GB2206413A - Determining a component in an organic solvent containing solution - Google Patents

Abstract

A method of measuring a component in a solution comprised of at least one organic solvent comprises; (a) passing continuously a sample of the solution at a predetermined rate to a mixing chamber (1); (b) passing continuously to the mixing chamber (1) at a predetermined rate a buffer solution comprising a predetermined amount of an indicator ion; (c) effecting homogeneous mixing of the buffer solution and the sample whilst continuously withdrawing the resulting solution mixture from the mixing chamber through a cell (6) comprising an electrode (15) selective for said indicator ion, a reference electrode (16) and an automatic temperature compensation probe; (d) continuously or continually measuring the cell potential and (e) comparing the measurement with calibration data for the system to determine the concentration of the component. Specified materials are water traces in various organic solvents and alcohol content of drinks. Carbonated drinks are degassed prior to entry into the mixing chamber. <IMAGE>

Description

APPARATUS AND METHOD FOR MEASURING AMOUNT OF A SELECTED COMPONENT USING THE ION-ISOCONCENTRATION TECHNIQUE The present invention relates to an apparatus and method for measuring the amount of a selected component under continuous flow conditions. The invention is concerned particularly with the measurement of such amounts by the use of the so-called Ion-Isoconcentration Technique (IICT).

Particular examples of measurements with which the invention is concerned are the determination of trace water in organic solvents and the determination of individual organic solvents in mixtures thereof.

The IICT is based on the use of a cell comprised of an ion-selective electrode, a given reference electrode and a cell solution comprised of an organic solvent and an ion for which the indicator electrode is selective.

Under ion-isoconcentration conditions, potentials of cells with ion-selective electrodes vary systematically and reproducibly with variation in solvent concentration, thus enabling the latter to be determined rapidly. The Ion-Isoconcentration Technique is known and a detailed description can be found, for example, in Ion-Selective Electrode Rev.

1981 Vol.3 pp 127-187 (particularly pages 155-173).

In the case of a pH glass electrode as the ionselective electrode, the variation in cell potential is proportional to the variation in the moisture content of organic solvents at low water content. In many industrial processes the moisture content of material is an important factor in determining the quality of the final product. On-line measurement of moisture content can therefore provide cost effective crnvrol of a recess.

The use of IICT for batchwise measurement has already been described (see Ion-Selective Electrode Rev.

(above) and also Analyst, October 1978, Vol. 103 pp 1046-1052), but to our knowledge its use for the on-line measurement of a selected component under continuous flow conditions has not been developed.

According to a first aspect of the present invention there is provided a method of measuring a component in a solution comprised of at least one organic solvent, the method comprising; (a) passing continuously a sample of the solution at a predetermined rate to a mixing chamber; (b) passing continuously to the mixing chamber at a predetermined rate a buffer solution comprising a predetermined amount of an indicator ion; (c) effecting homogeneous mixing of the buffer solution and the sample whilst continuously withdrawing the resulting solution mixture from the mixing chamber through a cell comprising an electrode selective for said indicator ion, a reference electrode and an automatic temperature compensation probe;; (d) continuously or continually measuring the cell potential and (e) comparing said measurement with calibration data for the system to determine the concentration of said component.

According to a second aspect of the invention there is provides apparatus for measuring in a solution containing at least one organic solvent, the apparatus comprising a mixing chamber having two inlets and an outlet, an electrode cell connected to said chamber and having an ion-selective electrode, a reference electrode, an automatic temperature compensation probe and means for measuring the cell potential generated between said ion-selective electrode and said reference electrode.

The method of the invention is applicable to the determination of a wide range of components (e.g.

trace water in an organic solvent, or one organic solvent in a mixture of solvents) although it will be appreciated that the choice of ion-selective electrode, reference electrode and indicator ion is dependent on the system under investigation.

Particular examples of reference electrodes, ionselective electrodes and indicator ions to be used in particular determinations will be detailed below.

In all cases, however, the determination is effected by taking a sample of the solution under investigation (after degassing if necessary, e.g.

carbonated solutions) and passing it as a continuous flow through the apparatus of the invention.

In order that the method of the invention may be used successfully for on-line monitoring, it is essential for the sample of the solution to be mixed as thoroughly as possible with the buffer solution (containing the indicator ion) before the mixture is introduced into the cell for measurement of the potential. We have found that the most suitable mixing arrangement is a comparatively small chamber (e.g. of glass) into which the sample and indicator ion solutions are introduced and from which the mixture may be continuously withdrawn, and which is also provided with a magnetic stirring device to enhance the mixing.A particular example of such an arrangement is one having two upper inlet tubes (one for the sample and one for the buffer solution) angled downwardly towards each other so that the respective streams issuing from the inlet tubes flow to r base c- the chamber. Such a chamber also comprises ar, outlet tute (e.g. through the wall of the chamber) through which the mixed solutions may be passed to the cell (detailed more fully below).

A chamber of the type described in the preceding paragraph (with magnetic stirrer) has been found to be superior to conventional mixers (e.g. Tee or Kenics mixers) in providing a greater degree of mixing of the two liquid streams, particularly when dealing with viscous solvents (e.g. ethanediol).

Peristaltic or piston pumps may be used for supplying the sample and buffer solutions to the mixing chamber at the required predetermined rate and for supplying the solution mixture from the mixing chamber to the cell.

The cell is preferably of glass and comprised of first, second and third chambers through which the solution passes sequentially before leaving the cell.

Preferably each of the chambers is formed from a separate tube or the like into the top of which is fitted an electrode or temperature probe as detailed below. The first chamber has an inlet about half-way up its body connected to the mixing chamber and has an outlet tube connecting its lower region to the upper region of the second chamber. Similarly a connecting tube is provided between the lower region of the second and upper region of the third chamber, the latter having an outlet from the lower region rising about half-way up its body. Preferably also such a cell has an air-vent (e.g. in the first chamber) to assist depulsing and debubbling.

A cell constructed as described above has the advantage that solution homogeneity is improved due to the fact that the liquid is forced up and down during its passage through the three chambers.

For preference, the ion-selective electrode is provided in the first chamber, the reverence electrode in the second chamber and an automatic temperature compensation probe in the third chamber.

Preferably also, the cell is held in a liquid bath to minimise temperature fluctuations.

The evaluation of the cell potentials generated between the reference and ion-selective electrodes to give the amount of the component being measured lends itself readily to computer processing (as described more fully below). The automatic temperature compensation probe is used to determine the absolute temperature of the solution in the electrode cell.

This absolute temperature is compared to a datum temperature (e.g. 25 C). The magnitude of any difference between the measured and datum temperatures causes a compensation to be applied to the potential measured by the electrodes in the cell.

To measure the amount of the selected component, the sample is passed continuously through the apparatus until such time as the cell potential has substantially equilibrated. Measurement of the equilibrium potential and comparison with calibration data will give the amount of selected component in the sample.

Calibration of the apparatus can be effected by a number (e.g. 3) of standard solutions each comprised of the same components as the sample under investigation but containing different, known amounts of the component to be determined in the sample.

Each calibration solution may be passed in turn through the apparatus and the potential of the cell measured (after it has been allowed to equilibrate).

A series of potentials is obtained (one for each calibration solution), thus allowing a calibration curve (potential vs amount) tc be obtained. The procedure is then repeated for the sample to allow the amount of the selected component to be obtained from the calibration curve.

A preferred embodiment of apparatus in accordance with the invention is adapted for repeated calibration and sampling and has its mixing chamber connected via a multi-way selector valve to a plurality of calibration solutions as well as to the source of the product to be sampled. The multi-way valve may be under computer control so -that at any one time either a calibration solution or the product to be sampled is passed to the mixing chamber for admixture with the buffer solution.Such an apparatus may be operated cyclically whereby the calibration solutions and the sample are passed through the apparatus in a predetermined cycl. In a particularly preferred mode of operation, the calibration solutions , will be formulated so that at least one has a concentration of the selected component greater than that of the sample and at least one has a concentration of that component lower than that of the sample. The calibration solutions are then passed through the apparatus in increasing (or decreasing) order of their concentrations in the selected component. The sample is passed through the apparatus between the calibration solutions of immediately lower and higher concentrations in the selected component.Use of this procedure allows a continual increase (or decrease) in cell potential during the calibration and sample measurements thus avoiding unnecessary fluctuations and allowing the cell potential to equilibrate as quickly as possible.

The ion-selective electrode, reference electrode and indicator ion are dependent on the system under investigation.

So far as the ion-selective electrode is concerned, a pH glass electrode is particularly suitable for determinations of trace water in organic solvents and trace formamide in water due to its high sensitivity to small changes in the water content and formamide content, respectively, under conditions of constant acid concentration. A pH glass electode should preferably be produced from low impedance glass and may require conditioning in an appropriate buffer solution prior to monitoring.

Fluoride and chloride electrodes (selective for fluoride and chloride ions, respectively) are also suitable for many determinations.

So far as reference electrodes are concerned, those which are commercially available may not be suitable. Such electrodes usually have aqueous filling solutions and can give rise to instability of potential at the high solvent concentrations found in the method of this invention. Satisfactory performance at high solvent concentrations has been achieved by using silver-silver chloride and calomel reference elements with the following filling solutions: (a) Silver-silver chloride reference element: Saturated lithium chloride in methanol, ethanol, propan-2-ol, ethanediol/propan-2-ol (3:1) and sulpholane, respectively.

Saturated tetraethylammonium chloride in propan-2-ol and propylene carbonate, respectively.

(b) Calomel reference element: Saturated lithium chloride in acetic acid, methanol and sulpholane, respectively.

Saturated tetraethylammonium chloride in mropan-2-ol.

The reference electrode can be replaced by another ion-selective electrode, e.g. solid-state chloride electrode, which improves the stability and reproducibility of the cell potential in PICT. For use in aggresive solvents (e.g. butan-2-one) the chloride electrode body is made from PTFE material and the membrane sealant from non-epoxyresin adhesive.

The indicator ions are supplied in a buffer solution which may be formulated with various components to facilitate measurement of the potential and/or flow of the sample or calibration solution through the apparatus.

The function of the buffer solution may be fourfold: (i) providing ions to which the indicator electrode is selective, viz. proton in PICT hydroxide ín HICT and fluoride in FICT (proton-isoconce,-ltratiGn technique, hydroxide isoconcentration technique and fluoride-isoconcentration technique, respectively); (ii) increasing conductance for solvents of relatively high impedance, (e.g. propanone, butan-2-one, nitrobenzene, tetrachloroethylene) by adding methanol and/or glacial acetic acid; (iii) improving the sensitivity of potential response, e.g.

by adding propan-2-ol to ethanediol, or by adding propylene carbonate to the buffer; (v) increasing the Renold numbers when dealing with viscous solvents by adding methanol or ethanol.

Flow rates of the solutions through the cell may be kept relatively low, e.g. 9cc/min to minimise solvent wastage, and this is suitable for the majority of solvents. Alternatively, a higher flow rate may be used to improve monitor performance.

This is necessary for the following solvents: (a) ethanediol, characterised by high viscosity, has been monitored at a flow rate of 27cc/min. (b) Butan-2-one has been analysed at the increased rate of 1Scc/min to minimise diffusion of the aqueous filling solution from the calomel reference electrode into the cell.

(c) When using two high impedance electrodes (e.g.

glass electrode and chloride electrode) increase in flow rate to 18cc/min gives improved stability of potential.

It follows from the above that the reference electrode, buffer solution, flow rate and mixing are important factors in continuous monitoring by PICT.

Particular examples of determinations which may be effected by the invention, and the conditions under which they are carried out are given by the following.

1) Determination of trace water in the following solvents by PICT: (a) Methanol, ethanol, propan-l-ol and propan-2-ol: Flow rate, 9cc/min; buffer solution, 10-2mol dim~3 perchloric acid in the dry solvent under investigation; reference electrode filling solution, saturated lithium chloride in the respective dry solvent.

(b) Ethanediol: Flow rate, 27cc/min; buffer solution, 10~2mol dim~3 perchloric acid in 20% dry propan-2-ol and 80% dry ethanediol; reference electrode filling solution, saturated lithium choride in ethanediol/propan-2-ol mixture (3:1); reinforced mixing.

(c) Propanone: Flow rate, 9cc/min; buffer solution, 10-3mol dm-3 para-toluenesulphonic acid in 10% glacial acetic acid, 1% dry methanol and 89E dry propanone; reference electrode filling solution, saturated lithium choride in dry propan-2-ol.

(d) Butan-2-one: Flow rate, 18cc/min; buffer solution, 10-3mol dim~3 para-toluenesulphonic acid in 20% dry methanol plus 80% dry butan-2-one; reference electrode filling solution, saturated aqueous lithium chloride (calomel electrode).

(e) Tetrachloroethylene: Flow rate, 18cc/min; buffer solution, 10-2mol dm-3 perchloric acid in 50% dry methanol plus 50% dry tetrachloroethylene; reference electrode filling solution, saturated aqueous lithium chloride.

2(i). Determination of ethanol in drinks by FICT (a) Sherry, Vermouth: Flow rate, 9cc/min; buffer solution, 2x10-4mol dm-3 sodium fluoride plus Orion TISAB II buffer; reference electrode filling solution, 10% aqueous potassium nitrate.

(b) Carbonated drinks, e. beer, cider: Flow rate, 6cc/min; buffer solution 4xl04mol dm-3 sodium fluoride plus Orion TISAB IV buffer; reference electrode filling solution, 10% aqueous potassium nitrate.

2(ii). Determination of ethanol in drinks by HICT Flow Rate, 9 cc/min: buffer solutior, 2 x 10-3 mol dm-3 sodium hydroxide; reference electrode filling solution, 0.48 aqueous sodium hydroxide.

The invention will be further described by wait of example only with reference to the accompanying drawings, in which Fig. 1 illustrates one embodiment of mixing chamber (1); Fig. 2 illustrates one embodiment of electrode cell (6); Fig. 3 illustrates apparatus for carrying out the method of the invention; Figs. 4-8 illustrate results obtained with the method of the invention.

Fig. 9 illustrates an ultrasonic degasser.

The mixing chamber 1, shown in Fig. 1, is made of glass and comprises upper inlet tubes 2 and 3 angled towards each other so as to allow the liquid streams issuing therefrom to flow to the base of the chamber. About half way down the chamber is a liquid outlet 4 and contained within the bottom of the chamber is a magnetic follower 5. In use of the chamber, the sample/standard solution and buffer solution are supplied respectively through inlettubes 2 and 3 and flow to the base of the chamber before being mixed by use of the magnetic follower 5. The solution mixture exits through outlet 4 at a rate equal to the total rate at which the sample and buffer solutions are supplied through inlets 2 and 3.

Fig. 2 illustrates one embodiment of electrode cell 6 and will be seen to comprise a first chamber 7, a second chamber 8, and a third chamber 9. The first chamber 7 has a liquid inlet 10 (which in use is connected to the outlet 4 of mixing chamber 1) and the third chamber 9 has an outlet 11. Chambers 7 and 8 are connected by a connecting tube 12 and a similar tube 13 .connects chambers 8 and 9. Additionally, chamber 7 is provided with an air vent 14.

Within chamber 7 is ar. ionRselactive 'ecrr,3 15. A reference electrode 16 is providec in cramner 8 and an automatic temperature compensation probe 17 is provided in chamber 9.

Fig. 3 is a diagrammatic illustration of an arrangement for carrying out the method of the invention. In the illustrated arrangement, a multi-way selector valve 18 is connected to a conduit 19 from which the product to be analysed is sampled and is also connected to reservoirs 20-22 containing calibration solutions. At any one time, liquid from conduit 19 or any one of reservoirs 20-22 is supplied via valve 18 to the inlet 2 of mixing chamber 1. The inlet 3 of the chamber 1 is connected to a reservoir 23 of buffer solution. The solutions from conduit 19 or reservoirs 20-22 may be supplied by a peristaltic or piston pump (not shown). From chamber 1, the mixed solutions pass to chamber 7 of electrode cell 6 and successively through chambers 7, 8 and 9 before passing to waste through outlet 11.

If measurements are being made on carbonated drinks the high carbon dioxide content gives rise to unstable potential readings when monitoring ethanol by FICT. It is therefore necessary to degas the carbonated drink prior to its entry into the mixing chamber. This may be done by passing the drin through an untrasonic degasser (Fig. 9) without interrupting the continuous flow.

Briefly, the carbonated drink is pumped through a glass coil 26 immersed in an ultrasonic bath 27.

The liberated carbon dioxide is released to the atmosphere via a vent 28 after passing through a watercooled condenser 29 to prevent loss of ethanol vapour. The degassed drink 30 is collected in a cylindrical glass vessel 31 from which it is supplied by a peristaltic or piston pump to the mixing chamber (Fig. 1). The copper cooling coil 32 prevents a rise in bath temperature.

The ion-selective electrode 15, reference electrode 16 and automatic temperature compensation probe 17 are connected to a computer control system 24 associated with a printer 25. When using two ion-selective electrodes the leads from the electrodes are connected to the computer controlled system 24 via a differential amplifier (not shown in Fig. 3).

Computer control system 24 also controls selector valve 18 which may be switched to allow either the sample or one of the calibration solutions from reservoirs 20-22 to pass to mixing chamber 1.

Computer 24 is programmed to take and process measurements of the cell potential at suitable intervals (e.g. every thirty seconds) so that printer 25 may produce a graph of cell potential against time. From this information, the computer is able to calculate the equilibrium potential. More particularly, this equilibrium potential is taken to be the average of the cell potential over a certain period of time (e.g. over the period 7-10 minutes) during which the potential is most stable.

Figs. 5-8 illustrate results obtained using the apparatus shown in Fig. 3 for the determination of water in ethanol-water mixtures. Three calibration solutions were used each having a different concentration of water in ethanol as follows; Calibration solution 1 = 0.00 +,/- 0.01E m water Calibration solution 2 = 0.50 +/- 0.01% m/m water Calibration solution 3 = 1.00 +/- 0.019 m/m water The predetermined strength of sample solution was 0.90 +/- 0.01% m/m water. The electrode systems used were as follows: Low impedance pH glass electrode conditioned for one hour in the buffer solution (cited earlier); silver-silver chloride reference element with saturated lithium chloride in dry ethanol as filling solution.

The flow rate through the system was 9cc/min.

In this example, the sample solution is known to have a concentration of water between that of the second and third calibration solutions. Therefore, in operation of the apparatus, measurements are firstly effected on the first calibration solution, for E ch the data are shown in Fig. 4.

Subsequently, measurements are effected on the second calibration solution, for which the data are shown in Fig. 5 and then on the sample solution (data shown in Fig. 6). Finally, measurement is effected on the third calibration solution (data shown in Fig. 7).

From the equilibrium potentials obtained for the calibration solutions over the period 7-10 minutes, a calibration graph may be plotted (Fig. 8) from which the concentation in the sample solution may be determined.

Claims (22)

CLAY'US

1. Apparatus for measuring a component in a solution containing at least one organic solvent, the apparatus comprising a mixing chamber having two inlets and an outlet, an electrode cell connected to said chamber and having an ion-selective electrode, a reference electrode, an automatic temperature compensation probe and means for measuring the cell potential generated between said ion-selective electrode and said reference electrode.

2. Apparatus as claimed in claim 1 wherein the inlets of the mixing chamber are provided in the upper region thereof and are angled downwardly towards each other.

3. Apparatus as claimed in claims 1 or 2 wherein the cell comprises first, second and third chambers through which liquid from the mixing chamber passes sequentially before leaving the cell.

4. Apparatus as claimed in claim 3 wherein the connections between communicating the chambers of the mixing cell are arranged to cause liquid to pass upwardly and downwardly on it's passage through the chambers of the cell.

5. Apparatus as claimed in claim 4 wherein the connections between the first and second chambers and between the second and third chambers, each comprise a tube inclined upwardly in the direction of flow through the cell.

6. Apparatus as claimed in claims 4 or 5 wherein the ion selective electrode is in the first chamber of cell, the reference electrode is in the second chamber, and the temperature compensation probe in the third chamber.

7. Apparatus as claimed in any one of claims 4 to 6 wherein the cell is provided with an air vent.

8. Apparatus as claimed in any one of claims 1 to 7 provided witn an ultrasonic deg ssiny unit upstream of the mixing chamber.

9. Apparatus as claimed in any one of claims 1 to 8 provided with a multi-way selector which may selectively provide communication between an inlet of the mixing chamber and any one of a plurality of vessels for holding calibration solutions or a source of the solution to be analysed.

10. A . method of measuring a component in -a solution comprised of at least one organic solvent, the method comprising; (a) passing continuously a sample of the solution at a predetermined rate to a mixing chamber; (b) passing continuously to the mixing chamber at a predetermined rate a buffer solution comprising a proCterrined amount of an indicator ion; (c) effecting homogeneous mixing of the buffer solution and the sample whilst continuously withdrawing the resulting solution mixture from the mixing chamber through a cell comprising an electrode selective for said indicator ion, a reference electrode and an automatic temperature compensation probe;; (d) continuously or continually measuring the cell potential and (e) comparing said measurement with calibration data for the system to determine the concentration of said component.

11. A method as claimed in claim 10 wherein the mixing of the sample and of the buffer solution is effected by passing these solutions into a mixing chamber through respective tubes downwardly angled toward each other.

12. A method as claimed in claim 10 or 11 wherein the cell comprises first, second and third chambers through liquid from the mixing cell passes sequentially.

13. A method as claimed in claim 12 wherein the mixture of sample and buffer solution flows upwardly in passing from each chamber of the mixing cell to the next chamber.

14. A method as claimed in any one of claims 10 to 13 wherein the calibration data is obtained using a plurality of calibration solutions containing the same components as the sample but different concentrations of the component being measured, each of said calibration solutions being individually mixed with the buffer solution and the mixture being subject to a cell potential in said cell.

15. A method as claimed in claim 14 wherein at least one of said calibration solutions has a concentration of the component being measured greater than that in the sample and at least one has a lesser concentration of said component than the sample, and the measurement of the cell potential on the sample is effected between the measurements of cell potential on the calibration solution of the next most higher and the next most lower concentration of the component being measured.

16. A method as claimed in claim 15 wherein cell potential measurements on the calibration solutions and the sample are carried out repeatedly in a predetermined cycle.

17. A method as claimed in any one of claims 10 to 16 wherein the component being measured is trace water in an organic liquid.

18. A method as claimed in claim 17 wherein the determination is by PICT.

19. A method as claimed in any one of claims 10 to 16 wherein the component being measured is ethanol in an alcholic beverage.

20. A method as claimed in 19 wherein the determination is by FICT.

21. A method as claimed in any one of claims 10 to 16 wherein the buffer solution serves in addition to providing the indicator ion, to increase conductance, inprove the sensitivity of potential response, and/or increase Renold numbers.

22. A method as claimed in any one of claims 10 to 21 wherein the solution to be analysed is degassed before admixture with the buffer solution.