous reaction, for example of the cyanide ions with hydro-gen peroxide, in the reaction vessel which, as mentioned herein-before, is provided with a stirrer so that the reactants are mixed instantaneously. Even despite the additional time required for emptying in the discontinuous process time is still gained as compared with the continuous method.
~ oreover, the apparatus per se is substantially simpler and less prone to trouble since no streams of liquids but volumes are mixed and therefore, no flowmeters are required.
The measuring method according to the invention is also applicable to potentiostatic systems.
The present invention will be further illustrated by the following Examples:
Exarnple 1: Continuous Method A solution having a CN concentration of 12 mg per litre and a proportion of catalyst of 0.02% by volume (see German Patent 2,352,856) is used as a partial flow of liquid to be tested. The solution has a temperature of 95C and a pH value of 11.5.
A flow of approximately 100 litres of the starting solution per hour is fed continuously through the pipes 1, la and lb into the reaction zone 2. After the starting solution has passed through the reaction zone 2 its redox potential is rneasured with the electrode 3 in the first interval. For the subsequent second interval the valve 4 opens for a period of approximately 5 minutes and 0.5 litre of a 3.5% by weight H2O2 solution flows per time interval from the vessel 5 through the pipes 5a and lb into the reaction zone 2. Oxygen liberated by the decomposition of H2O2 is removed from the measuring apparatus via the air vent pipe lc. During the second interval the vari~
able redox potential which corresponds to the CN zero concentra-tion because of the complete conversion of the cyanide into ~3~6t~
cyanate, is measured. For the starting solution a potential difference of 336mV is obtained from the measurement of the two redox potentials. Because of the fixed cycle-controlled valve 4, which takes into account the hold-up in the reaction zone, the time required by the measuring process is approximately 10 minutes.
Example 2: Discontinuous ~ethod i The discontinuous testing apparatus with the fixed-cycle-controlled valves is shown in E`igure 3.
A solution, having a CN concentration of 120 mg per ~itre and a proportion of catalyst of 0.02% by volume (see German Patent 2,352,856) is used as a partial flow of liquid to be tested. The solution has a temperature of 90C and a pH
value of 11.3.
With the valve 2a closed and with the opening of the valve 2b delayed the 1000 ml measuring volume 2 is filled with the solution. After closing the valve 2b and delaying the open-ing of the valve 2c, the solution flows into the stirring vessel 3 in the first cycle. The redox potential of the starting solu-tion is measured with the electrode 5. From the measuring volume 8 of 30 ml, which has been filled up from the vessel 6 via the ` pipe 7 in the meantime, a 3.5~ by weight H2O2 solution is added ~ ~y closing the valve 8b and by delayed opening of the valve 8a.
-~ In the second cycle the stirrer 4 causes the two liquids to be ~; mixed thoroughly. The H2O2, which is present in excess over the stoichiometric amount, comple~ely reacts the amounts of cyanide present to cyanate and a redox potential corresponding to the CN zero concentration is measured. For the starting solution a potential difference of 458 mV is obtained. After the measure-ment the vessel 3 is emptied in the third cycle. A time of approximately 2.5 minutes is required for the measurement. For the equialization of pressure the vessels 3 and 6 are ventilated ., : ~
'Z~6~
via the pipes 10 and 11 respectively.
Example 3 The measuring method was tested in different test series with effluents having different CN concentrations. The non-detoxicated starting solution had a CN concentration of 120 mg per litre. This solution was diluted in stages to 1.2 mg per litre and the potentials were recorded. Furthermore, solutions having cyanide ion concentrations of 1 mg per litre, 0.1 mg per litre and 0.01 mg per litre were produced in a laboratory. The pH value was adjusted to 12. The amount of activator of 0.02%, as prescribed in the German Patent 2,352,856, was added to the solution. Figure 4 shows the relationship between change in potential and CN concentration in the range from 0.01 to 100 mg per litre. In this singly logarithmic representation an approximately linear relationship is obtained over a wide range of concentrations.
In order to determine whether the change in potential ; of 10 mV measured at 0.01 mg per litre is due to the CN concen- -tration, a test with effluent without CN was carried out. A
change in potential was observecl, its amount is certainly smaller. This value cannot be plotted on this curve in Figure 4 because of the logarithmic representation of the CN concentra-; tion. When defining this 5 mV measured at CN concentrations - of zero as zero point instability thenCN concentrations to ; approximately 0.01 mg per litre can be detected therewith with .. :
~ adequate certainty.
.''' .
. ' . ., : ' ..
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a method for at least one of measuring, controll-ing and adjusting chemical processes to determine the concentra-tion of a specific dissolved component by means of redox- or ion-sensitive electrodes in which the zero point of the potential is subject to changes during the measuring procedure, the improve-ment in which the measurement of the concentration of said com-ponent by a measuring chain is carried out on a partial flow of the system such that the potential corresponding to the instan-taneous concentration of said component is determined first, followed by the addition of a reagent which either does not influence the potential of the measuring chain or influences it only to a negligibly small extent, so that the electrode shows the potential corresponding to the concentration zero of said component whereupon the difference of the two potentials indicates the actual concentration of said component.
2. A method according to claim 1, in which the reagent is added in an amount which is at least equivalent to what re-quired for reacting said component with the reagent.
3. A process as claimed in claim 2 in which the reagent is added in excess.
4. A method according to claim 1 or 2 in which the determination of the instantaneous concentration and the establish-ment and determination of the zero potential is cyclically repeated.
5. A method according to claim 1, 2 or 3 in which the partial flow to be measured is passed through a reaction zone without the addition of a reagent while the corresponding potential of the measuring chain is determined, whereupon after a selected length of time the reagent is continuously fed into the reaction zone during a second interval while the resulting zero potential of the measuring chain is measured.
6. A method according to claim 1, 2 or 3 in which a partial flow in the form of a defined amount is passed discontin-uously into a reactor and the instantaneous concentration is determined with a measuring electrode, whereupon after a certain interval a defined amount of the reagent to be used is added to the reactor so that the component is completely reacted and the measuring chain determines the potential corresponding to the zero concentration, whereafter the vessel is emptied during a further interval.
7. A method according to claim 1, 2 or 3 which is used for measuring a component in an aqueous solution.
8. A method according to claim 1, 2 or 3 which is used for measuring the content of cyanide ions in a solution.
9. A method according to claim 1, 2 or 3 which is used for measuring the content of cyanide ions with the aid of hydrogen peroxide as the reagent.
10. A method according to claim 1, 2 or 3 which is used for measuring the content of cyanide ions in a solution detoxicated with hydrogen peroxide.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE2917714A DE2917714C2 (en) | 1979-05-02 | 1979-05-02 | Procedure for measuring with redox or ion-sensitive electrodes |
| DEP2917714.8-52 | 1979-05-02 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1132660A true CA1132660A (en) | 1982-09-28 |
Family
ID=6069774
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA351,040A Expired CA1132660A (en) | 1979-05-02 | 1980-05-01 | Method of measuring with redox- or ion-sensitive electrodes |
Country Status (12)
| Country | Link |
|---|---|
| JP (1) | JPS55151255A (en) |
| AT (1) | AT381395B (en) |
| BE (1) | BE883046A (en) |
| CA (1) | CA1132660A (en) |
| CH (1) | CH645990A5 (en) |
| DE (1) | DE2917714C2 (en) |
| DK (1) | DK154458C (en) |
| FR (1) | FR2455740A1 (en) |
| GB (1) | GB2049951B (en) |
| IT (1) | IT1128438B (en) |
| NL (1) | NL8001823A (en) |
| SE (1) | SE8003310L (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| NL8303792A (en) * | 1983-11-03 | 1985-06-03 | Cordis Europ | Apparatus provided with an measuring circuit based on an ISFET; ISFET SUITABLE FOR USE IN THE MEASURING CIRCUIT AND METHOD FOR MANUFACTURING AN ISFET TO BE USED IN THE MEASURING CIRCUIT |
| SE9000166L (en) * | 1990-01-17 | 1991-07-18 | Eka Nobel Ab | redox |
| JP5116068B2 (en) * | 2004-09-07 | 2013-01-09 | Ｊｘ日鉱日石金属株式会社 | Method for stabilizing electroless gold plating solution |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2773019A (en) * | 1954-07-26 | 1956-12-04 | Exxon Research Engineering Co | Control of hypochlorite treating operation |
| NL292885A (en) * | 1963-05-16 | |||
| DE1523018A1 (en) * | 1964-10-08 | 1969-04-24 | Siemens Ag | Measuring device for the automatic measurement of the cyanide content and / or the pH value of industrial waste water |
| DE2043176A1 (en) * | 1969-11-12 | 1971-05-27 | Zellweger Uster Ag | Slide rule to evaluate non meter readings - for concentration measurements |
| DE2018514B2 (en) * | 1970-04-17 | 1980-01-31 | Guenter Dr.Techn. 4630 Bochum Schierjott | Method and device for the continuous determination of a component in a flowing medium by electrochemical indication |
| DE2352856C3 (en) * | 1973-10-22 | 1978-09-28 | Deutsche Gold- Und Silber-Scheideanstalt Vormals Roessler, 6000 Frankfurt | Process for the detoxification of waste water containing cyanides and / or cyanohydrins and / or nitriles |
| US4002428A (en) * | 1976-05-21 | 1977-01-11 | The United States Of America As Represented By The Secretary Of The Interior | Deductive method for measuring ion concentration electrically |
| US4049382A (en) * | 1977-02-16 | 1977-09-20 | Orion Research Incorporated | Total residual chlorine |
-
1979
- 1979-05-02 DE DE2917714A patent/DE2917714C2/en not_active Expired
-
1980
- 1980-03-27 NL NL8001823A patent/NL8001823A/en not_active Application Discontinuation
- 1980-04-17 FR FR8008655A patent/FR2455740A1/en active Granted
- 1980-04-25 DK DK179980A patent/DK154458C/en not_active IP Right Cessation
- 1980-04-29 BE BE6/47152A patent/BE883046A/en not_active IP Right Cessation
- 1980-04-30 AT AT0231980A patent/AT381395B/en not_active IP Right Cessation
- 1980-04-30 IT IT67687/80A patent/IT1128438B/en active
- 1980-04-30 SE SE8003310A patent/SE8003310L/en not_active Application Discontinuation
- 1980-05-01 CH CH338380A patent/CH645990A5/en not_active IP Right Cessation
- 1980-05-01 CA CA351,040A patent/CA1132660A/en not_active Expired
- 1980-05-02 JP JP5803180A patent/JPS55151255A/en active Granted
- 1980-05-02 GB GB8014859A patent/GB2049951B/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB2049951A (en) | 1980-12-31 |
| SE8003310L (en) | 1980-11-03 |
| JPS6329218B2 (en) | 1988-06-13 |
| AT381395B (en) | 1986-10-10 |
| CH645990A5 (en) | 1984-10-31 |
| ATA231980A (en) | 1986-02-15 |
| DE2917714A1 (en) | 1981-03-26 |
| GB2049951B (en) | 1983-05-25 |
| FR2455740B1 (en) | 1983-12-09 |
| JPS55151255A (en) | 1980-11-25 |
| DK179980A (en) | 1980-11-03 |
| IT8067687A0 (en) | 1980-04-30 |
| FR2455740A1 (en) | 1980-11-28 |
| DK154458C (en) | 1989-04-10 |
| BE883046A (en) | 1980-10-29 |
| DK154458B (en) | 1988-11-14 |
| NL8001823A (en) | 1980-11-04 |
| IT1128438B (en) | 1986-05-28 |
| DE2917714C2 (en) | 1983-12-22 |
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