GB2370641A - Coulometric titration system - Google Patents

Abstract

The system has electrolysis electrodes (3) inserted in a mixture (2) of reagent and sample to be tested. Current supply (4) provides electric current at any level between predetermined limits to pass between the electrodes (3). The current actually passing between the electrolysis electrodes is measured by meter (5). A control unit (8) sets at any instant the level at which the current is maintained and in response to the meter (5) detecting the actual current passing between the electrolysis electrodes (3) is below the level then set by the control unit (8), reduces the set level to substantially the highest current that the current supply (4) is able to maintain. A charge counter (9) calculates the total electric charge passed between the electrolysis electrodes (3).

Description

COULOMETRIC TITRATION SYSTEM The present invention relates to the field of coulometric titration apparatus, and more particularly, but not exclusively, to coulometric titration apparatus for determining the water content of a water containing mixture. It is particularly relevant to mixtures of water and oil, especially crude oil and other petroleum products. The present invention further relates to a method for determining water contents using such apparatus.

It is well known to determine low concentrations of water in materials such as crude oil by titrating the water with iodine in the presence of a reagent (mixture). The method is generally known as the Karl Fischer method and the many different cocktails of reagents which may be used are collectively referred to as Karl Fischer reagents.

The titration can be carried out by conventional volumetric methods, but particularly in the case of water determination on crude oil and other petroleum products, coulometric methods are now widely accepted as being the preferred method. In a coulometric titration, iodine to react with the water is generated in-situ by electrolysis. The iodine reacts to reduce the concentration of water and the endpoint, i. e when there is a minute excess of free iodine, is determined The exact amount of iodine generated up to that endpoint can be calculated from the quantity of electric current passed between electrolysis electrodes of the titration apparatus, and from this amount, the water content can be found.

This method has the drawback that, in some cases, the endpoint can be overshot if the electrolysis current remains at a substantially constant high level throughout the titration. If the current is kept low, the titration is inconveniently lengthy and may lead to errors because the drift rate is assumed to be constant for the duration of the titration.

Known coulometric titration apparatus attempts to overcome this by pulsing the electrolysis current with the pulses becoming progressively shorter as the endpoint is approached. This method can still lead to the generation of an excessive titre of iodine at the endpoint, and incorrect results due to electrical resistance are likely.

Such apparatus also has a particular problem with determining water in crude oil. The standard methods, such as A. S. T. M. (American Society for Testing and Materials) Method D-4928, specify that the reagent be modified with anhydrous xylene before testing to ensure that the crude oil does not deposit high molecular weight solids in the titration apparatus The addition of anhydrous xylene to the reagent can raise the resistance of the sample to a point at which the apparatus is unable to produce the required electrolysis current. The amount of iodine generated thus falls. Since the"counter"of the apparatus determines the amount of iodine produced by multiplying the pulse durations by the theoretical set current, it will register an erroneously high titre of iodine.

An attempt was made to address this problem in the apparatus described in UK Patent Number GB 2237387. This also employs pulsed electrolysis current, but when a set current level cannot be achieved, lower current settings are available. When the resistance is high, when the detector signal is decreasing, or when there is low power supply, the control circuitry automatically steps down the current to the next lower setting and the counter determines the iodine generated thereafter on the basis of the new set current. Relatively low current settings are available, which are triggered-as the titration approaches the endpoint.

However, such an apparatus has been found to fall short of providing an acceptable solution.

The crude steps down in current lead to a prolonged titration time. The actual current being passed between the electrolysis electrodes can vary measurably before the control circuitry intervenes to step the set current down to a more reliable level, and so the calculated iodine titre can still be significantly in error. As a result, it is necessary to run frequent titrations on standard test samples of known water content, to check for errors, and to make compensations and corrections to the water content measurements accordingly.

Because of the shortcomings described above, the efficiency of coulometric titration apparatuses must be checked with standards, and the reagent replaced if the obtained results are outside specified limits. There is also a need for a portable, reliable water content meter which can be operated in the field by a non-specialist user.

It is therefore an object of the present invention to provide a coulometric titration device which obviates the above disadvantages and provides an accurate, reliable and straightforward means for the determination of the water content of materials, such as crude oil It is a further object of the present invention to provide a method for determining such water contents, which method is simple, accurate and suited for use either in a laboratory or in the field by non-specialist staff.

According to a first aspect of the present invention, there is provided a coulometric titration device comprising electrolysis electrodes adapted to contact a mixture of reagent and sample to be tested, means to generate electric current at any level between predetermined limits and to pass it between said electrodes, metering means adapted to determine the current actually passing between the electrolysis electrodes, control means to set at any instant the level at which said current generating means maintains said current, and in response to the metering means detecting that the actual current passing between the electrolysis electrodes is below the level then set by the control means, to reduce said set level to substantially the highest current that the current generating means is able to maintain, and counting means adapted to calculate the total electric charge passed between the electrolysis electrodes.

Advantageously, the current is generated as a series of pulses of fixed duration, such that the counting means may calculate the time that a current of any level has flowed from the number of said pulses at that level which have been generated.

In a preferred embodiment, the titration device comprises detection electrodes to be inserted into the solution and measuring means adapted to determine the polarisation potential between said detection electrodes, and in response to any change in said polarisation potential to signal the control means, said control means being adapted to change accordingly the set level of the current passing between the electrolysis electrodes.

Advantageously, when the measuring means detects that the polarisation potential has fallen to a level at or below that corresponding to an endpoint of the titration, the control means sets the current passed between the electrolysing electrodes at such a level as to maintain the baseline at a stable, known condition.

The control means may signal the counting means during and at the end of the titration, and when the titration is complete may communicate data on the total electric charge passed during the titration to a calculating means, said calculating means being adapted to calculate a titration result, such as a water content of the sample, from said total electric charge.

Preferably, some or all of said control means, said counting means, said metering means, said detecting means and said calculating means comprise software programs running on suitable microprocessor means. The titration device may be so dimensioned and configured as to fit into a conveniently portable carrying case. The titration device may comprise input means, such as a keyboard or keypad, to enable a user to control the device. The titration device may comprise display means which may comprise a printing means and/or a display screen to display the progress of the titration and to display results. The titration device may comprise a memory means to store alternative control programs and to store results. The titration device may comprise means, such as an RS232 data interface, to electronically interface with other apparatus. The titration device may be adapted to be used for Karl Fischer titrations to determine the water content of a solution. Optionally, the titration device may be used to determine the water content of a sample of crude oil or other petroleum product. According to a second aspect of the present invention, there is provided a method of coulometric titration comprising the steps of inserting electrolysis electrodes into a solution of reagent, adding a sample to be tested, passing an electric current level between said electrolysis electrodes in order to electrolytically generate a titration reagent, metering the current actually passing between said electrolysis electrodes and, if the current actually passing between said electrolysis electrodes changes, changing the level of the electric current to substantially the highest current that can be maintained. Preferably, the method comprises the step of calculating the total electric charge theoretically passed between the electrolysis electrodes from the actual current passed between said electrodes.

The method may comprise the steps of inserting detection electrodes into the solution, determining the polarisation potential between said detection electrodes and changing the set level of the current passing between the electrolysis electrodes in line with any change in the polarisation potential. The method may also comprise the step of terminating the current passing between the electrolysis electrodes when the polarisation potential has fallen to a level at or below that corresponding to an endpoint of the titration. Preferably, the method comprises the step of calculating a titration result, such as water content of the solution, from the total electric charge passed between the electrolysis electrodes. An embodiment of the present invention will now be more particularly described, by way of example and with reference to the accompanying drawing, in which : Figure 1 is a schematic diagram of a titration device embodying the invention. A coulometric titration device embodying the present invention is particularly useful as a "Karl Fischer"titrator, to determine the water content of a sample of organic material, such as crude oil. The following description is of such a"Karl Fischer"titrator embodying the present invention, though the present invention is not necessarily limited to titrations using the Karl Fischer method. A cell 1 contains a mixture 2 of a Karl Fischer reagent having an excess of iodine and a sample of the solution to be tested. A pair of electrolysis electrodes 3 is inserted into the mixture 2, and is connected to a controllable current supply 4. A current meter 5 is connected between the electrodes 3 and the current supply 4, to measure the actual current flowing in the circuit including the electrodes 3 A pair of detector electrodes 6 is also immersed in the mixture 2 and is connected to a potential measuring device 7, which measures the polarisation potential of the mixture 2 by applying on alternating current across the detector electrodes 6 and detecting the potential induced across the electrodes 6 thereby.

The potential measuring device 7, the current meter 5 and the current supply 4 are all connected to a control unit 8 as shown. The current meter 5 is also connected to a charge counter 9, which in turn is connected to a calculator unit 10. The control unit 8 is also linked to a keyboard 11, a LCD display screen 12 and a memory unit 13.

In use, a measured sample to be tested is placed in the cell 1 with the reagent; this mixture is conventionally a solution in anhydrous xylene, to prevent less soluble components of the sample leaving solid deposits in the cell 1. The titration is started by a command entered to the control unit 8 either automatically or via the keyboard 11. A current level to be passed between the electrolysis electrodes 3 is set by the control unit 8 and the current supply 4 supplies a series of pulses of current at the set current level. Each pulse has a standard duration and so the electric charge passing through the mixture 2 during each pulse is dependent on the current alone, as therefore is the quantity of iodine generated by electrolysis of the Karl Fischer reagent in the cell 1. The current meter 5 detects the actual current being passed for each pulse and communicates this data to the charge counter 9, which sums the total charge passed since the start of the titration.

In normal operation, the potential measuring device 7 follows the polarisation potential of the mixture 2, which is a measure of the conductivity of the solution, which is largely due to the water from the sample. As the iodine generated by the electrolysis electrodes 3 and the Karl Fischer reagent reacts with the water, the conductivity and the polarisation potential fall. At the endpoint of the titration, the potential measuring device 7 signals the control unit 8, which stops the current supply 4 and thereby stops the electrolysis. The control unit 8 also instructs the charge counter 9 to cease counting. The calculator unit 10 establishes a final count of the total electric charge passed between the electrodes 3 during the titration and then calculates therefrom the total quantity of iodine generated. This is the quantity needed to react with all the water, and therefore can be used to give a measure of the water content. The result is passed to the memory unit 13, which may record it with any other data. During the titration, as the polarisation potential falls, the control unit 8 also instructs the current supply 4 to pass current between the electrolysis electrodes 3 at a proportionately lower level, such that smaller quantities of electric charge are passed in each pulse, and smaller aliquots of iodine are generated. The titration rate thus slows as the end point is neared. The meter 5 detects the new current being passed and communicates this data to the charge counter 9 appropriately.

In practice, the resistance of the solution can be such that the current, set by the control unit 8 for the current supply 4 to pass, cannot be achieved. This can be due to different formulations of reagents, to the dilution of the sample with anhydrous xylene, or intermittent interference by barely soluble components of the crude oil. The control unit 8 detects that the current measured by the meter 5 is not at the level that it has specified, and reduces the current setting to the highest setting at which the current supply 4 can maintain a current at the specified set level. The meter 5 detects this new current level and the charge counter 9 is informed appropriately.

The titration device embodying the present invention thus operates at all times at the maximum electrolysis current consistent with accurate titration and with stable operation. It is thus faster and more accurate in operation than known devices. As all charge calculations are based on measured current flows, rather than on set, assumed current flows, as in the case of known devices, the titration device embodying the present invention is also more accurate and reliable, since it automatically corrects the errors to which known devices are prone. It may therefore be referred to as ACE (Automatically Corrected Errors).

Claims (13)

1. A coulometric titration device comprising electrolysis electrodes adapted to contact a mixture of reagent and sample to be tested, means to generate electric current at any level between predetermined limits and to pass it between said electrodes, metering means adapted to determine the current actually passing between the electrolysis electrodes, control means to set at any instant the level at which said current generating means maintains said current, and in response to the metering means detecting that the actual current passing between the electrolysis electrodes is below the level then set by the control means, to reduce said set level to substantially the highest current that the current generating means is able to maintain, and counting means adapted to calculate the total electric charge passed between the electrolysis electrodes.

2. A device as claimed in claim 1, wherein the means to generate electric current produces a series of pulses of fixed duration, such that the counting means may calculate the time that a current of any level has flowed from the number of said pulses at that level which have been generated.

3. A device as claimed in either claim I or claim 2, wherein the titration device comprises detection electrodes to be inserted into the sample/reagent mixture and measuring means adapted to determine the polarisation potential between said detection electrodes, and in response to any change in said polarisation potential to signal the control means, said control means being adapted to change accordingly the set level of the current passing between the electrolysis electrodes.

4. A device as claimed in any one of the preceding claims, wherein the measuring means is adapted to detect that the polarisation potential has fallen to a level at or below that corresponding to an endpoint of the titration, and in response thereto the control means is adapted to set the current passed between the electrolysing electrodes at such a level as to maintain the baseline at a stable, known condition.

5. A device as claimed in either claim 4, wherein the control means is adapted to signal the counting means, at the end point of the titration, and in response thereto the counting means communicates data on the total electric charge passed during the titration to a calculating means, said calculating means being adapted to calculate a titration result, such as a water content of the sample, from said total electric charge.

6. A device as claimed in any one of the preceding claims, adapted to be used for Karl Fischer titrations to determine the water content of a sample, optionally a crude oil sample.

7. A coulometric titration device substantially as described herein with reference to the Figure of the accompanying drawings.

8. A method of coulometric titration comprising the steps of inserting electrolysis electrodes into a solution of reagent, adding a sample to be tested, passing an electric current at a set level between said electrolysis electrodes in order to electrolytically generate a titration reagent, metering the current actually passing between said electrolysis electrodes and, if the current actually passing between said electrolysis electrodes changes, changing the level of the electric current to substantially the highest current that can be maintained.

9. A method as claimed in claim 8, further comprising the step of calculating the total electric charge theoretically passed between the electrolysis electrodes from the actual

current passed between said electrodes.

10. A method as claimed in either claim 8 or claim 9, further comprising the steps of inserting detection electrodes into the sample determining the polarisation potential between said detection electrodes and changing the set level of the current passing between the electrolysis electrodes in line with any change in the polarisation potential.

11. A method as claimed in claim 10, further comprising the step of terminating the current passing between the electrolysis electrodes when the polarisation potential has fallen to a level at or below that corresponding to an endpoint of the titration.

12. A method as claimed in any one of claims 8 to 11, further comprising the step of calculating a titration result, such as water content of the solution, from the total electric charge passed between the electrolysis electrodes.

13. A method of coulometric titration substantially as described herein with reference to the Figure of the accompanying drawings.