CS261774B1 - Method of nitritometric determination of azide anions - Google Patents

Abstract

The titration determination with nitrite anion is carried out under the simultaneous action of iodide ions and a catalyst based on sulfide-bound sulfur. As a compound with iodide ion, potassium iodide is preferably used in a concentration of the order of 10"i mol/1, based on the initial volume of the titrated solution. The catalyst is dissolved in the titrant and preferably a salt with thiosulfate anion is used, e.g. sodium thiosulfate, in a molar concentration 2 orders of magnitude lower than the molar concentration of the titrant, or it is thiourea in a molar concentration 1 order of magnitude lower than the molar concentration of the titrant.

Description

The invention relates to the nitritometric determination of the azide anion by simple titration with a volumetric solution of a nitrous acid salt, with a visual indication of the equivalence point, especially in protein solutions.

Sodium azide is also used to preserve substances that are easily subject to microbial contamination, such as protein diagnostic preparations. Its content is limited by technical standards and must be analytically controlled. The azide ion can be determined volumetrically in simple aqueous solutions using argentometry, cerimetry, iodometry and titration with sodium nitrite. In protein solutions that often also contain sodium chloride as an isotonizing agent, simple titration determination is problematic and therefore instrumental methods such as potentiometry and polarography are used. For immunoglobulins for diagnostics, the polarographic method is particularly recommended and used, although it is laborious, lengthy and, moreover, burdened by a relatively significant methodological error of determination.

The above-mentioned shortcomings are solved by the method of nitritometric determination of the azide anion by titration with a volumetric solution of a nitrous acid salt according to the present invention, the essence of which is that the determination is carried out in the simultaneous presence of a salt of hydroiodic acid and a compound containing sulfide-bound sulfur. The salt of hydroiodic acid is best used in a concentration of 10 ¹ mol/li·tr, based on the initial volume of the titrated solution, and the compound with sulfide-bound sulfur is dissolved in a volumetric reagent, i.e. a solution of a nitrous acid salt. It is preferably a salt of thiosulfuric acid in a molar concentration two orders of magnitude lower than the molar concentration of the titrant itself, or it is thiourea in a molar concentration one order of magnitude lower than the molar concentration of the titrant, a salt of nitrous acid. This allows using a simple visual indication based on the principle of iodometry to reliably determine the equivalence point and using a fast and ready titration method without complex instrumentation to determine the azide ion.

When determining the azide anion with nitrite ion, the reaction proceeds according to the following equation:

N + NO + 2H ⁺ = N ₂ + N ₂ O + H ₂ 0 (1)

The equivalence point is commonly determined potentiometrically. A simple visual indication, according to the present invention, is iodometric and uses starch grease as a color indicator. In the first phase of the determination, the reaction mechanism according to the above-mentioned equation 1 predominates, but in the second half of the determination, before equivalence, the reaction according to the equation djA ' begins to apply

NO~ + 2 j + 4 H ⁺ = Jj + 2 NO + 2 HjO (2)

The two reactions do not proceed consecutively as would be desirable. Although reaction 1 has a significantly higher rate in the first phase of titration than reaction 2, due to the shift of thermodynamic equilibria, premature iodine excretion distorts the correct result of the determination. For this reason, a compound with sulfide-bound sulfur is added to the reaction mixture, which acts as a catalyst for the third reaction, which proceeds according to the equation

Nj + J ₂ - 3 N ₂ + 2 j” (3)

This reaction takes place before the equivalence point is reached, thus eliminating the premature action of reaction 2 and allowing the correct titration consumption to be obtained. Only in this final phase is reaction 2 correctly applied and the first excess of titrant indicates the exact equivalence point. With respect to the substance to be determined, the azide ion, all three of the above equations are stoichiometrically equivalent. By simultaneously using these reactions according to the invention, their synergic action is achieved, although they are individually competitive. The result of this common higher t^jínek is the accurate and undisturbed achievement of the equivalence point and the possibility of using a simple, fast and accurate titration method for the determination of the azide ion even in samples containing a significant percentage of proteins, without the need for complex analytical instrumentation. Sodium azide is used for preservation in a concentration of about 0.1% and diagnostics are relatively expensive. Therefore, 1 ml of the preparation is used for analysis and under these conditions, a suitable titrant concentration of 10” ² mol/liter is suitable. When using thiosulfate, as a substance with sulfide-bound sulfur, its required concentration is 10 ⁴ mol/liter.

It can be assumed that, in addition to reaction 3, iodine released by reaction 2 could act on thiosulfate to form tetrathionate. This is a general iodometric reaction. Theoretically, this would increase the results by 1% rel. However, it was shown on a model with an order of magnitude higher thiosulfate content that this only happens partially. Under the given conditions, _{the x} disturbing effect of this reaction is practically not evident. When using thiourea, a similar phenomenon did not occur at all. The actual working technique may cause a higher error.

Example

Analytical reagents required

1. Alkalized titration solution 10 ^-2 mol/1 NaNO2 and 10~ ⁴ mol/1 Na2S ₂ C> ₃ contains in 1000 ml 5.0 g of anhydrous sodium carbonate, 0.025 g of sodium thiosulfate pentahydrate and 0.690 0 g of sodium nitrite, previously dried to constant weight at 105 °C.

2. The alkalized KJ solution contains 5.0 g of anhydrous sodium carbonate and 50.0 g of potassium iodide in 1,000 ml.

3. Glacial acetic acid.

4. Starch grease, approximately 4%.

As test solutions, immunoglobulins containing 2% and 5% protein were prepared from human normal immunoglobulin containing 14.8 S protein by dilution with saline. Sodium azide was added in graded amounts.

Analytical procedure for determination

To 1.00 ml of the sample, 5.0 ml of KJ solution, 10.0 ml of distilled water, drops of starch tallow and, after mixing, 2.0 ml of glacial acetic acid were gradually added. With intensive mixing, the titration solution was added dropwise until the first blue coloration.

Determination results (3 polarographic determination results are also shown for comparison).

Sample No. 1 2 3 4 5 6 Protein content 2 2 2 2 2 5 Actual content of NaN^-g/1 0.50 1.00 1.50 2.00 3.00 2.00 Determined by titration -g/1 0.52 1.03 1.52 1.97 3.01 2.00 Determined polarogx. -g/1 0.86 2.80 - 1.54 Example 2 Necessary analytical reagents 1. Alkalized titration solution IO ^-2 mol/1 NaNO ₂ and 10 ^-3 mol/1 (nh ₂ ) ₂ cs contains in 1,000 ml

5.0 g of anhydrous sodium carbonate, 0.076 g of thiourea and 0.690 0 g of sodium nitrite, previously dried to constant weight at 105 °C.

The other reagents are identical to reagents 2-4 in Example 1.

10 mol/l sodium azide was prepared as a test solution. 1.0 g of anhydrous sodium carbonate and 0.650 1 g of sodium azide, previously dried to constant weight at 100 °C, were dissolved and made up to 1000 ml with water.

Determination procedure

To 6.00 ml of sodium azide test solution was gradually added 5.0 ml of KJ solution,

1.0 ml of water, 4 drops of starch ointment and after mixing 2.0 ml of glacial acetic acid. With intensive stirring, the titration solution was added dropwise until the first blue coloration. The determination was carried out in quadruplicate.

Determination results

The following titration solution consumptions were obtained:

6.02 - 6.00 - 6.00 - 6.02 ml.

The use of the invention is not limited or limited by the examples given.

The analytical procedure according to the present invention can be used for the rapid and accurate determination of the azide anion in any common analytical laboratory without the need for potentiometric or polarographic equipment. The recommended polarographic method works with a mercury drop electrode and concentrated sulfuric acid under ice cooling and the analysis takes approximately one hour. In this case, the error of the determination is significant, as can be seen from Example 1. The method according to the invention does not require working with either poisonous mercury or caustic acid, one determination takes only a few minutes and the results are very accurate. It is therefore advantageous in terms of labor productivity, analytical accuracy and work safety.

Claims (3)

A method for the determination of azide anions, characterized in that the determination is carried out in the presence of iodide ions, preferably potassium iodide, in the order of 10 µM, based on the initial volume of the titrated solution and the sulphide-bound sulfur catalyst. 2. The process of claim 1 wherein the catalyst is an anionic thiosulfate, preferably sodium thiosulfate, dissolved in the titration solution at a molar concentration 2 orders of magnitude lower than that of the titration reagent. 3. The process of claim 1 wherein the catalyst is thiourea dissolved in the titrating agent solution at a molar concentration one order of magnitude lower than that of the titrating agent.