AU610378B2 - Multi-cell potentiometric sensing device - Google Patents

Description

\PPl'CAT'ON ACCEPTED AND AMENDMENTS

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1 610378 COMMONWEALTH OF AUSTRALIA Patent Act 1952 r n M P Ot F T E S P ECIFICATION

(ORIGINAL)

Class Int. Class Application Number Lodged PI7728 13 April 198

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8 This document contn tile amendments made w Section49and is correct for printing.

o 0 00 0 e 0000 0 o« 0 0 0 0 0 00 00 o o 0 04 09 0 0 6 4 0 4 4 C Complete Specification Lodged Accepted Published Priority: Related Art Name of Applicant Address of Applicant Actual Inventors Address for Service UNISEARCH LIMITED 221-227 Anzac Parade, Kensington, New South Wales, Commonwealth of Australia Peter W. Alexander and D. Brynn Hibbert F.B. RICE CO., Patent Attorneys, 28A Montague Street, BALMAIN. 2041.

Complete Specification for the invention entitled: "Multi-Cell Potentiometric Sensing Device" The following statement is a full description of this invention including the best method of performing it known to Us:- 11/04/8 L S i /ned Sta/ MANAGER, INTELLECTIUAL 'KUezKI- S i g n e d S t a t u s MICHAEL REID Declarant's Nam EL I F. B. RICE CO PATET ATTORNEYS This form is suitable for any type of Patent Application. No legalisation required.

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nr 2 This invention relates to an apparatus and method for the analysis of ionic species in aqueous solution using ion selective electrodes and more particularly an apparatus and method that utilizes an ion selective electrode or other potentiometric sensing electrodes, such as coated wire-electrodes, metallic electrodes, and field-effect transistor electrodes, disposed in a flow cell disposed in a flow cell.

Electrochemical analysers that utilize ion selective electrodes, in combination with a reference electrode are well known in the art. Such analysers are normally used where the potential developed by the electrodes is Nernstian over the range of concentration of species in solution to be analysed. It therefore follows that as a e* 15 general rule, such analysers cannot be used when a concentration of ionic species of interest is so low as to be outside the Nernstian response range.

99 In order to improve the sensitivity of such S o0 S0o 2 electrochemical analysers, it has been disclosed by K. Suzuki et al., Anal. Lett. 20, 1773-1779 (1987) that a number of cells, each containing a reference and an ion selective electrode may be connected in series to obtain a potential equal to the number of cells used multiplied by the potential for an individual cell. The advantage of 25 this arrangement is that the sensitivity to the ionic species of interest is greatly increased.

However, in the arrangement disclosed, each reference and ion selective electrode pair were maintained in separate containers, the electrodes being appropriately connected so that they were all in series. A maximum of three series connected cells were disclosed wherein it was shown that for a single cell, the slope was 58 mV per decade, whilst for two and three cells, the slope was found to be 116 mV and 174 mV respectively.when potassium ions in a concentration of 1 x 10' 5 to 1 x 10- 1 were i

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-3o IlS 000 00 09 0O o 0000 O @0o 00 0 0 II 00 4 0 Pi 1000 00I 4 0 used.

In an earlier paper by R. Stepak, Fresenius Z. Anal.

Chem., 315, 629-630 (1983) it was shown that amplification of the potential developed in series connected cells could be used to determine potentiometric end point in the analysis of dilute solution. However, clearly the solution in each cell had to be treated in precisely the same manner. That is, the amount of titrant added to each cell would have to be the same and relatively large volumes of sample would be required. On this basis and the fact that a number of automatic burettes corresponding to the number of cells used would be required, it is clear that this method as disclosed is of no practical value.

The present inventors have recognized these difficulties inherent in the prior art apparatus and methods and have therefore sought an improved method and apparatus that whilst making use of the improved sensitivity obtained by connecting cells in series would be a method that could be readily used in the analytical art.

Surprisingly, the present inventors have found that through the use of at least two flow cells, each having a reference and an ion selective electrode in the flow cell path connected in series, each reference and ion selective 25 electrode can be substantially electrically isolated in the flow system such that the potential developed is substantially the multiple of the number of electrode pairs by the potential for an individual cell.

Accordingly, the present invention consists in a method for the analysis of an ionic species in a solution comprising:passing a sample of the solution through at least two flow cells, each flow cell including an electrode pair comprising a reference and an ion selective electrode to the ionic species, the sensing surfaces of the electrodes 1

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-I _-irr ~lr PILIIIC-UX1 I1Yt--il- -I 0, 4 being in contact with said solution, said electrodes being connected electrically in series; electrically isolating the solution in a flow cell pair from the solution in at least one other flow cell pair, in a manner such that the potential developed by the electrode pairs electrically connected in series is substantially equal to the number of reference and ion selective electrode pairs multiplied by the potential developed by an individual electrode pair; measuring the potential developed by the ,e electrode pairs in series; and c determining the concentration of said ionic I species by relating the measured potential to that obtained when suitable standard solutions of the ionic species are similarly measured.

SIn another aspect, the present invention further Sconsists in an apparatus for use in the determination of the concentration of an ionic species in a solution, comprising at least two flow cells, each flow cell having an inlet and an outlet for the solution and including an C electrode pair comprising a reference electrode and an ion cccc

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cc selective electrode to the ionic species, disposed between IC the inlet and the outlet, the sensing surfaces of the c r electrode pair being disposed in a passage in a flow cell in a manner such that they will be in contact with the CC(Csolution flowing therethrough, means to electrically C Cserially connect the electrode pairs, means to electrically isolate the solution in a flow cell from the at least one other flow cell such that the potential developed by the electrode pairs, is substantially equal i to the number of electrode pair13 multiplied by the potential developed by an individual electrode pair and tube means to permit the solution to be passed through the flow cells.

The flow cells mav comprise any such apparatus known 4 .4.I~1 P i. i.

in the art that is capable of mounting the reference and the ion selective electrode in the flow passage thereof to an extent sufficient to enable solutions flowing through the passage to contact the sensing surfaces of the electrode. Such flow cells would be connected by appropriate tubing.

Alternatively, a plurality of such flow cells may be formed in a single block with the interconnection being accomplished by passages formed in the block. In either case, preferably the diameter of the passages or tubing does not exceed two millimetres.

To satisfactorily electrically isolate pairs of electrodes from each other, two means may be employed.

In a first embodiment, the means comprises gas 15 bubbles, preferably air, introduced into the solution e flowing through the cells in a manner such that an air U o bubble is present in an inlet stream and an outlet stream o o of each of the cells, with the electrode pair disposed 0 Po SO between the air bubbles in each cell. In this embodiment, by segmenting the solution in a flow cell, the air bubbles provide a high resistance path thereby effectively ooO electrically isolating the pairs of electrodes from each g Sother.

In a second embodiment, the inventors have most S 25 surprisingly found that by limiting the diameter of the interconnecting tubing or passages between the cells, a sufficiently high resistance is obtained to substantially electrically isolate the cells, To achieve this, the 41,14 inventors have found that tubing having a diameter of 0.5mm is acceptable.

A variety of electrode combinations may be used as appropriate. Additionally, it is within the scope of the invention to include a plurality of electrode pairs in each cell to allow for the determination of a plurality of ionic species.

Z4 6 The ionic species that may be analysed by the inventive method and apparatus preferably comprise anions, cations, reducing agents or oxidising agents, either inorganic or organic, most preferably inorganic anions.

Such anionic species include chloride, hypochlorite and hydroxide. Note that since hydroxide may be measured, pH may also be determined.

To further describe the invention, reference will now be made to a number of illustrative experiments in figures showing two embodiments of the invention in which:- Fig. 1 is a schematic diagram showing the arrangement of apparatus for a continuous analysis system based on the inventive method and apparatus; Fig. 2 is a schematic diagram showing a three cell 15 arrangement according to one embodiment of the invention; Fig. 3 is a schematic diagram showing the embodiment 4 of Fig. 2 in the wash mode; Fig. 4 is a schematic diagram showing a three cell o 00 arrangement according to a second embodiment of the o invention; Fig. 5 is a continuous-flow recording of change in electrode potential versus time for the analysis of 4 chloride samples using the arrangement of Fig. 2; Fig. 6 is a graph of change in electrode potential 64*1' 25 versus log concentration; Fig. 7 is a continuous-flow recording of change in electrode potential versus time for the replicate analysis of 2 x 10- 4 M chloride solution; and 'Fig. 8 is a continuous-flow recording of change in electrode potential versus time for the replicate analysis of 5 x 10- 6 M chloride solution.

In Fig. 1, it is shown that sample or wash solution is pumped through a mixing coil 12 by pump 11, through at least two flow cells 13 of the invention and then to waste 19. Air bubbles are introduced into the solution at

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llj 7 an appropriate rate. The pump 11 is a ten channel peristalsic pump obtained from Watson-Marlow Ltd. fitted with tygon tubes of 1.0mm ID. The maximum flow rate through each channel is 1.2mL min.'.

The potential developed by the cells is determined using an Activon digital pH/mV meter 14 supplied with an analogue voltage output connected to a Houston Omniscribe strip chart recorder (not shown). The analogue output is also connected to an amplifier 15 with an offset controller and interfaced to an Apple IIE microcomputer 16 via an 8-bit analogue-to-digital converter. Data is acquired in real time with the microcomputer and stored on a floppy disk. Later the data may be either printed on a dotmatrix printer 18 or on an X-Y plotter 17.

In Fig. 2, there is shown the cell arrangement used in one embodiment of the invention. Arrows indicate direction of flow whilst 26, 27, 28 represent the reference electrodes and 23, 24, 25 the ion selective SI, .T electrodes. As shown, air bubbles 29 are present in the inlet and outlet stream 36 of each cell 20, 21, 22 effectively segmenting the solution stream. Solution is S'l: (introduced into cell 20 via tubing 30 at inlet end 31.

t Solution exits cell 22 via tubing 30 at outlet end 32.

t r Reference electrode 26 and indicator electrode 23 are 25 in cell Reference electrode 27 and indicator electrode 24 are in cell 21.

Reference electrode 28 and indicator electrode 25 are in cell. 22.

Reference electrode 26 is connected by lead 34 to indicator electrode 24. I Reference electrode 27 is connected by lead 35 to indicator electrode Thus, the electrodes are effectively series connected.

The reference and indicator electrodes used were 4 -8silver-silver chloride electrodes constructed from silver wire of 0.5mm diameter immersed in 0.5M ferric chloride solution according to the method of Bates et al.

"Measurement of Electrode Potential and pH" Wiley Sons, New York 1965, p. 279.

Electrodes 23, 28 were connected by leads 33 to the mV meter after fitting the flow cells described in P.W.

Alexander et al. Anal. Lett., 17, 309-320 (1984) using Omnifit connectors threaded to screw into the flow cell to prevent leakage.

The reference electrodes were fitted into the flow cells with a plug of agar prepared from 1M KNO 3 solution, to isolate the electrode from the flowing solution.

As shown in the figure, each pair of indicator and (cc 15 reference electrodes in a cell are at right angles to each other, preferably as close as possible, but no further than 1cm apart.

o The flow cells 20, 21, 22 were constructed from o 0° perspex blocks 5 x 3 x icm, with outlets and inlets being connected by tubing 30 20cm in length, of 0.5mm internal diameter.

O* In Fig. 3, there is shown the arrangement of Fig. 2 with the apparatus in the wash mode. In this mode, a 4 cr single air bubble 29 at the inlet 31 of the first cell 25 and a single air bubble 29 at the outlet of the last cell rit are used to isolate sample solution 36 from wash solution 37.

In Fig. 4, the arrangement of the apparatus is the c, same as for Fig. 2 except that capillary tubing 38 is used to connect outlets and inlets of the flow cells 20, 21, 22 rather than using air bubbles to isolate each electrode pair.

Using the apparatus shown in Fig. 2, a number of experiments were conducted in which all the reagents were AR grade and water was Milli Q purified. The chloride 9 sample solutions were prepared by serial dilution of a 0.1MKC1 stock solution to yield concentrations in the range of 1 x 10-6 to 2 x 10- 2 M. The wash/blank stream used was 0.1MKNO 3 The wash stream was initially pumped through the system at a rate of 0.64mL min 1 after a steady baseline potential was observed, with the chart recorder on 100mV full scale, chloride sample solutions were pumped into the system with a blank reagent wash between each sample. In between each sample and wash solution, an air segment was pumped into the system to separate the sample from the reagent wash solution. Sampling and wash time intervals were usually twenty seconds with five second air segment between each sampling and wash period.

15 In Fig. 5 there is shown the peaks obtained with the a three-cell arrangement of Fig. 2 for chloride samples in o 00 the concentration range of 1 x 10 5 to 2 x 10 4 M. It 0004 oo o is noted that a distinct shoulder was observed for each a 00 0. 0cell and a peak occurred at the point where the blank wash 20 solution reached the first flow cell. Some noise was observable when air bubbles passed the electrodes but this oL was insufficient to cause serious peak distortion.

o Similar peaks were observed for two and one cell 0 V modes of operation.

,t 25 In Fig. 6 there is shown the calibration curves observed when the peak heights are plotted against the log of the chloride concentrations in the range 1 x 10-6 to 2 x 10- 2 M for a one, two and the three cell arrangement of Fig. 2. The slopes for the linear portions of these plots were found to be 54, 106 and 147mV respectively, which are in close agreement with the slopes expected from i the sum of the Nerstian cell constants for each cell in series in the flow system. It is therefore evident that the sensitivity of the method, using the inventive apparatus, to chloride ions is improved by the use of air i -ibubbles to segment the solution and therefore electrically isolate the electrode pairs. The improvement obtained is equivalent to that obtained by Stepak.

The effect of the sample front after the initial air bubble reached the first electrode was that the observed electrode potential initially increased to a steady state value equivalent to the single cell potential. When, however, the air segment passed through the second cell, a second air bubble now isolated the cells from each other.

Hence the observed potential adds on and a second shoulder was observed. This effect is repeated for each cell connected in series.

It was found that only a single air bubble was required between each sample and the following wash e 15 solution. If, however, the cells were too far apart, then a. the observed potential changes were lower than Nernstian.

o o° The effects of the flow velocity, tubing diameter, and 00da Mo 0 tubing length between cells affect the response. In the 0 0 0 '00 present design, it was found that approximate Nernstian 00 O o 20 response could be achieved with a flow rate of 0.6mL min- 1 and tubing of 20cm length and 0.5mm internal g0*0 diameter. An increase in length to 40cm caused a marked 00 decrease in the potential change observed for the second 0s 1 and third cells connected.

25 The above desired factors also control the rate of response of the cell system. The flow rate of 0.6mL min- 1 used gave a response time of less than one l second for the potential reading to reach each shoulder.

The response was therefore sufficiently rapi'd for a c 30 shoulder to be observed for each cell in series, using a twenty second sampling time with five second for air segmentation between each sample and wash; the total peak width depends on the total sampling, wash and segmentation times. The response time for the cell design used was approximately ninety seconds as shown in Figs. 5 and 6.

t L. 11 The response time can be improved by using faster flow rates, but ultimately at the expense of poorer sensitivity. At a flow rate greater than approximately lmL min 1 the shoulders disappeared and the total peak height decreased due to the shorter time of exposure of the electrodes to the sample solution.

In Figs. 7 and 8, the precision of the air segmentation method was tested using chloride concentrations of 2 x 10-4M and 5 x 10- 6

M

respectively. Considerable noise was observed in the latter case, as shown in Fig. 8, where the amplifier gain used to acquire the data was increased. However, the peak shapes were reproducible and the peak heights and standard deviations were determined as i.s shown in Table 1. The 15 results gave relative standard deviations of 1.5% and 6.9% respectively for the two concentrations tested. The limit q_ ,of detection calculated in Table 1 from 2 x O- for the low .o@a concentration was 7 x 10- 7 M. This is approximately ten times improved on the recognized limit of detection of S 20 chloride using the silver-silver chloride electrode.

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-12 TABLE 1 Precision of Replicate Peak Measurements for 3-cell System Peak Heights (mV) 2 x 10-4M 5 x 10-6M 178 15.5 176 14.3 178 14.3 174 13.7 174 14.9 172 12.7 172 t 15 0 P 0O' 0

CBD

Mean 175 14.2 S.D. 2.5 0.98 R.S.D. 1.5 6.9 o Although not wishing to be bound by theory, the present inventors believe that whilst they have shown that S 20 cell potentials are essentially additive and therefore it would be expected that the greater the number of cells in series the greater the sensitivity achieved, and it is 04 4 believed that the electrical resistance between cells 4 1 4 determines the overall slope of the calibration curve such o~o; 25 as shown in Fig. 6, nevertheless it is likely that no advantage is to be gained when more than about five to six cells are used.

4 However, in view of the fact that a close to theoretical value of 3 x 59mV per decade has been observed for three cells, it is clear that the present invention offers a substantial increase in analytical sensitivity.

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Claims (15)

1. A method for the analysis of an ionic species in a solution comprising: passing a sample of the solution through at least two flow cells, each flow cell havingj an inlet and an outlet for the solution and including an electrode pair comprising a reference and an ion selective electrode to the ionic species disposed between the inlet and outlet, the sensing surfaces of the electrodes being in con-tact with said solution, the electrode pairs being connected Q cooelectrically in series; electrically isolating the solution in a flow cell pair from the solution in the at least one other fl~ow cell pair in a manner such that the potential developed by C C the electrode pairs electrically connected in series is Csubstantially equal to the number of reference and ion selective electrode pairs multiplied by the potential developed by an individual electrode pair; measuring the potential developed by the a a 0 a00electrode pairs in series; and o 0 0000 determining the concentration of said ionic species by relating the measured potential to that :00 obtained when suitable standard solutions of the ionic species are similarly measured.

2. A method as in claim 1 wherein the solution in the flow cells is electrically isolated by the introduction of gas bubbles into the solution in a manner such that a bubble is present in the solution flowing into the inlet Liand a gas bubble is present in the solution flowing out of the outlet of each of the flow cells.

3. A method as in claim 2, wherein a wash solution is passed through each flow cell, by introducing a gas bubble into the solution under analysis as it flows into the inlet of a first flow cell followed by the wash solution.

4. A method as in claim 2 or claim 3, wherein the gas is ir.

The following statement is a full description of this invention including the best method of performing it known to Us:- sOO6~-70 11''8 r:r I 14 A method as in claim 1, wherein the solution is passed through tubing into the inlet of one flow cell, through tubing out of the outlet of the flow cell, and into the inlet of the at least one other flow cell through tubing out of the outlet of said other flow cell, said tubing having an internal diameter to effectively electrically isolate the solution in each flow cell.

6. A method as in any one of claims 1 to 5, wherein the solution is passed through the flow cells by pumping.

7. An apparatus for use in the determination of the concentration of an ionic species in a solution, comprising at least two flow cells, each flow cell having €c an inlet and an outlet for the solution and including an S( electrode pair comprising a reference electrode and an ion selective electrode to the ionic species, disposed between c the inlet and the outlet, the sensing surfaces of the a C C 4 Celectrode pair being disposed in a passage in a flow cell in a manner such that they will be in contact with the solution flowing therethrough, means to electrically Sserially connect the electrode pairs, means to 0 0 C'0oo electrically isolate the solution in a flow cell from the 00 0 at least one other flow cell such that the potential oO0o developed by the electrode pairs is substantially equal to the number of electrode pairs multiplied by the potential developed by an individual electrode pair and tube means S C to permit the solution to be passed through the flow cells.

8. An apparatus as in claim 7, wherein the means for Cccr electrically isolating the solution in the flow cells comprises gas bubbles disposed in the solution in a manner such that a bubble is present in the solution flowing into the inlet and a gas bubble is present in the solution flowing out of the outlet of each of the flow cells.

9. An apparatus as in claim 8, wherein the gas is air.

C-i _U 0, i 15 An apparatus as in any one of claims 7 to 9, wherein the tube means between the flow cells is no more than in length.

11. An apparatus as in any one of claims 7 to 10, wherein the reference and ion selective electrodes are disposed in the flow cells perpendicularly to each other.

12. An apparatus as in claim 7, wherein the tube m-ans has an internal diameter such that the electrode pairs are effectively electrically isolated.

13. An apparatus as in any one of claims 7 to 12, wherein the internal diameter of the tube means is

14. An apparatus as in any one of claims 7 to 13 wherein the reference and ion selective electrodes are no more than 1cm apart. '4 s

15. An apparatus as in any one of claims 7 to 14 including three flow cells. a 84 0o o DATED this 11 day of April 1989 Q4 04 00 0 o0 UNISEARCH LIMITED Patent Attorneys for the Applicant: o (R C F.B. RICE CO.