GB2157438A - Ion concentration analysis - Google Patents

Abstract

Analysis of a series of samples is carried out in an cell 10 containing an ion-selective electrode 11. A known volume of base solution, to which a trace of the ion to be measured has previously been added, in run into the cell, and the electrode voltage (mV) or ion concentration (pX) measured. A small volume of standard solutions containing a known concentration of the ion is added to the base solution and the mV/pX measured. The same volume of sample solution is then added to the base solution from the same dispenser and the mV/pX again measured. The cell is drained without rinsing and refilled with base solution ready for the next analysis. Concentration is calculated from the three measurements. The order of standard and sample solution addition may be reversed and there may be more than one addition of either or both solutions. No blank determination is required. <IMAGE>

Description

SPECIFICATION lon concentration analysis This invention relates to ion concentration analysis using an ion-selective electrode and is directed to an improved incremental addition technique which considerably speeds and sim plifies such analysis.

By ion concentration is meant the total ionic concentration, free and chemically complexed, present in the sample solution.

In accordance with the present invention a method of making an analysis of ion concentration in a series of samples using an ion selective electrode in an electrode cell com prises the steps of: (a) Filling the cell with an approximately known volume of base solution doped with a trace of the ion to be determined, and taking an initial pX or mV reading or alternatively adjusting to a nominal pX value; (b) Adding from a dispenser a volume of liquid, small relative to the volume of the base solution, containing a known concentration of the ion to be measured (this solution is subsequently termed the standard solution); (c) Taking a first pX/mV reading; (d) Adding, from the same dispenser used in (b), the same volume of the first sample for analysis; (e) Taking a second pX or mV reading; (f) Draining the cell such as by suction without rinsing;; (g) Proceeding back to (a) above for the next sample analysis; (h) Calculating the results of the analysis.

Certain amplifications of the basic steps are given below: (i) The order of additions (b) and (d) can be exchanged for some applications. Also one or more further additions (b) and/or (d) can be made to eliminate the need to know the electrode slope.

(ii) If a pX measurement is made, either a temperature compensation probe can be included in the cell, or the temperature can be recorded manually and adjusted on the pX meter. If mV measurements are taken the temperature of the solution must be recorded.

(iii) There is no blank determination, this being corrected for inherently in the procedure.

(iv) The composition of the base solution, which has a typical volume of 20-100 ml, depends on the analyte and type of sample analysed, and is based on general chemical principles. However, it must always have the properties stated below: (A) it must be doped with a low concentration, which need be known only very approximately, of the analyte to be determined. This concentration must be on the linear response range of the electrode, be high enough to give a rapid response time, and give conve nient pX/mV changes when the additions are made, (B) it must have an ionic strength that is either not significally changed by addition of the sample/standard solution, or is similar to the ionic strength of the latter.

(C) it must contain chemical components which mask any interfering substances which may be present in the samples. In addition it is sometimes advantageous that the base material forms a chemical complex with the measured ion, releasing only a small constant fraction of the latter to be sensed by the electrode.

(v) After step (f) when the analysis of a sample is completed, there is no requirement to rinse the electrodes and cell before proceeding back to step (a). Any residual analyte is determined by addition step (b) and is automatically compensated for in the calculation.

(vi) The additions in (b) and (d) are made using plastic-tipped microlite pipettes. The volume delivered by the pipette is typically between 0.1 and 1% of the volume of the base solution, depending upon the application. If the identical pipette is used for the additions in (b) and (d), any inaccuracy in the nominal volume of the pipette is of no consequence.

(vii) The calculations required to produce an analytical result are fairly simple extensions of well-known "known-addition" formulae. However more complex numerical methods are required if further additions (b) and/or (d) are made. These require a micromputer.

(viii) The following parts of the procedure may, if desired, be microprocessor controlled using available technology; (1) the filling and draining of the electrode cell, (2) automatic storage of measurements, when these are stable to any predetermined criterion and (3) calculations of the analytical results.

Extension to the automatic additions of standard and sample is also feasible.

The advantages of the procedure, compared with conventional ion-selective electrode methodology, and microprocessor-controlled units to aid such conventional analysis, are given below: (a) There is no blank determination.

(b) No additional plastic/glass containers, other than the electrode cell, are required, any number of analysis being possible in the same cell.

(c) No cleaning/rinsing of the electrodes and cell is required before, after and during a series of sample analysis.

(d) Contamination problems are virtually eliminated. The only possible source of contamination, other than from the air is from the micropipette tips. As a previously unused tip is used for each addition, the risk of contamination is negligible, and no special precautions to prevent it are required.

(e) The problem of determining whether the sample concentration will bring the measured solution on to the linear response range of the electrode does not exist, as the base solution is previously doped to ensure this.

(f) There is better electrode performance.

This occurs because the electrode is always immersed in the base solution, the composition of which only changes marginally. Thus the electrode becomes permanently 'conditioned' to a particular solution, resulting in better stability and faster response time.

(g) In contrast to conventional known addition methods, a low sample concentration is associated with a small potential change, and a high concentration with a high change.

Thus the response is (very approximately) linear with concentration. This is intuitively more satisfactory. Detection limits and accuracy are similar, but precision higher, than conventional incremental methods.

(h) Because of (a) to (f) immediately above, analyses are faster and require much less skill than conventional ion selective electrode methods, virtually the only skill required being the correct use of a micropipette. An analysis takes typically between 1 and 5 minutes, and depends primarily on the response time of the electrode at the analyte concentrations measured. There is no additional preparation time, or delay time between consecutive analyses.

(i) The same procedure, with identical additions, can be used to cover a very wide concentration range, typically about 2000fold, from the limit of detection to the upper limit.

The invention will now be described further with reference to the accompanying drawing.

In the drawing there is shown an electrode cell 10, with an ion-selective electrode 11 and reference electrode 12 connected to a pX/mV meter 20 or a microprocessor control unit. A combination ion-selective electrode can replace the electrodes 11, 1 2 if desired. Also connected to the pX/mV meter or microprocessor is an automatic temperature compensator 13, but a manually read thermometer can replace this. The cell has an inlet 1 4 for the base solution and an inlet 1 5 for adding the standard solution and the sample to be analysed. An outlet 1 6 is provided and the cell contains a magnetic stirrer bar 17, actuated by a magnetic stirrer motor 18 beneath the cell.

Among many other applications the procedure is very useful for the rapid determination of acids and alkalis, at virtually any concentration in any type of sample the analysis being particularly fast (about 1 + minutes) because of the rapid response of the glass electrode. This is illustrated in the context of determining free nitric acid in uranyl nitrate solutions. Here the problem of hydrolysis of the uranyl ion is eliminated by complexing uranyl ion with sulphate at a pH less than or equal to 3.0 under which conditions hydrolysis of uranyl is insignificant. The ion-sensitive electrode used is a combination glass electrode, and an automatic temperature compensator is present.

A base solution, consisting of approximately 50 ml of magnesium sulphate soluton (2.5M), doped with about 100 mg/l nitric acid to give a pH of approximately 3.3 is used. This is supplied to the electrode cell and stirred.

When the reading at the pX/mV meter is stable, it is adjusted to an arbitrary value of 3.300 with the appropriate control knob.

0.2 ml of nitric acid (0.5M) is then added.

This constitutes a known amount of the ion to be measured. Stirring is continued and a first stable pH reading taken to the nearest 0.001 pH units. This reading is, for calculation purposes below, represented by "U,' and will be between pH 2.9 and 3.0. The same volume (0.2 ml) of sample is now added from the identical pipette, and a second stable pH reading then taken. This reading is represented by "V". The fall in pH depends on the free acidity of the sample.

The % w/v nitric acid (W) in the sample can then be calculated using the equation: 3. 1 5 (1 03.300-V 1 03.300-U) W= 1 03.300-U1 A nomogram can be provided which eliminates the above calculation, or an electronic calculator, programmable or otherwise, can be used.

After taking the "V" reading the stirrer 1 7 can be stopped and the cell drained. There is no need to rinse the electrodes or the cell before dealing with the next sample. An exact knowledge of the pipette volume is not required.

It is to be observed that in the above procedure hydrogen ion is also complexed by sulphate as the bisulphate ion (HS04-) and only a fraction of the total hydrogen ion is sensed by the electrode. The philosophy of the procedure and composition of the base solution ensures, however, that the total hydrogen ion concentration is determined. This is because the same fraction of total hydrogen ion concentration present in the standard and sample solutions, is measured in both cases.

The above procedure, using the doped magnesium sulphate solution, can also be used to determine the strength of any dilute mineral acid solution (of approximately 0.0015-5M).

Claims (2)

1. A method of making an analysis of ion concentration in a series of samples using an ion-selective electrode in an electrode cell comprises the steps of: (a) Filling the cell with an approximately known volume of base solution doped with a trace of the ion to be determined and taking an initial pX or mV reading or alternatively adjusting to a nominal pX value; (b) Adding from a dispenser a volume of liquid, small relative to the volume of the base solution, containing a known concentration of the ion to be measured; (c) Taking a first pX/mV reading; (d) Adding from the same dispenser used in (b), the same volume of the first sample for analysis; (e) Taking a second pX/mV measurement; (f) Draining the cell without rinsing; (g) Processing back to (a) above for the next sample analysis.

2. The method of Claim 1 in which the additions in (b) and (d) are reversed and/or more than one addition of standard solution and/or sample is made.