WO2009017461A1

WO2009017461A1 - Titration apparatus

Info

Publication number: WO2009017461A1
Application number: PCT/SG2008/000252
Authority: WO
Inventors: Arul Ramasamy
Original assignee: Arul Ramasamy
Priority date: 2007-07-30
Filing date: 2008-07-14
Publication date: 2009-02-05
Also published as: SG149728A1

Abstract

A titration apparatus having a sensor (5) to detect the end-point of a titration. The sensor (5) is a light sensor monitoring a change of colour in the analysis sample (2a) and may be calibrated to account for turbidity or colouration in the analysis sample (2a). The titrationapparatus therefore provides the possibility of overcoming interference from turbidity or colouration in the analysis sample (2a) to the end-point detection. This provides the advantage of relieving the need for extensive sample preparation before titration, allowing titration to be performed in the field.

Description

Titration Apparatus

Field of Invention

This invention relates to an apparatus for performing titration and in particular to photometric or colorimetric detection of titration end-point.

Background of the invention

Titration is a commonly used analytical chemistry method for determining the concentration of an analyte in an analysis sample. Titration involves determining the volume of a titrant, having a known concentration of a reactant, required to completely react with a predetermined volume of the analysis sample. The analyte and the reactant must have an established quantifiable relationship to provide the possibility of determining the concentration of the analyte in the analysis sample. Typically, the titrant is dispensed continually in small discrete volumes into the analysis sample. The point at which all the analyte has been completely reacted without excess titrant is known as the end-point of the titration. The end-point may be detected by a colour change in the reaction mixture, which may be due, for example, to a change of the oxidation state of all the analyte or to the presence of a colorimetric end-point indicator added to the analysis sample. An example of a colorimetric indicator is phenolphthalein for indicating change of pH. Phenolphthalein is colourless in acidic pH and purple in a basic pH. Other than pH indicators, electronic means of detecting titration end-point such as electrodes may be used. Traditionally, the titrant is added into the analysis sample using a burette. However, to automate titrations, automatic titration devices have been developed which can dispense the titrant into the analysis sample using a micropump.

Most of these automatic titration devices, or autotitrators, have been developed to use an electrode to detect the end-point. However, electrodes are difficult to maintain. For example, electrodes have to be kept soaked in buffer solutions when not in use. Furthermore, deterioration in electrode sensitivity is not immediately noticeable, which can lead to false reading of the end-point. Furthermore, most electrodes have a delicate membrane surface which deteriorates when used with in unclear or turbid analysis sample.

WO 1995/003537 discloses an auto-titrator that detects the end-point by spectrophotometric or colorimetric detection of indicator colour change. A light is shined through the analysis sample and the transmission thereof is monitored. As titration proceeds, a sudden reduction or increase of the light transmission indicates a change in colouration or clarity in the analysis sample, which indicates that the titration end-point. However, as with electrodes, spectrophotometric readings have a problem that they are subject to interference by turbidity in the analysis sample. This affects the accuracy of the end point detection. Furthermore, there is a further problem that spectrophotometric readings are also subject to interference by inherent sample colouration. Thus, to analyse an analysis sample using an electrode or spectrophotometric detector, the analysis sample must typically be pre-treated for titration, e.g. by filtration, centrifugation or solvent extraction, in order to remove turbidity or inherent colouration in the sample. However, this reduces analysis turnaround time and requires a skilled analyst and the support of a laboratory workspace to prepare the analysis sample and perform the titration. This also restricts auto-titrators from being useable in the field, i.e. outdoor of the laboratory, on samples taken from the environment for immediate titration, without elaborate sample preparation.

Therefore, it is desirable to provide an apparatus and/or method for automatic detection of titration end-point that is less prone to interference from turbidity or colouration in the analysis sample. Furthermore, it is also desirable to provide a titration apparatus and/or method which reduces the need for elaborate sample preparation.

Summary of the invention

In a first aspect, the invention proposes a titration apparatus comprising a light sensor to detect transmission of light through a titration analysis sample, the light sensor capable of providing an electrical indication representing an intensity of the light detected by the light sensor, such that the electrical indication changes when the intensity of detected light changes, wherein the level of the electrical indication representing the intensity of the light detected is adjustable. Thus, the pre-titration electrical indication of the light transmitted through the analysis sample may be adjusted higher if it is too low due to low light transmission through the sample, which may be due in turn to a lack of clarity, turbidity or inherent colouration in the analysis sample. Thus, the invention provides the possibility of detecting the colour change of an indicator in the analysis sample, despite the interference, by possibly providing a sufficient allowance for change in the electrical indication to represent the end-point. As a result, the invention also possibly reduces the need of elaborate sample preparation before titration. This translates to a possibility to use an auto-titrator in the field.

In a second aspect, the invention proposes a titration apparatus comprising a means, e.g. the tube 7, to introduce a drop of fluid into a titration analysis sample a pair of electrically conductive members which are spaced apart the pair of electrically conductive members arranged such that the drop of fluid passes between the pair of electrically conductive members wherein if the drop of fluid is electrically conductive, the drop of fluid electrically connects the pair of electrically conductive members.

This provides the possibility that the dispensation of the fluid is countable drop-by-drop, by the electrical connection between a drop of the fluid and the wires. This possibly provides a robust and rugged manner of measuring the volume of titrant used in a titration without requiring heavy, expensive or delicate precision pumps having the capability of monitoring volume of dispense titrant. This also translates to a possibility to use an auto-titrator in the field where the environment may be harmful to the maintenance of delicate auto-titrators having such expensive pumps. In a third aspect, the invention proposes a titration apparatus comprising a light source to transmit light through a titration analysis sample, and a light sensor to detect change in colour of the titration analysis sample, wherein the light source is positioned to be above the titration analysis sample, and the light sensor is positioned at the side of the titration analysis sample.

Thus, the invention provides the possibility that a vortex formed in an analysis sample during titration (due to stirring which is commonly employed) does not interfere with the detection of a colorimetric change in the analysis sample at end-point.

In a fourth aspect, the invention proposes a method of detecting the colour change of an indicator at the end-point of a titration, comprising the steps of providing a titration analysis sample, positioning the titration analysis sample such that light from a light source is transmitted through the titration analysis sample to be detected by a light dependent resistor, the light dependent resistor being connected in series with a potentiometer, providing a voltage across the light dependent resistor and the potentiometer, measuring the portion of the voltage divided to the potentiometer comparing the portion of the voltage divided to the potentiometer with a calibration reference voltage, adjusting the resistance of the potentiometer until the voltage divided to the potentiometer is at a pre-determined level as indicated by the calibration reference voltage, titrating the titration analysis sample until end-point, wherein the intensity of the light detected by the light dependent resistor at end-point changes the portion of the voltage divided to the potentiometer.

In a fifth aspect, the invention proposes A method of monitoring the volume of titrant used in a titration comprising the steps of dispensing the titrant into an analysis sample drop by drop, each drop of the titrant dispensed such that the drop passes between two wires to contact the two wires and to allow a current to pass through the two wires, recording the passage of current through the two wires as an indication of a drop of the titrant having been dispensed.

Brief description of the figures

It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention, in which like integers refer to like parts. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

Figure 1 is a perspective view of an embodiment of the present invention;

Figure Ia is a corresponding schematic front view of an embodiment of the present invention;

Figure 2 is the schematic illustration of a circuit used in the embodiment of Figure 1; Figure 3 shows schematically a drop counter used in the embodiment of Figure 1;

Figure 4 is a graph showing analysis data obtained, by way of example, by the embodiment of Figure 1; and

Figure 5 is a more elaborate schematic illustration of the circuit of Figure 2;

Detailed description of the invention

Figures 1 and Ia show a titration device comprises a light-proof housing 1. The housing 1 has an access door Ia by which the interior of the housing 1 may be accessed. A platform 12, a light source 4 and a light sensor 5 are provided in the housing 1 and positioned in such a way that an analysis sample 2a in a container 2 may be placed on the platform 12 to be illuminated by the light source 4. In this position, light transmitted through the analysis sample 2a may be detected by the light sensor 5. The container 2 may be a beaker or an Erlenmeyer flask, which is transparent to light from the light source 4. The light source 4 is positioned above the container 2 and the light sensor 5 is positioned at the side of the container 2 to detect light from the light source 4 dispersed by the analysis sample 2a laterally. In an alternative arrangement which is not illustrated, the light source 4 is positioned at one side of the container 2 and the sensor on the opposite side the light source 4 across the container 2. The light source 4 is typically, but not necessarily, a light emitting diode (LED) capable of emitting light in a monochromatic wavelength or a selected band of wavelength. In this embodiment, the light is preferably red in colour.

The light sensor 5 is a light dependent resistor (LDR) of which resistance changes when light is directed onto it. Normally, the resistance of a light dependent resistor 5 is very high, and may be as high as 1000000 ohms. However, when illuminated with light, the resistance drops drastically. Thus, the light dependent resistor 5 is usable to electrically sense light transmitted through the analysis sample 2a.

The housing 1 also houses a magnetic assembly 3 which provides a rotating magnetic field beneath the platform 12.

External of the housing 1 is a bottle of titrant 8a (not shown in Figure 1), a tube 7 and a pump 8. The tube 7 is positioned such that when the container 2 of analysis sample 2a is placed on the platform 12, the titrant may be pumped by the pump 8 through the tube 7 into the container 2 to perform a titration. The pump 8 may be a precision pump 8 or a peristaltic pump 8, and has means to measure the volume of basic solution dispensed for titration with sufficient precision.

The embodiment also comprises a control circuit 9. The control circuit controls the pump 8 (via connection 15 shown in Figure Ia), the light sensor 5 (via connection 13 shown in Figure Ia), the magnetic assembly 3 (via connection 8b shown in Figure Ia), as well as a calibrator 11 (via connections 16 and 17 shown in Figure Ia). The control circuit 9 is also connected to an LED display device 10 which can display the amount of titrant used to reach the end-point of a titration. A simplified schematic diagram of a part of the circuit is shown in Figure 2, e.g. the connection to the LED display 10 is not illustrated in Figure 2

The titration device has two phases of operation, a calibration phase and a titration phase. For this purpose, the control circuit 9 has a first comparator op-amp 201 (IC4 (1/4)) for calibrating the titration device during the calibration phase and a second comparator op-amp 206 (IC4 (¹A)") for controlling the pump 8 during the titration phase.

The control circuit also comprises a timer 211 which is activated by a switch SW2. When activated, the timer 211 operates the titration device in, firstly, the calibration phase and then the titration phase.

To operate the titration device in the calibration phase, the timer 211 supplies a signal to an AND gate 208 in the circuit for s pre-determined period of time, which permits operation of the calibrator 11. During the titration phase, the timer 211 supplies a signal to a transistor 212 to permit control of the pump 8. Preferably, but not necessarily, the timer 11 also pre-determines a period of time for the titration phase, in case the end- point cannot be reached and the container 2 overflows. During the calibration phase, the timer 211 does not supply any signal to the transistor 212 and during the titration phase, the timer 211 does not supply any signal to the AND gate 208. The calibration op-amp 201 has an inverse terminal which is connected to detect the voltage across an adjustable potentiometer VRl. The potentiometer VRl is connected in series with the light dependent resistor 5 and, when the titration device is in use, a predetermined voltage is applied across both the light dependent resistor 5 and potentiometer VRl. As a result, the applied voltage is divided between the light dependent resistor 5 and potentiometer VRl. Thus, the voltage across potentiometer VRl is directly affected by the resistance developed in the light dependent resistor 5. It is preferable, although not necessarily, that power supply in the titration device is provided by a portable battery, to provide the possibility that the titration device may be portable and usable in the field.

The non-inverted terminal of the calibration op-amp 201 is connected to a potentiometer VR2 and to a potentiometer VR3. Each of the potentiometers VR2, VR3 is connected in series with a resistor R8. When the titration device is in use, a pre-determined voltage is applied across the resistor R8 and either one of the potentiometers VR2, VR3. Selection between the potentiometers VR2, VR3 is made possible via a switch SW3.

Each of the potentiometers VR2, VR3 is pre-adjusted in the factory to pre-determine the voltage across resistor R8, accordingly to the selection on the switch SW3. In this way, the potentiometers Vr2, Vr3 and the resistor R8 are designed to provide a predetermined calibration reference voltage to the non-inverting terminal of the calibration op-amp 201. For example, to set either VR2 or VR3 for a specific titration, a solution having the mixture of the reagents at the end-point of an intended titration is prepared, along with an added dose of the intended indicator. This solution is placed in a transparent container 2 on the platform 12 in the housing 1. The housing 1 is then closed and the dispensing pump allowed to operate. VR2 is then adjusted until the dispensing pump stops pumping. Thus, if the intended analysis sample is titrated to the state of the standard solution, the end-point would be considered as being reached for the analysis sample.

If the voltage at the non-inverting terminal of the calibration op-amp 201 is greater than the voltage at the inverting terminal of the calibration op-amp 201, the calibration op- amp 201 produces a high state voltage output. Vice versa, the calibration op-amp 201 produces a low state output if the voltage at voltage at the inverting terminal is greater than the voltage at the non-inverting terminal. In the calibration phase, when the timer 11 supplies a high state signal to the AND gate, the high state output of the calibration op-amp 201 is useable to control the calibrator 11. Thus, the calibrator adjusts the potentiometer VRl as in a feedback loop until the voltage across the potentiometer VRl is greater than the voltage across the resistor R8.

Turning now to the titration op-amp 206, the inverting terminal of the titration op-amp 206 is also connected to tap the voltage across potentiometer VRl. However, the non- inverting terminal of the titration op-amp 206 is connected to a resistor R9. The resistor R9 is connected in series with another resistor R7. During operation of the titration device, a voltage applied to resistors R7 and R9 is divided such that there is a predetermined voltage across resistor R9. In this way, resistor R9 provides a predetermined end-point reference voltage Vre to the titrating op-amp 206, against which the voltage across potentiometer VRl may be compared during the titration phase.

The circuit 9 further comprises a first and a second NOT gate 207, 213. The first NOT gate 207 is connected to a titration transistor 212, which is in turn connected to the titrating op-amp 206. The second NOT gate 213, however, is connected in series with the first NOT gate 207. A switch SWl is provided for selectively connecting the pump 8 to either one or two NOT gates 207, 213. The switch SWl is set to the first NOT gate 207 to invert the output from the titration op-amp 206 between high and low states if, in an titration, the indicator at end-point is expected to reduce the initial amount of light transmitted through the analysis sample 2a, e.g. the colour of the indicator darkens. Conversely, the switch SWl is connected to the second NOT gate 213 to invert the signal from the titration op-amp 206 twice, if the indicator at end-point is expected to allow more light to transmit through the analysis sample 2a.

To perform a titration, an analysis sample 2a is prepared and contained in the container 2. By way of example herein, the analysis sample is an acidic and the titrant is an alkaline. Therefore, a few drops of a suitable indicator such as phenolphthalein is added into the acidic solution. The phenolphthalein will change from colourless to purple at the end-point. Thus, at the end-point, the purple phenolphthalein will absorb a large portion of the red light from the light source 4, reducing the amount of light transmitted through the analysis sample 2a to the light dependent resistor 5. This provides the possibility of end-point detection by the titration device. As the transmission of red light is reduced by the purple phenolphthalein at end-point, the switch SWl is set to connect the pump 8 to only the first NOT gate 207.

A magnetic stirrer is placed inside the container 2 and the container 2 is placed on the platform 12. The magnetic assembly 3 is then operated such that the magnetic stirrer is rotated to stir the analysis sample 2a. The access door is then closed to prevent ambient light from entering into the housing 1 and interfering with the reading by the light dependent resistor 5.

The light source 4 is then switched on to emit light into the analysis sample 2a. The light transmits through the analysis sample 2a and dispersed laterally in the illustrated arrangement is sensed by the light dependent resistor 5, causing a reduction in the resistance of the light dependent resistor 5. This lowers the voltage across the light dependent resistor 5 and increases the voltage across potentiometer VRl.

It is preferable that the light source 4 is positioned at the top of the container 2, instead of across the container 2 and opposite the light dependent resistor 5, so that a vortex formed in the analysis sample by the magnetic stirring will not interfere with the light transmission through the analysis sample 2a.

The switch SW2 is then pressed to activate the timer 211 to run the calibration phase and then the titration phase. As mentioned, in the calibration phase, if the initial voltage across potentiometer VRl is less than the voltage across the resistor R8, the output of the calibration op-amp 201 is in a high state. The AND gate therefore receives a high state signal from both the calibration op-amp 201 and the timer 211. This allows the calibrator 11 to operate, via a mechanical link 220, and adjust potentiometer VRl until there is no high state output at the calibration op-amp 201, at which point the calibrator 11 is no longer in operation.

In some variations of the embodiment, the output from the calibration op-amp 201 need not be adjusted to a zero voltage, i.e. a low state voltage from the calibration op-amp 201 will not cause the calibrator to adjust potentiometer VRl. This saves some calibration time. Preferably, however, the resistance of the potentiometer is re-set to the minimum resistance and adjusted incrementally until the voltage across the potentiometer VRl is greater than the voltage across R8. This ensures that the potentiometer VRl is set to just the right resistance as indicated by R8 each time. By way of example, the calibrator 11 may be a geared motor with a shaft attached to a spindle in potentiometer VRl via spur gears, which allows the potentiometer VRl to be adjusted (not illustrated). When the time stipulated for the calibration phase runs out, the timer 211 stops sending a signal to the AND gate 207, so that any output from the calibration op-amp 201 will not be able to adjust potentiometer VRl. The calibration phase is thus completed.

The titration phase then begins, in which the timer 211 supplies a signal to the base of the titration transistor 212. Thus, the titration transistor 212 is set to allow a signal from the titration op-amp 206 to operate the pump 8. During the preceding calibration phase, potentiometer VRl has already been adjusted such that the voltage across potentiometer VRl is also greater than the voltage across resistor R9. The output of the titration op- amp 206 is therefore in a low state at the beginning of the titration. This is converted to a high state signal by the first NOT gate 207. The high state signal is directed to operate the pump 8 to dispense titrant into the analysis sample 2a. The pump has a predetermined pump rate so that the amount of the titrant dispensed can be easily monitored. Preferably, the pump introduces the titrant in countable drops of known volume per drop.

Figure 3 shows how the drops 302 of titrant may counted by platinum wires 301 situated at the mouth of the tube 7 inside the housing 1. Each drop of titrant from the tube 7 will pass between the platinum wires 301 contacting both platinum wires 301. A current is applied constantly to the wires 301 such that when a drop of the titrant, which is ionic in nature, contacts both platinum wires 301, the current is able to flow through the wires 301. The platinum wires are connected to a counter device (not shown) in the circuit 9, via connection 14, to count and record a drop 302 of titrant each time the current flows through both of the platinum wires 301. The total number of drops of titrant, or their total volume, dispensed into the analysis sample 2a is thereby recordable and may be displayed at the LED display 10 when the end-point is reached. This provides a more robust and cost effective method of monitoring the volume of titrant used than an expensive and delicate precision pump.

The size of the tube 7, the speed of the pump 8 and the surface tension and other characteristics of specific titrants may be pre-established to pre-determine the volume of each drop of the titrant. Using this information, each drop of titrant may be accounted for volumetrically.

At the titration end-point, the phenolphthalein changes from colourless to purple. The purple phenolphthalein absorbs the red light from the light source 4 such that light transmission to the light dependent resistor 5 is suddenly reduced and the resistance of the light dependent resistor 5 increases. As a result, the voltage across potentiometer VRl decreases. Thus, the voltage at the inverting terminal of the titration op-amp 206 becomes lesser than the voltage across resistor R9. The voltage output of the titration op-amp 206 then becomes a high state signal and the output of the first NOT gate 207 becomes a low state signal. This causes the pump 8 to stop operating and the titration is complete. The LED display 10 then exhibits the amount of titrant used to reach the end- point.

Generally, the volume of the basic solution multiplied by the known molarity of the basic titrant is indicative of the moles of basic reactants used to neutralised the acidic solution. This is indicative of the number of moles of the acidic analyte present in the acidic analysis sample 2a. The number of moles of the acidic analysis sample 2a divided by the volume of the acidic analysis sample 2a provides the molarity of the acidic analysis sample 2a. This calculation is well-known and commonly performed by the chemical analyst and may be automated by using a microprocessor.

Accordingly, a calibration phase has been described which provide the possibility of calibrating the sensitivity of the potentiometer VRl to detect the titration end-point despite interference from sample cloudiness, turbidity or colouration, i.e. the pre- titration voltage across potentiometer VRl may be adjusted if it is too low due to low light transmission through the analysis sample 2a. This provides a sufficient allowance to detect a fall in the pre-titration voltage to the end-point reference voltage Vre.

In other words, if the initial amount of light transmitted through the analysis sample before titration is less than pre-determined by the resistor R8, the resultant high state output from the calibration op-amp 201 causes the calibrator 11 to adjust the potentiometer VRl until the voltage across potentiometer VRl is at least the same or greater than the voltage across the resistor R8. This provides that the pre-titration voltage across potentiometer VRl may be at least as great as the case when the analysis sample is clear.

An example of analysis samples which are turbid or coloured is environmental water, e.g. water in chillers from cooling towers and so on.

Experimental example

The data of an experimental example is provided in Table 1 to show how the titration device may be used for measuring total hardness in water samples taken from the environment. "Calcium hardness" relates to concentrations of calcium, which can be quantified by titrating with a known concentration of a chelating agent, EDTA (ethylenediamine tetraacetic acid). The indicator used in this case is Hydroxy Napthol Blue which turns from red to blue on reaching the end point. The data provides an example of end-point detection of three types of water samples for the same analysis despite visual interference inherent in the analysis samples, i.e. a clear water sample, a turbid water sample and a slightly brownish 'dirty' water sample.

Indicator used : Hydroxy Napthol Blue Reagent : KOH 8N Titrant : EDTA 0.01M

Figure 4 is a graph of the same experiment of the Table 1, where graph lines A, B and C represent the voltage level across potentiometer VRl during the calibration phase and titration phase. The voltage level Va, Vb and Vc indicates the pre-calibration and post- addition-of-indicator voltages developed across potentiometer VRl due to the clarity, turbidly or colouration in analysis samples A, B and C. Vx is the calibrated voltage across potentiometer VRl for all samples A, B and C, as determined by the calibration reference voltage across R8. The end-point is detected when the voltage of potentiometer VRl falls below the end-point reference voltage level, Vre, of the resistor R9. It can be seen that the initial voltages Va, Vb and Vc across VRl, for the samples A, B and C respectively, have all been calibrated or adjusted to Vx before titration begins.

More specifically, for Sample A, the pre-calibration and pre-titration state of the analysis sample is a clear solution. Addition of Hydroxy Napthol Blue to Sample A produces a red colour in the clear analysis sample. Thus, red light from the light source 4 is easily transmitted by dispersion to reach the light dependent resistor 5. This causes the pre-calibration voltage across the light dependent resistor 5 to be almost at its minimum and the pre-calibration voltage Va across potentiometer VRl to be high. Nevertheless, the calibrator adjusts the potentiometer VRl until the voltage Va across the potentiometer VRl increases to voltage level Vx (calibrated value). This ensures that Va is above the end-point reference voltage Vre so that the end-point may be detected. After the calibration phase, the titration phase begins.

When the titration reaches the end-point, the Hydroxy Napthol Blue changes from clear red to clear blue and the amount of red light transmitted to the light dependent resistor 5 is reduced; the blue Hydroxy Napthol Blue absorbs much of the red light. Thus, the voltage Va across potentiometer VRl falls to a level which is slightly below that of the end-point reference voltage Vre across resistor R9. At this point, the titration op-amp 206 gives a high state output which the NOT gate converts into a low state signal. This stops the operation of the pump 8 to end the titration. The amount of titrant used is then indicated by the LED display 10. For sample B, the pre-titration analysis sample has some turbidity, which gives the pre- titration analysis sample a turbid red colour when mixed with Hydroxy Napthol Blue. The turbidity interferes with the transmission of light through the analysis sample 2a. Thus, the pre-calibration resistance level in the light dependent resistor 5 is higher than the case for Sample A. Correspondingly, the initial voltage Vb across the potentiometer VRl is lower than in the case of sample A. However, during the calibration phase, the calibrating op-amp 201 causes the calibrator 11 to adjust potentiometer VRl such that the voltage Vb across potentiometer VRl increases to the voltage level Vx.This ensures that Vb is above the end-point reference voltage Vre so that the end-point may be detected; this provides an allowance for the voltage Vb to fall to the titration end-point leverl Vre. After the calibration phase, the titration phase begins. Thus, at the end-point, when the colour of the analysis sample 2a changes from turbid red to turbid blue (see Table 1), there is a corresponding a fall in voltage Vb to the end-point reference voltage Vre.

Sample C has an inherent colouration which causes the analysis sample to be a reddish brown when mixed with Hydroxy Napthol Blue. This reduces some transmission of light through Sample C, as compared to Sample A, and leads to a low initial voltage Vc across potentiometer VRl. During the calibration phase, the calibrator 11 adjusts the resistance of the potentiometer VRl until the voltage Vc across potentiometer VRl is increased to Vx. . After the calibration phase, the titration phase begins. As the voltage Vc is elevated after calibration, this provides an allowance for the potentiometer VRl voltage to fall to the level of the end-point reference voltage Vre at end-point, when the colour of the analysis sample 2a changes from brownish red to turbid blue (see Table 1). In all Samples A, B and C, when Va, Vb or Vc < Vre, the high state voltage output at the titration op-amp 206 is converted to a low state value by the first NOT gate to stop the pump 8.

As can be seen in the description, it is preferable in this experimental example that the light from the light source 4 is red. In this way, the reduction in light transmission is made more drastic than in the case in which a polychromatic white light is used instead of the red light source 4; more light is absorbed by the blue colour of the Hydroxy Napthol Blue at the end-point if the light is red, compared to a white light. This helps to improve the sensitivity of the end-point detection by the light dependent resistor 5. Furthermore, red light having a longer wavelength than those of other colours has the advantage of penetrating through analyte samples more easily, especially when the sample may be laden with some particles in comparison to light of shorter wavelengths.

Picking up on the earlier example of titration using phenolphthalein, in a reverse situation where an acid may be used to titrate an alkaline, the phenolphthalein turns from purple to colourless at the end-point. To use the titration device on such a reaction, the switch SWl is set to connect the output of a second NOT gate 213 to the pump 8. The second NOT gate inverses the signal output of the first NOT gate 207 such that the pump 8 is operated only when the output of the titration op-amp 206 is in a high state. In other words, only if light transmission through the purple phenolphthalein in the analysis sample 2a is below a certain level will the pump 8 operate to titrate the analysis sample 2a. The titration end-point is detected when the voltage of the non-inverting terminal increases above the end-point reference voltage Vre at the inverting terminal. However, before calibration begins, the switch SW3 (see Figure 2) is set to apply a voltage across the potentiometer VR3 instead of the potentiometer VR2. The potentiometer VR3 divides the applied voltage such that the voltage across resistor R8 is lower than that of the voltage across the resistor R9. The potentiometer VRl is then adjusted by the calibrator 11 to the voltage level of the resistor R8. Thus, at the beginning of the titration phase, the voltage at the non-inverting terminal of the titration op-amp 206 is lower than the voltage at the inverting terminal, creating a high state output at the titrating op-amp 206 which is unchanged because there are two NOT gates in series between the titrating op-amp 206 and the pump 8. At the end-point of the titration, light transmission through the analysis sample 2a increases and the voltage at the inverting terminal of the titrating op-amp also increases. This stops the pump 8 and ends the titration. The LED display 10 then exhibits the amount of titrant used to reach the end-point.

Both the resistance level of potentiometers VR2 and VR3 are pre-determined in the factory. This is done by using standard solutions for specific titrations, mixed with specific indicators, to factory-calibrate the potentiometers VR2 and VR3 to obtain the desired voltage across the resistor R8. Thus, the titrator in this embodiment is, although not necessarily, designed for pre-determined titration reactions using specific indicators.

Figure 5 shows an expanded circuit diagram, which is the same as that of Figure 2 in function, where the timer 211 is actually two timers 211, one comprising IC 1(1/2)" and ICl(l/2) for timing the calibration phase and the other comprising IC2(l/2) and IC(l/2)" for timing the titration phase. Furthermore, the switch SWl is now replaced by transistors, Tr5, Tr7, Tr8, Tr9 and R12, R61, R46, R47, such that a common switch SW7 selects between VR2 and VR3 and between using the second NOT gate or not. Furthermore, the AND gate 20 is now replaced by the combination of Tr2, Tr3, R2 and R3, to control the calibrator 11. Furthermore, an oscillator IC3 (1/2) is used to drive the pump 8 when the signal reaching the pump from the titration op-amp 206 is in a high state.

Therefore, a titration apparatus has been described, comprising a light sensor 5 to detect transmission of light through a titration analysis sample 2a; the light sensor 5 capable of providing an electrical indication representing an intensity of the light detected by the light sensor, such that the electrical indication changes when the intensity of detected light changes; wherein the level of the electrical indication representing the intensity of the light detected is adjustable.

Furthermore, a titration apparatus has been described comprising a light source 4 to transmit light through a titration analysis sample 2a, and a light sensor 5 to detect change in colour of the titration analysis sample 2a, wherein the light source 4 is positioned to be above the titration analysis sample 2a, and the light sensor 5 is positioned at the side of the titration analysis sample

Furthermore, a titration apparatus has been described which comprises a means, e.g. the tube 7, to introduce a drop of fluid 302 into a titration analysis sample 2a, a pair of electrically conductive members 301 which are spaced apart, the pair of electrically conductive members 301 arranged such that the drop of fluid 302 passes between the pair of electrically conductive members 301, wherein if the drop of fluid 302 is electrically conductive, the drop of fluid 302 electrically connects the pair of electrically conductive members 301.

The titration apparatus therefore provides the possibility of overcoming interference from turbidity or colouration in the analysis sample to the end-point detection. This provides the advantage of relieving the need for extensive sample preparation before titration, allowing titration to be performed in the field.

Furthermore, a method of detecting the colour change of an indicator at the end-point of a titration have been described, comprising the steps of providing a titration analysis sample 2a, positioning the titration analysis sample 2a such that light from a light source 4 is transmitted through the titration analysis sample 2a to be detected by a light dependent resistor 5, the light dependent resistor 5 being connected in series with a potentiometer VRl, providing a voltage across the light dependent resistor 5 and the potentiometer VRl, measuring the portion of the voltage divided to the potentiometer VRl, comparing the portion of the voltage divided to the potentiometer VRl with a calibration reference voltage, adjusting the resistance of the potentiometer VRl until the voltage divided to the potentiometer VRl is at a pre-determined level as indicated by the calibration reference voltage, titrating the titration analysis sample 2a until end- point, wherein the intensity of the light detected by the light dependent resistor 5 at end- point changes the portion of the voltage divided to the potentiometer VRl. Furthermore, a method of monitoring the volume of titrant used in a titration have been described, comprising the steps of dispensing the titrant into an analysis sample 2a drop by drop, each drop 302 of the titrant dispensed such that the drop 302 passes between two wires 301 to contact the two wires 301 and to allow a current to pass through the two wires 302, recording the passage of current through the two wires 302 as an indication of a drop of the titrant having been dispensed.

While there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design, construction or operation may be made without departing from the scope of the present invention as claimed.

For example, which the embodiments and examples given uses an indicator to provide the colour change, it is possible that in some reactions such as redox reactions, the colour change is due to the change in oxidation state of the reactants. In these cases, there is no need for any colour indicator.

Furthermore, although red light has been mentioned as preferable, in variations of the embodiment, the light source 4 may provide white light, blue light, green light, U.V. radiation and so on.

Furthermore, it is possible that parts of the described embodiment may be employed independently of other parts. For example, the light sensor, the potentiometer VRl, the potentiometer VR2, the calibrator and other accompanying parts may be employed in an end-point detection device for a laboratory titration using a burette and a beaker, i.e without the pump 8.

Furthermore, although two potentiometers VR2 and VR3 have been described for selecting the mode of titration, there may be more than two of such potentiometers, each of these potentiometers being pre-set at factory to allow the potentiometer VRl to be calibrated to different resistance level for different titration reactions.

Furthermore, although the wires 301 at the mouth of the tube 7 inside the housing 1 have been described as being made of platinum, it is possible that the wires may be made of other conductive and inert material, such as gold, carbon and so on.

Furthermore, there may be at least one alternative resistor to the resistor R9 to provide the end-point reference voltage Vre and a switch may be provided to select between these resistors.

Furthermore, as would be known to the skilled man, other than using the potentiometer VRl, various alternatives such as capacitors or inductors which may be adjusted to calibrate their sensitivities to the detected light is possible.

Furthermore, as would be known to the skilled man, other than using the light dependent resistor, optoelectronic alternatives such as photodiodes, phototransistors, photocell, photovoltaic cell, photomultplier, photoresistor and so on which provide an electrical response in the presence or absence of light may be used in variations of the embodiment.

Furthermore, the op-amps in the circuit 9 may be replaced by using transistors and resistors arrangement as would be known to the skilled man.

Furthermore, although only an LED display 10 has been described for indicating the volume or drops of titrant used, it is understood that all manner of data display, computerisation, data storage, data transmission may be used in some embodiments.

Furthermore, although not described in detail, the skilled man will understand that features of the embodiment may be implemented in various forms of auto-titrators, including those which draw samples from an auto-sample for multiple samples.

Claims

1 A titration apparatus comprising a light sensor to detect transmission of light through a titration analysis sample; the light sensor capable of providing an electrical indication representing an intensity of the light detected by the light sensor, such that the electrical indication changes when the intensity of detected light changes; wherein the level of the electrical indication representing the intensity of the light detected is adjustable.

2 The titration apparatus as claimed in claim 1 wherein the light sensor comprises a light dependent resistor arranged in series with a potentiometer, a voltage applicable across the a light dependent resistor and the potentiometer; wherein the portion of the applied voltage divided to the potentiometer provides the electrical indication.

3 The titration apparatus as claimed in claim 2 wherein the portion of the voltage divided to the potentiometer is adjustable by adjusting the resistance of the potentiometer.

4 The titration apparatus as claimed in claim 3 further comprising an adjustment means for monitoring the portion of the voltage divided to the potentiometer; and the adjustment means being capable of making the adjustment to the resistance of the potentiometer.

The titration apparatus as claimed in claim 4 wherein the adjustment means comprises a comparator operational amplifier having an inverting terminal and a non-inverting terminal; the inverting terminal being arranged to tap the divided voltage across the potentiometer; the non-inverting terminal being arranged to tap a reference voltage; wherein the comparator operational amplifier is capable of providing an output based on the voltage tapped by the inverting terminal and a non-inverting terminal to drive the adjustment of the resistance of the potentiometer.

6 The titration apparatus as claimed in claim 5 wherein the inverting terminal of the comparator operational amplifier being arranged to tap one of a plurality of reference voltages.

7 A titration apparatus comprising as claimed in anyone of claims 1 to 6, further comprising a titration reference voltage, the titration reference voltage being an indication of the end-point of a titration.

A titration apparatus as claimed in anyone of claims 1 to 7, wherein the light source is positioned to be above the titration analysis sample; and the light sensor is positioned to be at the side of the titration analysis sample.

A titration apparatus as claimed in anyone of claims 1 to 8, further comprising a means to introduce a drop of fluid into the analysis sample; a pair of electrically conductive members which are spaced apart; the pair of electrically conductive members being arranged such that the drop of fluid passes between the pair of electrically conductive members; wherein if the drop of fluid is electrically conductive, the fluid electrically connects the pair of electrically conductive members.

A titration apparatus comprising a means to introduce a drop of fluid into a titration analysis sample; a pair of electrically conductive members which are spaced apart; the pair of electrically conductive members arranged such that the drop of fluid passes between the pair of electrically conductive members; wherein if the drop of fluid is electrically conductive, the drop of fluid electrically connects the pair of electrically conductive members.

A titration apparatus comprising a light source to transmit light through a titration analysis sample; and a light sensor to detect change in colour of the titration analysis sample; wherein the light source is positioned to be above the titration analysis sample; and the light sensor is positioned at the side of the titration analysis sample.

A method of detecting the colour change of an indicator at the end-point of a titration, comprising the steps of providing a titration analysis sample; positioning the titration analysis sample such that light from a light source is transmitted through the titration analysis sample to be detected by a light dependent resistor, the light dependent resistor being connected in series with a potentiometer; providing a voltage across the light dependent resistor and the potentiometer; measuring the portion of the voltage divided to the potentiometer comparing the portion of the voltage divided to the potentiometer with a calibration reference voltage; adjusting the resistance of the potentiometer until the voltage divided to the potentiometer is at a pre-determined level as indicated by the calibration reference voltage; titrating the titration analysis sample until end-point, wherein the intensity of the light detected by the light dependent resistor at end-point changes the portion of the voltage divided to the potentiometer.

A method of monitoring the volume of titrant used in a titration comprising the steps of: dispensing the titrant into an analysis sample drop by drop; each drop of the titrant dispensed such that the drop passes between two wires to contact the two wires and to allow a current to pass through the two wires; recording the passage of current through the two wires as an indication of a drop of the titrant having been dispensed.