R. D. Glukhovskaya, Yu. L. Lel’chuk

MERCURIMETRIC METHOD FOR THE DETERMINATION OF CHLORIDES IN ELECTROLYTES OF NICKEL PLATING BATHS

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

Mercurimetric methods for the determination of halides are finding ever wider application in the practice of chemical analysis. These methods are fairly accurate and are not associated with the use of such a scarce reagent as silver nitrate. Among the mercurimetric methods, the most justified have proved to be those based on the use of diphenylcarbazone and β-nitroso-α-naphthol as indicators; however, the former gives an indistinct color transition at the equivalence point, while the latter tends to form supersaturated solutions, which leads to premature precipitation and incorrect fixing of the equivalence point.

Previously [1] we proposed a new mixed indicator that proved to be quite suitable for the mercurimetric determination of chlorides, bromides, and iodides. This indicator is a mixture of two mercurimetric indicators: diphenylcarbazone and β-nitroso-α-naphthol. A detailed study showed that it lacks the disadvantages of each indicator taken separately. When titration is carried out in the presence of alcohol, it permits titration over a fairly wide range of acidity; the color transition at the equivalence point is sharp, even when titrating rather dilute (0.01 N) solutions, and no precipitation of any kind occurs.

The use of the mixed indicator enabled us to develop methods for the determination of chlorine, bromine, and iodine in organic compounds [2]. In the present work, a mercurimetric method is proposed for the determination of chlorides in electrolytes of nickel plating baths.

A number of methods have been proposed for the determination of chlorides in nickel electrolytes. The most widespread of these are volumetric argentometric methods using potassium chromate as the indicator (Mohr method), and indirect determination with ammonium thiocyanate in the presence of ferric ammonium alum (Volhard method) [3, 4]. These methods are fairly simple, but they are associated with the use of the scarce reagent silver nitrate. It is known, moreover, that the use of argentometric methods in routine analyses is extremely limited and is permissible only with subsequent regeneration of the silver residues, which complicates the procedure.

Recently a mercurimetric method has been proposed for the determination of chlorides in an alcoholic medium with diphenylcarbazone [3]. The essence

The method consists of the following: 2–3 ml of electrolyte is evaporated to dryness in a porcelain dish on a water bath. The cooled dry residue is dissolved in nitric acid; 20 ml of alcohol and diphenylcarbazone are added and, while stirring in the dish with a glass rod, the solution is titrated with mercuric nitrate solution until a violet color appears. We checked this procedure and obtained strongly divergent results. This is apparently explained by the fact that, owing to the gradual transition of the indicator color, it is difficult to fix the end point of the titration; moreover, the method is complicated by the additional evaporation of the electrolyte.

By using a mixed indicator, we succeeded in developing a rapid and accurate method for determining chlorides in nickel-plating baths. The color transition occurs rather sharply with 1–2 drops of approximately 0.03 N mercuric nitrate solution. The reproducibility of the results is good.

In order to determine the influence of various substances that may be present in a nickel electrolyte on the mercurimetric determination of chlorides with a mixed indicator, we carried out a series of titrations in the presence of foreign ions. To 1 ml of sodium chloride (Cl′ content 3.546 mg/ml) we added various amounts of solutions of salts and other additives, 2 drops of diphenylcarbazone, 2 drops of β-nitroso-α-naphthol, neutralized with nitric acid to a green color, and titrated with mercuric nitrate solution until the color changed from green to pink or red. The titration results are given in Table 1.

The approximate content of ions making up ordinary nickel electrolytes does not exceed: 300 mg/ml Ni, 130 mg/ml SO″₄, 7 mg/ml Cl′, 30 mg/ml H₃BO₃, 3 mg/ml saccharin, and 0.1 mg/ml furfural. As can be seen from Table 1, additives within the indicated amounts have little influence on the titration results, which makes it possible to determine chlorides in their presence. Large deviations are obtained when saccharin is added. The latter, as a blank test showed, is contaminated with chlorine ions.

The procedure was tested for the determination of chlorides in an electrolyte of the following composition:

Nickel sulfate — 250 g/l,
Nickel chloride — 20 g/l,
Boric acid — 30 g/l,
Saccharin — 3 g/l,
Furfural — 0.4 g/l,
Ethyl alcohol — 1.5 ml/l.

However, when chlorides were titrated in the electrolyte, at the moment the alcohol was added a green precipitate formed, which did not disappear until the very end of the titration. This led to a diffuse color transition, and the results were not reproducible. Meanwhile, when a sodium chloride solution was titrated in the presence of fairly large amounts of foreign ions, we did not observe such a phenomenon. This can apparently be explained by the large salt effect. Our supposition was confirmed by titrating the electrolyte with the addition of a certain amount of sodium nitrate solution or dry sodium nitrate salt. The results of the experiments are given in Table 2.

From the data presented it is evident that titration of chlorides in the presence of a certain amount (approximately 300 mg/ml) of sodium nitrate proceeds smoothly. The solution remains clear to the end and, at the equivalence point, the indicator changes color from green to pink with 1–2 drops of mercuric nitrate solution.

Table 1

Mercurimetric titration of sodium chloride with a mixed indicator
in the presence of various additives

Chlorine content in all experiments: 3.546 mg

Table 2

Mercurimetric titration of electrolyte in the presence of sodium nitrate

Required reagents: 1. An approximately 0.03 N solution of mercuric nitrate: 5–6 g of nitric-acid mercuric oxide are dissolved dropwise in concentrated nitric acid until the salt is completely dissolved, and the volume of the solution is brought to 1 liter. If, on standing for 24 hours, a precipitate separates, the solution is filtered. 2. A standardized solution of sodium chloride. 3. Dilute ($\sim 0.05$ N) nitric acid. The nitric acid used must be checked for the absence of chlorides. 4. A 1% alcoholic solution of diphenylcarbazone. 5. A 1% alcoholic solution of $\beta$-nitroso-$\alpha$-naphthol. 6. 96% ethyl alcohol.

Procedure: Into a 50 ml conical flask, transfer 1 ml of the electrolyte solution, add about 300 mg of sodium nitrate, 2 drops of diphenylcarbazone, 2 drops of $\beta$-nitroso-$\alpha$-naphthol, neutralize with nitric acid to a green color, and add 5 ml of alcohol. The contents of the flask are shaken and titrated with mercuric nitrate until the color of the solution changes from green to red. The titer of the mercuric nitrate is established in the same way, but instead of the electrolyte a sodium chloride solution is taken. The chlorine content is calculated by the formula

\[ B_{\mathrm{Cl}} = T \cdot V \cdot 1000, \]

where $B_{\mathrm{Cl}}$ is the chlorine content in g/l;
$T$ is the titer of mercuric nitrate with respect to chlorine;
$V$ is the amount, in ml, of mercuric nitrate used for the titration.

For comparison of the results, chlorides were simultaneously determined by the gravimetric method. The experimental data are summarized in Table 3.

Table 3

Verification of the mercurimetric determination of chlorides in a nickel electrolyte with a mixed indicator

As can be seen from the table, the results of the mercurimetric determination with a mixed indicator agree well with the data obtained by the gravimetric method. The average relative error is 0.18%. Reproducibility is good. Student A. N. Artsimovich participated in carrying out the experimental part of the work.

Conclusions

A mercurimetric method is proposed for the determination of chlorides in electrolytes of nickel plating baths. The method is simple, accurate, and gives well reproducible results.

References

1. R. D. Glukhovskaya, N. A. Ugolnikov, A new mixed indicator in mercurimetry. Izv. vuzov, Chemistry and Chemical Technology, 3, 49, issue 1, 1960.

2. R. D. Glukhovskaya, N. A. Ugolnikov, Z. M. Murav’eva. Mercurimetric method for determining halogens in organic compounds. Izv. TPI, 92, 169, 1960.

3. E. G. Kruglova, P. M. Vyacheslavov. Control of electroplating baths and coatings. Mashgiz, Moscow–Leningrad, 25, 1961.

4. G. T. Bakhvalov, A. N. Birktgan, V. P. Labutin. Handbook for the Electroplater. Metallurgizdat, Moscow, 287, 1954.