GB2231158A - Potentiometric determination of azide ions or hydrazoic acid - Google Patents

Abstract

The concentration of azide ions (N3<->) or hydrazoic acid (HN3) in a test solution, is determined by a pH electrode and a reference electrode both surrounded by a hollow body 5, the wall of which is partially formed by a semi-permeable diaphragm 2, which dips into the test solution 1, the interior of the hollow body 5 being filled with an inner filling solution 3, which is produced by dissolving 0.2 to 5 x 10<-3> mol of an easily soluble azide in one litre of water. The test solution has a maximum pH value of 2. Between 5 x 10<-3> and 1 mol per litre of a further salt may be added to the filling solution; the further salt being completely soluble in the solution and unreactive with azides or hydrazoic acid. Its purpose is to render the ionic strengths of the test and filling solution equal. A gas-sensitive sensor of the above construction is also claimed. <IMAGE>

Description

POTENTIOMETRIC DETERMINATION METHOD AND APPARATUS The present invention relates to a method for the potentiometric determination of azide ions (N3') or hydrazoic acid (HN3) in a test solution and to a gassensitive electrode suitable for carrying out such a method.

In certain circumstances, hydrazoic acid (HN3) may be formed in solutions, which contain nitrogencontaining compounds with nitrogen in various valencies.

Hydrazoic acid is produced, for example, from hydrazine and nitrous acid according to the equation [N2H5]+ + HN02 = HN3 + 2H20 + H+ The concentrations of HN3 and azide have to be monitored in such solutions for safety reasons.

Hydrazoic acid is particularly formed during the reprocessing of nuclear fuels in the PUREX-process.

It is disclosed in Atomkernenergie Kerntechnik", vol. 47 (1985), No. 2, pages 87 to 90, that azides in alkaline solution at pH 8 can be detected by using a silver azide solid-body electrode at concentrations of as low as 2 . 10-4 mol/l. In this case, however, hydrazine, hydroxylamine and ammonium interfere with the detection. Azides must be separated by anion exchange in a preparatory method step.

The present invention seeks to provide a simplified method of detecting hydrazoic acid without the need for such a prior separation step. The present invention further seeks to provide a method which enables lower concentration levels to be detected than has hitherto been possible and to increase the sensitivity of such detection. The present invention also seeks to provide a method in which hydrazoic acid can be detected by direct measurement in an acidic PUREX or in a feed solution containing high concentrations of uranium and hydrazine. In a further aspect, the present invention seeks to provide a gas-sensitive electrode which can be used for carrying out such a method.

According to the present invention, there is provided a method for the potentiometric determination of the concentration of azide ions (N3-) or hydrazoic acid NH3)in a test solution, wherein a pH electrode and a reference electrode are immersed in the test solution, wherein the pH electrode and the reference electrode are surrounded by a hollow body, the wall of which is partially formed by a semi-permeable diaphragm, which dips into the test solution, the interior of the hollow body being filled with an inner filling solution, which is produced by dissolving 0.2 to 5.0 leo 3 mol of an easily soluble azide in one litre of water; and the test solution having a maximum pH value of 2.

Also according to the present invention, there is provided a gas-sensitive electrode for the potentiometric determination of ions in a test solution,comprising a pH electrode and a reference electrode in a hollow body, which dips into the test solution and has a wall which, in the immersed portion, is partially formed by a semi-permeable diaphragm, said hollow body being filled with an inner filling solution,wherein the inner filling solution contains 0.2 to 5. 10-3 mol/l of an easily soluble azide.

In order to determine the concentration of hydrazoic acid or azide present in a test solution, a gassensitive electrode having a pH single-bar measuring chain is used which is surrounded by a hollow body filled with a solution containing a 0.2 to 5.10'3 molar solution of an easily soluble azide. The pH value of the test solution is set to a maximum value of 2.

Sodium or calcium azide is desirably used as the easily soluble azide for the inner filling solution within the hollow body.

If the test solution contains a relatively high concentration of other salts it is advisable to add a further salt to the inner filling solution. This salt may be added in a concentration of from 5 . 10-3 to 1 mol/l.

If the salt concentration from approximately 1.5 mol/l in the outer filling solution, water vapour may condense into the inner solution through the diaphragm, may dilute the azide solution thereby cause inaccurate measurements. The addition of the further salt prevents this effect.

The salts which may be used,are any salts which do not react with azide ions they are completely soluble, and they neither complex with nor precipitate azide ions.

NaCl, KCl,NaNO3 or KN03 are highly suitable for this purpose.

If higher ion concentrations are present in the test solution, it may be advantageous to add a complexing agent for cations thereto. Citric acid, for example, is a suitable complexing agent. It is also advantageous for the test solution to be buffered.

The ionic strengths of the inner ad outer solutions should be of the same order of magnitude, in order to prevent vapour diffusion and other osmotic effects.

A particularly simple and effective device for carrying out the method according to the invention resides in the fact that the inner filling solution of a conventional gas-sensitive electrode, having a pH singlebar measuring chain, is exchanged for an inner filling solution which contains an easily soluble azide having a concentration of between 0.2 and 5 . 10-3 mol/l. In such case, the conventional electrodes which are commercially available on the market may be used, wherein the inner solution and the test solution are separated by means of a semi-permeable diaphragm. The diaphragm may be formed from Teflon (Registered Trade Mark), polyethylene, polypropylene or any other suitable substance. Its thickness determines the response period of the electrode, so that thin diaphragms are preferably used for rapid measurements.

The method of the invention and the gassensitive electrode of the invention have proved satisfactory for determining the concentration of hydrazoic acid or azide ions in PUREX or feed solutions obtained during the reprocessing of nuclear fuels.

The range of measurement extends from 1 . 10-1 to 5 . 10-7 mol/l azide or hydrazoic acid. The characteristic curve extends linearly between 1 . 10-i and 5 . 10-6 mol/l. Compared with prior art methods, the range of measurement is improved by 1 to 2 orders of magnitude. Apart from possibly setting the pH value in the test solution to values ss 2, additional preparatory steps are unnecessary, provided that the test solution contains no gas-releasing constituents, which influence the dissociation equilibrium of the water.

According to the invention, hydrazoic acid may be measured directly in an acidic PUREX or feed solution.

Experiments have shown that uranium concentrations of up to 180 g U/I and hydrazine concentrations of up to 0.2 mol/l do not adversely influence the measurement. In respect of sensitivity, the measuring method of the invention is superior even to ion chromatography. An additional advantage resides in the fact that reliable measuring and evaluating apparatus may be used.

The present invention is illustrated, by way of example, in the accompanying drawings, in which: Fig. 1 shows a schematic representation of the method according to the invention.

Fig. 2 shows a calibration curve of a gassensitive electrode in accordance with the present invention in the form of a diagram CHN3 = f (potential value).

The invention will be further illustrated, by way of example, with reference to the following Examples.

Example 1 (Direct measurement) A maximum of 88ml of an accurately measured test aliquot is mixed with 10 ml of a TISAB solution in a glass beaker (150 ml).

The TISAB solution is produced in the following manner: A first solution is initially produced, whereby 21.01 g citric acid are mixed with 200 ml of a l-molar sodium hydroxide solution and is made up to 1 litre.

A second solution is subsequently produced, in that 126 g KNO3 is mixed with 100 ml of a 1-molar hydrochloric acid solution and, in turn, is made up to 1 litre.

The TISAB solution is produced by mixing 198 mol of the first solution and 802 ml of the second solution.

The pH value of the resultant solution is 1.4.

In the case of relatively small test aliquots, prior to the addition of the TISAB solution, the sample to be tested is diluted with water so that a volume of substantially 85 ml are obtained. The pH value of the test solution is then ascertained by a pH electrode and a suitable pH meter. The pH value is preferably set to 1.5 by the addition of concentrated,usually approximately 8molar, nitric acid. If, however the pH value ascertained reveals that the solution is too acidic after the addition of the TISAB solution, the pH value is set to 1.5 by the addition of concentrated, usually approximately 8-molar, carbonate-free sodium hydroxide solution.

Including the addition of the nitric acid or the sodium hydroxide solution, whichever is necessary, it must be ensured that the total volume does not exceed 100 ml. If necessary, an exact volume of 100 ml is then made up by adding water.

The inner filling solution is removed from a conventional ammonium electrode having a Teflon diaphragm, such as is commercially available from ORION, for example, and replaced by an inner filling solution containing 0.01 mol KN3/1 and 0.01 mol NaCl/l.

The electrode, which has been prepared in this manner, is inclinedly immersed in the prepared test solution, so that no gas bubbles can become adhered to the diaphragm, since such bubbles would falsify the measured result.

The potential change is awaited with constant stirring. The use of a potentiometric recorder with a correspondingly set inclination has proved satisfactory for monitoring the potential change. Depending upon the content of analyte to be measured, change periods of 1 to 2 minutes (c = 10-2 to leo 1 mol/l)or of more than 10 minutes (c = 5 x 10-6 molll) are required.

By reading-off the potential of the electrode on a suitable potentiometer, the analysis concentration (AKO) of the HN3 in the analysis solution may be determined.

This is effected by simply reading-off from a previously produced straight calibration line in the concentration range between 10'1 and 5 x 10-6 mol/l.

The calculation of the AKO is likewise easily possible by way of the derived formula E E, S Cx = 10 where: S = gradient of the electrode in mV; Ex = measured potential in mV; Eo = standard potential in mV; Cx = total concentration in mol/l.

The values for E0 = + 98.6 and S = 58.4 are obtained from the linear regression of the calibration curve in the form Pot gem. = f (lg cx). Programmable computers are used therefor.

Accordingly, an analysis concentration (AKO) of cx = 2.00 x 10-4 mol/l is calculated for a measured potential of - 117.4 mV.

It is necessary for the electrode to be rinsed prior to a new sample being measured. For this purpose, it is initially immersed with constant stirring in a solution which has been set to a pH value of approx. 8 by the addition of NaOH. After a potential of the order of250 mV has been reached, the electrode is placed in H2O and rinsed for a further 2 to 3 minutes. Thereafter it is prepared for a new measurement. If this procedure is not consistently followed, incorrect measurements are preprogrammed.

A pre-requisite for the direct measurement with the electrode is that the analysis solutions are pure waters which are not charged with salt, and the ionic strengths scarcely differ from one another. In such case, the ionic strength of the measuring solution is determined as a result of the introduction of salt in the TISAB solution.

Likewise, the calibration curve which has been ascertained applies only to a predetermined, external air pressure. Changes in air pressure produce incorrect measurements by parallel displacement of the calibration curve lg Cx = f (Pot gem.)* A possibility for correction is produced when a calibration solution with a definite content is measured and a change in the ordinate portion is effected on the basis of this measured value. This is very easily possible with the programmable computers.

Example 2 (Method of simple standard addition) Better and more accurate results are obtained by effecting the measurement according to the method of simple standard addition, even in solutions charged with salt. The mode of operation is described hereinafter.

The gradient S of the electrode has to be determined prior to the actual measurement, so that, with reference to the simplified Nernst equation E x = E0 + S x lg Cx and after a standard addition, two defining equations with the two unknown quantities Eo and Cx have to be solved according to the current mathematical solution methods.

The gradient S of the electrode is advantageously so determined that the change in concentration amounts to a molar power of ten and is effected from c = 1 x 10 3 to c = l x 10-2 mol/l. The measured change in potential corresponds to the gradient of the electrode and is +58 mV. Anions produce negative gradient values, since the N 3/HN3 determination here is causally attributed, however, to a pH measurement equal to H30+ determination if the gradient value is positive.

Determination of the gradient of the electrode Exactly 80 ml water and 10 ml TISAB solution are introduced into a 150 ml glass beaker at a pH of substantially 1.5. Added, whilst stirring with a magnetic stirrer, are 1.0 ml of a O.l-molar azide solution diluted with H20 to exactly 10 ml by means of a dosimeter. The total volume amounts to 100 ml with an analysis concentration of c = 1.0 x 10-3 mol/l.

Potential adjustment is effected with a potentiometric recorder, which has been adjusted in an appropriately inclined manner. In the event of potential stability, the mV value which is read-off at the potentiometer (ion meter)is noted. Added thereafter are exactly 1.0 ml of a c = 1.0 mol/l azide solution diluted with H20 to exactly 10 ml by means of a dosimeter.

After the standard addition, the total volume amounts to 110 ml with a concentration of c = 1 x 10-2 mol/l.

After the potential adjustment, the second potential value is determined as described above, and the difference is noted as the gradient value.

Determination and calculation of the analysis concentration As described above, a test aliquot is diluted with H20 and TISAB solution, and the pH value is set to substantially pH 1.5. The analysis volume amounts to exactly 100 ml.

The electrode is immersed in the test solution, and the potential adjustment is awaited (see above for the conditions). The potential of E1 3 -93.8 mV is read-off.

The estimated concentration cx = 5.08 x 10-4 mol/l is calculated according to the formula E E, S Cx = 10 The standard addition is calculated on condition that twice the amount of measuring ions as is originally present in the solution is added, so that a change in potential of approx. 25 mV occurs. An addition of the same amount of measuring ions produces a potential change of approx. 15 mV, and half the original amount produces a potential change of approx. 10 mV.

The standard addition is then calculated according to the formula c1 = cx x 100 x 2 = 1.02 x 10'1 Accordingly, 1 ml of a 0.1 -molar azide solution, either rounded-up or rounded-down, must be added as the standard. Since the additional metering is effected with an automatic metering system, which is so adjusted that the same amount of H20 is rinsed-out, 2 ml of a 5 x 10'2-molar solution are produced as the standard addition.

After the standard addition, the potential adjustment is again awaited, and thereafter the potential E2 =-66.7 mV is noted.

The current calculation formulae only permit predetermined analysis volumes and fixed dilution ratios (e.g. 1, 5 or 10% relative to the analysis volume). The subsequent calculation is effected according to a formula, where there are no restrictions regarding conditions because of the derivation.

Formula statement for simple standard addition with the following conditions: - any desired dilution ratio; - any desired standard concentration; - any desired aliquot volume; - any desired analysis volume;

V1 = Vol. standard addition in ml; c1 = standard conc. in mol/l; Vo = aliquot vol in ml; V = analysis vol in ml; E1 = initial potential in mV; E2 = potential after standard addition in mV; Cx = sought-after concentration in mol/l; S = gradient of the electrode in mV.

With the above-mentioned conditions - standard addition, analysis volume, initial potential and end potential - cx 3 5.02 x 10-4 is produced; the concentration c = 5.0 x 10-4 moll was sought-after.

Claims (9)

1. A method for the potentiometric determination of the concentration of azide ions (N3-) or hydrazoic acid (HN3) in a test solution, wherein a pH electrode and a reference electrode are immersed in the test solution, wherein the pH electrode and the reference electrode are surrounded by a hollow body, the wall of which is partially formed by a semi-permeable diaphragm, which dips into the test solution, the interior of the hollow body being filled with an inner filling solution, which is produced by dissolving 0.2 to 5 . 10 3 mol of an easily soluble azide in one litre of water; and the test solution having a maximum pH value of 2.

2. A method as claimed in claim 1, wherein 5 . 10-3 to 1 mol per litre of a further salt are additionally added to the inner filling solution, which salt is nonreactive with azides or hydrazoic acid; is completely soluble in the solution; and the constituent of which can neither precipitate nor form complexes with azide ions.

3. A method as claimed in claim 1 or 2, wherein cationic complexing agents are added to the test solution.

4. A method as claimed in any preceding claim, wherein the test solution is buffered.

5. A method as claimed in claim 1, wherein the ionic strengths of the test solution and inner filling solution are adapted to one another.

6. A method for the potentiometric determination of the concentration of azide ions or hydrazoic acid in a test solution as claimed in claim 1 substantially as hereinbefore described with reference to the foregoing Examples.

7. A gas-sensitive electrode for the potentiometric determination of ions in a test solution, comprising a pH electrode and a reference electrode in a hollow body, which dips into the test solution and has a wall which, in the immersed portion, is partially formed by a semipermeable diaphragm, said hollow body being filled with an inner filling solution, wherein the inner filling solution contains 0.2 to 5. 10-3 molIl of an easily soluble azide.

8. A gas-sensitive electrode as claimed in claim 7, wherein 5 . 10-3 mol/1 of an additional salt, which salt is non reactive with azides or hydrazoic acid; is completely soluble in the solution; and the constituents of which can neither precipitate nor form complexes with azide ions is additionally added to the inner filling solution, the ionic strength of the inner solution being adapted to the ionic strength of the test solution.

9. A gas-sensitive electrode as claimed in claim 7 constructed and arranged to operate substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.