WO1992005451A1

WO1992005451A1 - A method for measuring the ion concentration of aqueous samples and an arrangement for carrying out the method

Info

Publication number: WO1992005451A1
Application number: PCT/SE1991/000637
Authority: WO
Inventors: Torbjörn ANFÄLT
Original assignee: Tecator Ab
Priority date: 1990-09-24
Filing date: 1991-09-24
Publication date: 1992-04-02
Also published as: SE467074B; SE9003014D0; GB2263780B; GB9305797D0; GB2263780A; SE9003014L

Abstract

A method of determining the concentration of a given ion in an aqueous sample with the aid of an ion-selective electrode (4) and a reference electrode (5) placed in a measuring cell (3) by delivering the sample to the cell in an air-segmented form; effecting measurement with the electrodes; flushing the sample from the measuring cell subsequent to said measuring operation and replacing said sample with a conditioning solution (7) of known ion concentration; and immediately standardizing the electrode system, by measuring with the electrodes in the conditioning solution. A measuring arrangement including a measuring cell (3) with an ion-selective electrode (4) and a reference electrode (5) and constructed so that air bubbles in the sample delivered thereto will not come into contact with the active surfaces of the electrodes; intermittently working pumps (1, 2) for delivering air segmented sample solution and a conditioning solution (7) to the cell (3) and for evacuating the cell; and a sample storage loop (11) disposed between an inlet (19) to the cell and a tube (14) delivering the conditioning solution (7).

Description

A Method for Measuring the Ion Concentration of Aqueous Samples and an Arrangement for Carrying Out the Method

The present invention relates to a method of measuring the ion concentration of aqueous samples by metering the samples into a flow system in which, with a com¬ bined conditioning solution and standardizing solution, the samples expose an ion-selective electrode and a reference electrode, and also relates to apparatus for automatically determining the ion concentration poten- tiometrically.

Automatic analyzing systems which are based on pumping the sample, treating the sample with reagent and de¬ tecting the sample in a flow system have long been known. Skegg's classic "AutoAnalyzer" is best described in the U.S. Patent Specification Nos. 2,797,149 and 2,879,141. Sample and reagent are pumped by means of a peristaltic pump through mixing and reaction coils in which the chemical reaction is able to take place, whereafter the mixture is conducted to a flow-through detector. Because the samples are air segmented at precisely the time at which they are introduced into the system, it is possible to prevent the samples from becoming dispersed one in the other, i.e. to prevent said samples from contaminating one another. This enables the analysis frequency to be increased. Ruzicka and Hansen are the authors of a number of patent speci¬ fications and scientific articles in which the so- called Flow Injection Analysis (FIA) technique is found described. The sample is injected in the form of a plug into a carrier flow, or directly into the reagent. Mutual contamination between the samples is prevented, by selecting the flow conditions such as to obtain a laminar flow in the system. Compared with the air segmented systems, the analysis frequency can be increased still further, while enabling the sample volume and reagent volume to be decreased at the same time.

The conditions fundamental to the FIA-technique are described in Swedish Patent Specification No. 7701696-2. The Swedish Patent Specification No. 7610221-9 describes the use of ion-selective electrodes in combination with FIA. The sample is injected into a carrier stream which is caused to meet the electrode surface tangentially. The reference electrode is placed in a container in which the liquid level is held con¬ stant, at a level which just reaches the first edge of the measuring electrode, so as to enable electric contact to be established. This arrangement has many advantages:

The electrode surface is conditioned by the carrier flow. - Only a small sample volume is required for analysis (10-200 μl) .

- The response time is short; the result can be read after only some few seconds.

The arrangement also has a number of disadvantages:

- Measurements are made under dynamic conditions and over very short time periods.

Flushing of the electrode surface varies with res- pect to effectiveness (flow dependent) .

- In order to maintain laminar flow, it is necessary for the internal diameter in the various parts of the system to be small, which renders the system sensitive to particles present in the sample solu- tion.

The injector is mechanically sensitive and includes moveable parts which jeopardize long-term use.

A number of these drawbacks can be overcome by using several pumps in a FIA-system. For example, described in U.S. Patent Specification No. 4,597,298 (DK-B

160 268) is a method of introducing samples into a FIA- system with the aid of two pumps which are controlled timewise for taking-up a sample and for introducing the sample into the system. This enables the injector to be omitted as a system component and also enables larger sample quantities to be injected.

Consequently, it would appear at first sight that a combination of the inventions set forth in SE 7610221-9, SE 7701696-2 and U.S. 4,597,298 would solve those problems which occur when wishing to mea¬ sure the ion concentration of sample solutions auto¬ matically and reliably in operation. Such is not the case, however. The flow pattern of the electrode system according to SE 7610221-9 is disturbed by particles.

The active component of the electrode membrane is often an ion-exchanger and is quickly depleted by the carrier flow. Contamination between samples has been observed. Temperature variations have a significant effect on the dynamic measuring principle and hysteresis effects have been observed.

US-A-4,797,191 and DE-A1-30 33 680 describe methods of standardizing ion measuring systems by alternatingly delivering sample and standard solutions. The elec¬ trodes are disposed in thin flow conduits prone to clogg by particles present in the samples. The sample and standard solutions are circulated in closed sys¬ tems.

The purpose of the present invention is to provide a new method for mesuring the concentration of one or more ions in an aqueous sample and to provide a new arrangement for carrying out this method. By the new method and arrangement the problems of the known tech- nique should be eliminated. Especially clogging and interferences by particles present in the samples or carrier solutions should be avoided.

This purpose is accomplished by a novel method of determining the concentration of a given ion in an aqueous sample by measuring the sample solution and a standard solution with an ion selective electrode and a reference electrode in a measuring cell open to the en¬ vironment, the cell being integrated in a flow system, the aqueous sample being delivered to the flow system so as to be air segemented, the measuring cell being emptied, whereafter the air segmented sample is delive¬ red to the evacuated cell in such a way that the air segments do not come into contact with the active surfaces of the electrodes, a conditioning solution being used as carrier solution to deliver the air segmented sample to the cell, said conditioning solu¬ tion also being used as flushing solution for the measuring cell and as standardizing solution for the measuring system.

The air segments introduced in the sample solution help to flush the solution from the delivering conduit to clean this and also to blow the particles present in the sample solution from the conduit walls and from the walls in the measuring cell. It is not necessary to maintain a laminar flow in the present method, nor to use narrow conduits.

US-A-4,798,803 describes a measuring cell integrated in a flow system. This cell is closed to the environment. It is constructed in such a way that air bubbles are not trapped on the walls of the cell and thus do not in a non-reproducible way interfere with the analytical accuracy. However, the air entering this system is obtained by operator misstakes and/or by liberation of the air dissolved in sample, reagent and carrier solu¬ tions. The amounts of air are small and accidental and pass at random through the cell. The large amounts of air intentionally introduced into the sample flow according to the present invention would not give a satisfying result in the closed cell of US-A-4,798,803.

The present invention also provides an arrangement for automatically delivering air segmented samples to a measuring cell containing an ion-selective electrode and a reference electrode for determining the ion con¬ centrations in said samples, said arrangement compri¬ sing an intermittently working pump for pumping a non air segmented sample liquid from a tube positioned in the measurement object, in through a tube to a first branch point; the measuring cell being an open cell and being connected to the first branch point via a tube; a sample storage loop which has the first branch point at one end and a second branch point at the other end thereof; a tube connected to the second branch point and to an intermittently working pump for evacuating the storage loop and the measuring cell to an outlet; a tube connected to an intermittently working pump for delivering conditioning solution to the second branch point; and a tube connected to an intermittently work¬ ing pump for evacuating liquid from the measuring cell and delivering said liquid to an outlet.

It has surprisingly been found possible to solve the following problems with the aid of the inventive ar¬ rangement and its method of operation: Contamination between the samples is totally negli¬ gible, since the measuring cell is emptied com¬ pletely prior to the introduction of a new sample. Particles present in the sample solution do not effect the measuring result or the operation of the system, not even when the system is operated con¬ tinuously over several weeks.

It has been possible to increase markedly the use¬ ful life of the electrode membranes, since the exposure principle does not lead to depletion of the active component.

Temperature variations do not effect the accuracy and precision of the measuring results. No hysteresis effects occur. - It has been possible to improve the accuracy of the analysis result by always following each measuring process with a standardizing process.

The invention will now be described in more detail with reference to the accompanying drawing (Figure 1) which illustrates the arrangement, and also with reference to Table 1 which briefly states the modus operandi of the arrangement. Practical working examples will also be described (Figures 2, 3 and 4).

Description of the Drawings

Figure 1 illustrates the inventive arrangement.

Figure 2 illustrates the response curves obtained as a function of time in the arrangement for the nitrate concentration of two sewage water samples.

Figure 3 illustrates the response curves obtained by the apparatus during operation in practice for about 5 hours for nitrate in sewage water.

Figure 4 illustrates a specific form of execution of the present arrangement.

Figure 1 illustrates the arrangement. This comprises a pump (2) which functions to pump samples from a tube (8) , placed in the measuring object, in through a tube (9) leading to a branch point (10) , a measuring cell (3) in which an ion-selective electrode (4) and a reference electrode (5) are placed, a sample storage loop (11) having at the ends thereof a respective branch (10, 12), a tube (17) connected to a pump (2) for evacuating the contents of the storage loop and the measuring cell to an outlet (13) , a tube (14) connected to a pump (1) for introducing standardizing solution (7) to the branch point (12) , a tube (19) which con¬ nects the branch point (10) with the measuring cell (3) , and a tube (15) for evacuating liquid in the measuring cell (3) connected to a pump (1) and further to outlet (16) .

The electrodes (4, 5) are connected to an amplifier (6) in an electronic measuring circuit which produces an output signal of low impedance and mounted in the measuring cell (3) is a magnetic rod (18) which enables the sample to be stirred when an external magnetic field is applied. Reference signs a, b, c and d indi¬ cate flow rates (expressed, e.g. in ml/min) .

Table 1 discloses the working sequence of the arrange¬ ment. Before starting the sequence the cell (3) is filled with liquid. Table 1. Schematic Description of the Modus Operandi of the Arrangement

Sequence 1 2 3 4 5 6 7 8 Pump (1) OFF ON ON OFF OFF OFF OFF ON Pump (2) ON OFF OFF OFF ON OFF ON OFF

1 = A sample is introduced into the storage helix

(11) . The liquid present in the measuring cell (3) is emptied therefrom, since od. The sample in the storage helix will be air-segmented since od.

2 = When pumping of standard (7) commences, the sample will be conducted from the storge helix into the measuring cell (3), until the level reaches the suction outlet to drain (16) . This enables a constant level to be maintained. The inlet tube (19) leading to the measuring cell with the elec¬ trodes (4, 5) is arranged so that the air segments which bubbles up in the cell do not come into contact with the sensitive surfaces of the elec¬ trodes. A measuring process is carried out. The solution is stirred or agitated in the cell by means of a stirrer (18) , during the measuring period. Contamination, i.e. non-desirable mixing of standard/sample during the moment of delivery, is reduced by the air segments, which block con¬ vection and diffusion between the segments.

3 = Pumping of standard (7) is continued until the sample has been flushed from the measuring cell. The liquid in the cell is now standard solution. A measuring process is carried out and the sample signal is compared with the standard signal.

4 = Conditioning state. The electrodes are conditioned by the standard solution.

5 = Preparation of the rest state of the system. The liquid present in the measuring cell (3) is evacu- ated therefrom. The membrane of the ion-selective electrode is depleted to a lesser extent when exposed to air, and the useful life of the mem¬ brane is increased.

6 = Rest state.

7 = Same as state 1. Sample is introduced into the storage helix (11) .

8 = Same as state 2. Sample is introduced into the measuring cell (3) , etc.

Steps 5 and 6 can be omitted from the working sequence, when the number of samples per unit of time is in- creased.

Example 1

The arrangement has been tested in practice for measur- ing the nitrate concentration of sewage water. Figure 2 shows the measured potential difference between a nitrate-selective electrode and a reference electrode as a function of the time during a measuring cycle. A 40 mg/litre nitrate solution was used as a conditioning solution. It is assumed that this nitrate solution has already been introduced into the measuring cell at the time of commencing the measuring cycle. The starting times of the various steps (1-10) have been marked in the Figure. Steps 5 and 6 have been omitted, although these steps may, of course, be carried out during the 40 minute-long waiting time. The samples are exchanged for a standard solution of known nitrate concentration when calibrating and checking the system. Evaluation is effected by calculating the deviation, i.e. peak level, exhibited by a sample in comparison with the condition- ing solution (40 mg/litre) .

Figure 3 illustrates the measured potential difference between a nitrate-selective electrode and a reference electrode as a function of time during continuous long- term operation. The sample is sewage water taken from a sewage purification plant. The waiting times between the peaks (40 in.) have not been recorded. Although wide variations in temperature occurred, it is possible to eliminate the effect of such differences by appro- priate construction of the system.

During a continuous run of one month, a maximum signal deviation of 9 mV for the system operated with a conditioning solution of 40 mg nitrate/litre was ob- served when comparing with a control solution of 10 mg nitrate/litre. This deviation is surprisingly small. The results set forth in Figures 2 and 3 have been obtained by measuring directly on the sample solution.

Example 2

Figure 4 shows an arrangement according to the inven¬ tion for measurement of the nitrate concentration in sewage water which has been used in long time opera- tion.

In the figure

1 is an analysis pump with a capacity of about 7 ml/min; 2 is a sample pump with a capacity of about 15 ml/min; 3 is measuring cell; 11 is a sample storage loop;

the sample storage loop 11 being wound round the measu- ring cell 3 in order to thermostat the contained li¬ quid;

9 and 14 are tubes with inner diameters of 3.2 mm; and 13, 15, 16, and 17 are tubes with inner diameters of 4.0 mm.

Due to the difference in diameter between the tubes 9 and 14, on one hand, and the tubes 15 and 17, on the other, the flow rates in the tubes 9 and 14 are lower than in the tubes 15 and 17.

The cell 3 containes an ion-selective nitrate electrode and a reference electrode (not shown) .

The ion-selective nitrate electrode

- is of a double reference type having a standard gel KCl/AgCl as internal electrolyte and an antibacterial solution as external electrolyte; - respond to non-complex bound nitrate ions within the range of 1 to 7*10^"^ M (62,000 to 0.43 mg nitrate/litre; 14,000 to 0.1 mg nitrate nitrogen/1itre) ;

- shows a linear measuring range for concentrations of non-complex bound nitrate ions that are greater or equal to 10"⁵ M (0.62 mg nitrate/litre; 0.15 mg nitrate nitrogen/litre) ;

- operates within a pH-range of 4 to 11 at concentra- tions of nitrate from 0.62 to 620 mg nitrate/litre and within a pH-range of 2 to 12 for higher concentrations of nitrate;

- responds to a concentration change within 10 seconds; and

- operates within a temperature range of 0 to 50°C.

A sampling is carried out on nitrate containing sewege water. The arrangement is filled with standardizing solution of 40 mg nitrate/litre prior to sampling. The ambient temperature is within the range of 0 to 50°C. The sampling time is l to 5 minutes.

The sample pump 2 pumps the nitrate containing sample from a sewage plant through a tube 8, via tube 9, and into the sample storage loop 11. The standardizing solution is simultaneosly pumped out from the measuring cell 3, the flow rate in tube 17 being higher than that in the tube 9. For the same reason the sample is air segmented in the sample storage loop 11. The standardi¬ zing solution passes through the sample storage loop 11, a branch point 12, tube 17 and tube 13 to outlet. When the sample storage loop 11 is completely filled with the air segmented sample the sample pump 2 is stopped and the analysis pump 1 is started in order to pump standardizing solution through the the tube 14, via the branch point 12, towards the sample storage loop 11. The sample contained in the sample storage loop 11 is thereby, via a branch point 10 and a tube 19, conveyed into the measuring cell 3. The level in the measuring cell 3 rises until it reaches the suction outlet, where the liquid via the tube 15 is pumped to drain 16 by the analysis pump 1. In the measuring cell 3 the sample is, whilst stirred, subjected to a easu- ring process by means of the ion-selectiv nitrate electrode and the reference electrode. Standardizing solution is continously pumped in so that the liquid in the measuring cell 3 eventually consists of standardi- zing solution. By then a new measuring process is carried out, the signal of which is compared to the one obtained from the sample. The analysis pump l is switc¬ hed off and the electrodes are conditioned by the stan¬ dardizing solution. After this conditioning sequence the arrangement is ready for a new sampling.

Stability at long time operation: E₀ = 1 mV/day.

By making a simple modification to the system, it is also possible to effect sample matrix modification and to carry out chemical reactions prior to measuring ion concentration with the electrode system. An example, in this regard, is the adjustment of pH and ion strength when measuring with fluoride-selective electrodes. In this case, pump 2 in Figure 1 is provided with an additional conduit to which a combined pH and ion buffer is delivered and which is connected with conduit (9) so as to obtain a constant ratio of sample/buffer in the storage helix (11) . Another example is the analysis of aluminium, where a pH-buffer and fluoride standard can be delivered to the sample in the afore- described manner. Measuring is then effected with fluoride-selective electrodes, which detect the degree of sequestration between aluminium ions and fluoride ions, whereafter the aluminium concentration can be calculated.

Claims

1. A method of determining the concentration of a given ion in an aqueous sample with the aid of an ion- selective electrode and a reference electrode placed in a measuring cell by delivering the aqueous sample to the measuring cell in an air-segmented form, such that the air segments do not come into contact with the active measuring surfaces of the electrodes; effecting measurement with the electrodes while stirring or agitating the sample in the measuring cell; flushing the sample from the measuring cell subsequent to said measuring operation and replacing said sample with a conditioning solution of known ion concentration; and immediately standardizing the electrode system, by measuring with the electrodes in the conditioning solution, whereafter the ion concentration of the sample can be established.

2. A method according to Claim 1 in which several ion-selective electrodes are disposed in the measuring cell for simultaneously measuring the concentration of several types of ions in the sample.

3. A method according to Claim 1 or 2, in which the the sample is mixed in constant proportion with at least one solution prior to introducing the sample into the measuring cell in order to enable an optimal detec¬ tion with the electrode system.

4. A method according to any one of the preceding Claims, in which the electrode system is standardized immediately prior to each measuring process on samples instead of immediately after, i.e. the sample delivery and conditioning solution delivery are reversed time- wise.

5. A method according to any one of the preceding Claims, in which delivering and evacuating sample solution and conditioning solution to and from the measuring cell are performed with intermittently wor- king pumps whose working sequences can be defined exactly with respect to time and repeated each time a sample analysis shall be made.

6. A method according to any one of the preceding Claims, in which sample solution is repeatedly taken from one and the same measurement object, so as to monitor continuously the ion concentration of said object.

7. A method according to Claim 6 being used to moni¬ tor the nitrate content i sewage water.

8. An arrangement for automatically delivering air segmented samples to a measuring cell (3) containing an ion-selective electrode (4) and a reference electrode (5) for determining the ion concentrations in said samples, said arrangement comprising an intermittently working pump (2) for pumping a non air segmented sample liquid from a tube (8) positioned in the measurement object, in through a tube (9) to a first branch point (10) ; the measuring cell (3) being an open cell and being connected to the first branch point (10) via a tube (19) ; a sample storage loop (11) which has the first branch point (10) at one end and a second branch point (12) at the other end thereof; a tube (17) con¬ nected to an intermittently working pump (2) for evacu¬ ating the storage loop (11) and the measuring cell (3) to an outlet (13); a tube (14) connected to an inter¬ mittently working pump (1) for delivering conditioning solution (7) to the second branch point (12) ; and a tube (15) connected to an intermittently working pump (1) for evacuating liquid from the measuring cell (3) and delivering said liquid to an outlet (16) .

9. An arrangement according to Claim 8, c h a r - a c t e r i z e d in that separate pumps are used for respectively delivering and evacuating sample solution and conditioning solution respectively.

10. An arrangement according to Claim 8 or 9, in which the same pump (1) is used for delivering conditioning solution (7) and for evacuating liquid from the measu¬ ring cell (3) .

11. An arrangement according to any of Claims 8-10, in which the same pump (2) is used for pumping the non air segmented sample solution and for evacuating the stor¬ age loop and the measuring cell (3) .

12. An arrangement according to Claim 11, in which the tube (8) delivering sample liquid from the measurement object is more narrow than the tube (17) evacuating the storage loop (11) so that a higher flow rate is achie¬ ved in said last mentioned tube (17) .

13. An arrangement according to any of Claims 8-12, in which an agitator or stirrer (18) is mounted in the measuring cell (3) .

14. An arrangement according to any of Claims 8-13 in which one or more solutions are mixed automatically with the sample prior to introducing the sample into the sample storage loop (11) .