FR2645963A1 - METHOD AND DEVICE FOR THE POTENTIOMETRIC DETERMINATION OF THE CONCENTRATION OF AZOTOIC ION OR AZOTHYDIC ACID - Google Patents

Abstract

a) Method and device for the potentiometric determination of the concentration of azide ions or azothydric acid. b) characterized in that: 1. the pH electrode and the reference electrode are surrounded by a hollow body 5, the wall of which is partly formed by a semi-permeable membrane 2 immersed in the sample solution 1, 2 . the internal space of the hollow body 5 is filled with an internal filling solution which is prepared by dissolving, in 1 liter of water, 0.2 to 5 x 10 ** - ** 3 moles of an azide readily soluble.

Description

"Method and device for potentiometric determination of the

azide ion concentration or

of azohydric acid ".

  The invention relates to a process for the potentiometric determination of azide ions (N3-) or azohydric acid (HN3) in a sample solution, in which a pH electrode and a reference electrode are immersed in the sample solution, a gas-sensitive electrode for the potentiometric determination of ions in a sample solution, consisting of a pH electrode and a reference electrode in a hollow body which is immersed in the sample solution, whose wall, in the part immersed, consists partly of a semi-permeable membrane, and which is filled with an internal filling solution, and its use. Under certain conditions, azohydric acid (HN3) may form in solutions containing nitrogen compounds containing

nitrogen with various valences.

  Azohydric acid is formed for example from hydrazine and nitrous acid, according to the equation: [N 2 H 5] + + HNO 2 -> HN 3 + 2H 2 O + H + In such solutions, the concentration of HN 3 or Azide should be monitored for safety reasons. Hydrogen azide may be formed especially during the reprocessing of fuels

  in the PUREX process.

  According to D. Ertel, "Atomkernenergie-

  Kerntechnik "(Nuclear Energy - Nuclear Technology), vol 47 (1985), No. 2, pp. 87-90, it is known that alkaline solutions at pH 8 can be decelerated to a limiting concentration of about 2 10-4 mol / l by means of a silver azide solid electrode, in which case the hydrazine, the hydroxylamine and the ammonium interfere with the determination, but the azide must be separated by exchange.

  of anions in a process preparatory step.

  The object of the invention is to determine azohydric acid by a simple process, without preliminary separation. The limiting concentration should be brought to lower values and the sensitivity of the determination should be increased. The determination of azohydric acid by direct measurements of an acid feed or PUREX process solution with high levels of uranium and hydrazine should be made possible. In addition, a gas sensitive electrode must be provided for

implementation of the method.

  The object is achieved according to the invention, by a method characterized in that: a) the pH electrode and the reference electrode are surrounded by a hollow body whose wall is partly constituted by a semipermeable membrane immersed in the sample solution, b) the internal space of the hollow body is filled with an internal filling solution which is prepared

  by dissolution, in 1 liter of water, from 0.2 to 5 -

  3 mol of a readily soluble azide; c) the sample solution is adjusted to a pH of 2 at the most. The subject of the invention is also a gas-sensitive electrode, characterized in that the internal filling solution contains 0.2 to 5

  10-3.5 at 10-3 mol / l of a readily soluble azide.

  According to the invention, it is proposed to use, for the determination of the azothydric acid or azide concentration, a gas-sensitive electrode comprising a bar-shaped pH measuring channel, the internal solution of which is Filling contains a 0.2 to 5 x 10 -3 molar solution of a readily soluble azide. The pH of the sample solution is adjusted according to the invention to a value

up to 2.

  As a readily soluble azide for the internal filling solution, it is possible to use

  for example, sodium azide or calcium azide.

  In the case where the sample solution to be determined contains foreign salts at a relatively high concentration, it is recommended to add further salt to the internal filling solution. This salt can be added at a concentration of 5 x 10-3 to 1 mol / l. At salt concentrations from about 1.5 mol / l in the external filling solution, water vapor can condense in the internal solution through the membrane, dilute the azide solution and cause erroneous measurements. The addition of salt prevents this effect. Any salt can be used as a salt, provided that it does not react with the azide, is completely soluble and can not precipitate or complex. azide ions. NaCl, KCl, NaNO3 or KNO3 are suitable. In the case of relatively high concentrations of ions in the sample solution, it may be convenient to add a cation complexing agent to the sample solution. For example, citric acid o10 is suitable as a complexing agent. In addition, it is

  advantageous to buffer the external solution.

  The ionic strengths of the internal solution and the external solution must be in the same order of magnitude, in order to avoid a diffusion of

  steam and other osmotic effects.

  A particularly simple and effective device for carrying out the process according to the invention consists in replacing the internal filling solution of a gas-sensitive electrode comprising a bar-shaped pH measuring chain with a solution. filling solution containing an easily soluble azide at a

  concentration between 0.2 and 5 x 10-3 mol / l.

  In this case, it is possible to use the usual electrodes available on the market, in which the inner membrane and the sample solution are separated by a semi-permeable membrane. The membrane may be made of Teflon, Polyethylene, Polypropylene or other suitable material. Its thickness determines the response time of the electrode, so for fast measurements it is preferable to use

thin membranes.

  The process according to the invention and the gas-sensitive electrode according to the invention have proved to be well suited to the determination of the concentration of azohydric acid or of azide ions in PUREX feed or process solutions, in the

  reprocessing of nuclear fuels.

  The measurement range is from 1 x 10-1 to 5 x 10-7 mole / l of azide or azohydric acid. The

  profile is linear between i x 10-1 and 5 x 10-6 mol / l.

  The measurement range is improved by 1 to 2 orders of magnitude compared to the state of the art. Apart from any adjustment of the pH of the sample solution to values of 2, further preparation steps are unnecessary as long as the sample solution does not contain any gas-releasing components having an effect on the dissociation equilibrium of the sample solution. water. The azohydric acid can according to the invention be measured directly in the PUREX feed or process solution. Tests have shown that uranium concentrations of 180 g U / l and hydrazine concentrations of 0.2 mol / l have no effect on the measurement. Regarding sensitivity, the measurement method according to the invention is even superior to ion exchange chromatography. Another advantage is that it is possible to use measuring devices and

  evaluated.

Figure i is a representation

  schematic of the method according to the invention.

  FIG. 2 shows a calibration curve of a gas-sensitive electrode according to the invention, in the form of a diagram CE3 = f

(potential value).

  The present invention is illustrated using

  descriptive and nonlimiting examples hereinafter.

  Example 1 (Direct measurement) In a glass beaker (150 ml), an accurately measured sample of 88 ml is available.

  maximum, with 10 ml of a TISAB solution.

  The TISAB solution is prepared as follows: A solution i is first prepared by adding 200 ml of a 1M sodium hydroxide solution to 21.01 g of citric acid and increasing the volume of the solution.

to 1 liter.

  A solution 2 is then prepared by adding 100 ml of a 1M hydrochloric acid solution to 126 g of KNO3 and again raising the volume.

to 1 liter.

  The TISAB solution is obtained by mixing 198 ml of solution 1 and 802 ml of solution 2, the

pH is 1.4.

  In the case of relatively small sample volumes, prior to the addition of the TISAB solution, the sample should be diluted with water to a volume of approximately 25 ml. The pH of the sample solution is then monitored using a pH electrode and a suitable pH meter. The pH is preferably adjusted to 1.5 by addition of concentrated nitric acid, approximately 8 M. If the pH is too acidic after addition of the TISAB solution, the pH is adjusted to 1.5 by addition of a concentrated solution. about 8 M sodium hydroxide,

carbonate free.

  In the case of the addition of nitric acid or sodium hydroxide solution, it must be ensured that the total volume does not exceed 100 ml. After that, we can adjust the volume

to exactly 100 ml with water.

  The internal filling solution is removed from a commercially available Teflon membrane ammonium electrode, for example, from ORION, and replaced with an internal filling solution containing 0.01 mole of KN3. /liter

  and 0.01 mole NaCl liter (NaCl / liter).

  The electrode prepared in this way is dipped obliquely into the prepared sample solution, so that no bubble of gas which falsifies the membrane can be deposited on the membrane.

result of the measurement.

  The adjustment of the potential is under constant stirring. The use of a properly adjusted compensation and range recording apparatus has proved suitable for controlling the potential setting. Depending on the body content to be measured, adjustment times of 1 to 2 minutes (c = 1-2 to 10-1 mol / l) and more than 10

  minutes (c = 5 x 10-6 mol / l) are required.

  By reading the potential of the electrode on a suitable potentiometer, one can determine the analytical concentration (COA) of HN3 in the analyzed solution. This is done by simply reading on a previously constructed calibration curve in the concentration range between 10-1 and 5x

-6 mol / l.

  The calculation of the COA is also easily achievable by means of the derived formula: Ez - Eo

C1 = 10

  S S = slope of the electrode in mV Ex = potential measured in mV Eo = reference potential in mV

  C = total concentration in mole / l.

  The values for Eo = + 98.6 and S = 58.4 are obtained from the linear regression of the calibration curve as Potg = f (lg cx). For this purpose, a programmable computer is used. For a measured potential of -117.4 mV, according to this calculation, the concentration analyzed

(COA) cx = 2.00 x 10-4 mol / l.

  Before measuring a new sample, the electrode must be rinsed. For this purpose, it is first immersed under constant stirring in a solution which has been adjusted to about pH 8 by addition of NaOH. Once the potential of about -250 mV is reached, the electrode is placed in water and rinsed for another 2-3 minutes. After that, she is ready for a new measure. If we do not take care of

  consequent false measurements are pre-programmed.

  The necessary condition for the direct measurement by means of the electrode is that, as far as the solutions to be analyzed are concerned, they are solutions with a purely aqueous base which are not loaded with salts and which practically do not vary. others in ionic strength. The ionic strength of the solution to be measured is in this case determined by

  the introduction of salts of the TISAB solution.

  Likewise, the determined calibration curve is valid only for a determined external air pressure. Variations of the air pressure cause erroneous measurements by parallel displacement of the calibration curve log C = = f (Pot gem) To a correction possibility consists of measuring a known solution with a defined content and to be carried out on the base of this value a modification of the coordinates. This is possible very

  simple with programmable calculators.

  Example 2 (Single addition method of substance

reference).

  Measurement by the single addition of reference substance gives better and more accurate results, even in salt-laden solutions. The procedure is described below. Before the actual measurement, the slope S of the electrode must be determined, so that after a reference substance addition, two equations of determination with the two unknowns Eo and Cx are solved according to the usual mathematical resolution methods, using the simplified Nernst equation: E, = Eo + SX log C, the slope S of the electrode is suitably determined so that the concentration variation is 10 moles and c = 1 x 10 -3 to c = ix 10-2 mol / l. The measured potential variation corresponds to the slope of the electrode and is + 58 mv. Anions give negative slope values, but, since in this case the determination of N-3 / HN3 must be causally attributed to a pH measurement = determination of

  H30 +, the value of the slope is positive.

  Determination of the slope of the electrode: In a 150 ml glass beaker, exactly 80 ml of water and 10 ml of TISAB solution (approximately pH 1.5) are introduced. While stirring with a magnetic stirrer, 1.0 ml of a solution of 0.1 M azide diluted to exactly 10 ml with H 2 O is added using a Diluter / Dosimat measuring apparatus. The total volume is 100 ml with a concentration of

  analyte equal to c = 1.0 x 10-3 mol / l.

  The potential setting is performed on a compensation recording apparatus and set to a suitable range. At constant potential, we note the

  value in mV read on the potentiometer (ionometer).

  After that, exactly 1.0 ml of a solution of azide at c = 1.0 mol / l, diluted to exactly 10 ml with H 2 O, was added using a Diluter / Dosimat measuring apparatus. After the addition of reference substance, the total volume is 110 ml, with a concentration

c = 1 x 10-2 mol / l.

  After setting the potential, the second potential value is determined as described above,

  and note the difference as a slope value.

  Determination and Calculation of the Concentration of the Analyzed Substance: A sample is diluted as described above with H 2 O and TISAB solution and the pH is adjusted to about 1.5. The volume of the analyte is

exactly 100 ml.

  The electrode is immersed in the sample solution and the adjustment of the potential is awaited (conditions: see above). We read El potential

= -93.8 mV.

According to the formula: Ex - Eo

C. = 10

  The calculated concentration is calculated to obtain

cX = 5.08 x 10-4 mol / l.

  The addition of the reference substance is calculated by admitting that a quantity of ions to be measured equal to twice that initially present in the solution is added, so that it

  a potential variation of about 25 mV occurs.

  An addition of the same amount of ions to be measured gives about 15 mV, half the initial amount

  gives a potential variation of about 10 mV.

  After that, the addition of reference substance is calculated according to the formula: Cl = cx x 100 x 2 = 1.02 x 10-1

  It is therefore necessary to add judiciously

  The reference substance is approximately 1 ml of a 10-1 M azide solution. The addition is carried out using an automatic dosing device which is adjusted in such a way that the same amount of water is then introduced for washing, 2 ml of a 5 x - 2 M solution results as reference substance. After addition of the reference substance, the adjustment of the potential is again expected, then we

note the potential E2 = -66.7 mv.

  Current calculation formulas allow only determined analytical volumes and fixed dilution ratios (eg 1.5 or 10% of the volume of the sample to be analyzed). The following calculation is made according to a formula in which, because of the derivation, there is no

limitation of conditions.

  Formula for single addition of reference substance with the conditions: - any standard dilution rate - any concentration of the reference substance - any volume of the sample

  - any volume of the sample analyzed.

  Vl C1 Vo cx = E2 - E1 (10. * (l + V / V)) - i V1 = volume of the addition of reference substance in ml c1 = concentration of the reference substance in mole / 1 VO = volume of the sample in ml V = volume of the sample analyzed in ml E1 = initial potential in mv E2 = potential after addition of reference substance in mV cx = desired concentration in mole / 1

S = slope of the electrode in mV.

  With the above-mentioned conditions of the addition of reference substance, the volume of the sample to be analyzed, the initial potential and the final potential. gets cx = 5.02 x 10-4; the

  The desired concentration was c = 5.0 x 10-4 mol / l.

  List of references 1. sample solution 2. semi-permeable membrane 3. internal filling solution 5.4. pH measuring chain in single bar form

5. hollow body.

Claims (1)

CLAIMS l1j Method for the potentiometric determination of the concentration of azide ions (N3-) or azohydric acid (HN3) in a sample solution, in which a pH electrode and a reference electrode are immersed in the sample solution, characterized in that: a) the pH electrode and the reference electrode are surrounded by a hollow body (5) whose wall is partly constituted by a semipermeable membrane (2) immersed in the sample solution (1), b) the internal space of the hollow body (5) is filled with an internal filling solution which is prepared by dissolving, in 1 liter of water, 0.2 to 5 × 10 -3 mol of an azide easily soluble.   (c) the sample solution is set to a value of pH of up to 2.   Process according to claim 1, characterized in that 5 x 10-3 to 1 mole per liter of another salt which satisfies the following conditions is additionally added to the internal filling solution: not in reaction with azide or azohydric acid, - the salt is completely soluble in the solution, - the salt components can neither precipitate nor complexing azide ions.   3 ') Process according to claim 1, characterized in that one adds to the solution   sample a complexant for cations.   Process according to Claim 1, characterized in that the sample solution is buffered.) Process according to Claim 1, characterized in that the ionic strengths of the sample solution and the internal filling solution are 6 ') Gas sensitive electrode for the potentiometric determination of ions in a sample solution consisting of a pH electrode and a reference electrode in a hollow body which is immersed in the sample solution and whose wall, in the immersed part, consists partly of a semi-permeable membrane and which is filled with an internal filling solution, characterized in that the internal filling solution contains   0.2 to 5 x 10-3 mol / l of a readily soluble azide.   7 ') gas-sensitive electrode according to claim 6, characterized in that 5 x 10-3 mol / l of another salt according to claim 2 is additionally added to the internal filling solution, the ionic strength of the internal solution being granted to   the ionic strength of the sample solution.   8 *) Using the sensitive electrode   gas according to either of claims 6 or 7,   for the determination of the concentration of azide ions or azohydric acid in a sample solution.