Determination of Nitrate and Nitrite Ions in an Alkaline Electrolyte

E. S. Novikova, G. M. Yakunina

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

No satisfactory data on titrimetric methods for determining the nitrate ion in the simultaneous presence of large concentrations of the nitrite ion could be found in the literature.

Most existing methods for the quantitative determination of the nitrate ion amount to reducing the \(\mathrm{NO_3^-}\) ion to ammonia, absorbing it with sulfuric acid, and titrating the excess of the latter with a standard alkali solution [1].

In some cases the nitrate ion is reduced by the action of an excess of a solution of an \(\mathrm{Fe^{2+}}\) salt in an acidic or alkaline medium, after which the \(\mathrm{Fe^{3+}}\) formed or the excess \(\mathrm{Fe^{2+}}\) is determined [2].

However, determination of nitrate in this way is greatly hindered by the presence of the nitrite ion.

Our attempt at preliminary removal of the nitrite ion by the action of sulfanilic acid gives unsatisfactory results (Table 1).

Table 1

The magnitude of the normal potential

\[ E_0 \frac{\mathrm{Ti}^{3+}}{\mathrm{Ti}^{4+}} = 0.1\ \mathrm{V} \]

indicates the strong reducing properties of salts of trivalent titanium. The use of solutions of chloride and sulfate of \(\mathrm{Ti^{3+}}\) as a standard reducing solution has long been known [3], but in the literature there are no indications of the possibility of using \(\mathrm{Ti^{3+}}\) compounds for determining the nitrate ion in the presence of the nitrite ion.

Our attempts to apply the ability of \(\mathrm{Ti^{3+}}\) salts to interact with the nitrate ion in the analysis of an alkaline electrolyte encountered the need to determine the effect of the medium on reduction of the nitrate ion in an alkaline electrolyte. Verification showed that the best conditions are a pH interval from 2 to 3.

The results obtained are well explained by the fact that, in weakly acidic and alkaline media, \(\mathrm{Ti^{3+}}\) reduces, at the boiling tempera-

the hydrogen ions to free \(H_2\), which significantly distorts the results of the determination and makes the method unsuitable for analysis of the electrolyte; an increase in acidity raises the value of the potential

\[ E_0 \frac{\mathrm{TiO}^{2+}+2\mathrm{H}^{\cdot}}{\mathrm{Ti}^{3+}+\mathrm{H}_2\mathrm{O}} \]

and decreases the reducing action of the \(\mathrm{Ti}^{3+}\) salts, which also has a very unfavorable effect on the course of the reduction process.

The working solution of the \(\mathrm{TiCl}_3\) salt was prepared in the usual manner: 30 ml of the 19% commercial solution was diluted to 1 liter with freshly boiled distilled water. In establishing the titer of the titanium solution, we observed that direct titration of potassium dichromate with a \(\mathrm{Ti}^{3+}\) titanium-salt solution using diphenylamine as indicator gives the most constant results. However, experiments showed the necessity of establishing the titer of \(\mathrm{Ti}^{3+}\) against a standardized nitrate solution under conditions analogous to the conditions of the determination in the course of the electrolyte analysis (see below). The working solution of the \(\mathrm{Ti}^{3+}\) salt was stored in an atmosphere of hydrogen generated in a small generator placed above the bottle.

Table 2

During titration the solutions were protected from the action of atmospheric oxygen by passing a strong stream of carbon dioxide gas through them.

We rejected the technique of protecting the solution during titration by adding small amounts of \(\mathrm{NaHCO}_3\) from time to time, in view of the possibility that the reaction mechanism might change with a change in the reaction of the medium.

Table 3

(In alkaline solution \(\mathrm{Ti(OH)}_3\) can reduce nitrogen-oxygen salts to ammonia.) A substantial influence on the direction of the reaction of reduction of the \(\mathrm{NO}_3^{1-}\) ion by a \(\mathrm{Ti}^{3+}\) solution is exerted by the rate of addition of the working \(\mathrm{Ti}^{3+}\) solution, i.e., by the ratio of the concentrations of oxidizing agent and reducing agent at the moment of interaction.

Rapid addition of the working \(\mathrm{Ti}^{3+}\) solution promotes the formation of oxide, which, being readily oxidized, is the cause of the occurrence of various intermediate reactions and of distortion of the analytical results. The titanium-salt solution should always be added strictly dropwise, at a definite rate.

Procedure of determination. 5 ml of the electrolyte taken with dilution (1:10), containing nitrate and nitrite ions, is neutralized in a 100-ml volumetric flask to methyl orange and diluted to the mark. An aliquot portion of the resulting solution is transferred to a 250–300 ml conical flask, into which an approximately necessary amount of is introduced beforehand.

ml of 0.1 N \(K Mn O_4\). Then 4 N \(H_2SO_4\) is added dropwise to the same vessel until the solution clears, and the resulting mixture is titrated with potassium permanganate to a pink coloration. (The volume of \(K Mn O_4\) used for the titration makes it possible to calculate the content of the \(NO_2^{1-}\) ion in g/liter of electrolyte.)

The solution titrated with permanganate is heated to boiling, 10–15 drops of a 0.1% solution of alizarin red are added, and it is titrated with the working solution of \(Ti^{3+}\) salt.

During titration, owing to hydrolysis of the \(Ti^{4+}\) salt, its hydroxide is formed, which precipitates already at \(pH = 1.5\). \(Ti^{4+}\) hydroxide adsorbs the red indicator. The red coloration changes to gray-green, but is gradually restored if the solution has not been fully titrated. Near the equivalence point the precipitate ceases to acquire a gray-green coloration and the red coloration ceases to return even when the solution is boiled.

Thus the total amount of \(NO_3^{1-}\) contained in the sample and obtained by oxidation of the nitrate ion is determined.

In the calculations, the corresponding correction must be made. The amount of nitrite ion contained in the electrolyte solution has almost no effect on the results of the determination (Table 3).

The discrepancy between the results proved to be within the limits of permissible errors; consequently, the content of alkali nitrite and nitrate ions in the electrolyte can be determined from a single sample.

The method was repeatedly tested on artificial mixtures; provided that all conditions for storage of the working solutions and titration are observed, it is readily reproducible.

REFERENCES

1. E. G. Kruglova, P. M. Vyacheslavova. Control of Electroplating Baths and Coatings, Mashgiz, p. 79, 1961.

2. D. S. Abramson, S. I. Orlova. Control of Electrolytes and Quality of Electroplated Coatings, Mashgiz, p. 51, 1950.

3. E. Knecht, E. Hibbert, Ber. 36, 166, 1903.