I. P. Onufrienko, A. G. Borodulina, N. I. Gavryusheva

A New Method for Determining Trivalent Chromium in the Electrolyte of an Acid Chromium Plating Bath

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

The determination of trivalent chromium in the electrolyte of an acid chromium plating bath is carried out according to the following procedure. A 1 ml sample of the electrolyte (with dilution) is taken into a 500 ml conical flask, diluted with water to 200 ml, and 5 ml of \(H_2SO_4\) (specific gravity 1.84), 5 drops of a 1-percent solution of \(MnSO_4\), 10 ml of a 0.25-percent solution of \(AgNO_3\), and 20 ml of a 10-percent solution of \((NH_4)_2S_2O_8\) are added; the mixture is heated to boiling and then for an additional 5 minutes. Next, 5 ml of a 5-percent solution of \(NaCl\) is added and the solution is boiled until the pink color disappears and the \(AgCl\) precipitate coagulates; it is cooled, 20 ml of Zimmermann–Reinhardt mixture is introduced, and the mixture is titrated with 0.1 N Mohr’s salt solution to a green color. The excess Mohr’s salt is back-titrated with 0.1 N \(KMnO_4\) solution [1].

In a second sample, \(Cr^{6+}\) is determined, and \(Cr^{3+}\) is found from the difference.

In determining \(Cr^{3+}\) by this method, we observed a number of shortcomings; for example, the added portion of ammonium persulfate solution is not always sufficient to oxidize all the trivalent chromium, so this solution has to be introduced additionally. Further, it is difficult to establish when the crimson color of \(MnO_4^-\) has completely disappeared against the background of a large amount of hexavalent chromium. This prompted us to undertake the development of a simpler and more reliable method for determining \(Cr^{3+}\), which we were fully able to accomplish. Below we give the procedure of the method we propose for determining chromium in an acid chromic electrolyte.

Course of the analysis. A 1 ml sample of the electrolyte (with dilution) is taken into a 500 ml conical flask; about 200 ml of water and 5 ml of \(H_2SO_4\) (specific gravity 1.84) are added, after which the solution is heated and, during heating, 1 N \(KMnO_4\) solution is added dropwise until a stable pink coloration appears. The solution is boiled for 3 minutes. It is cooled to 50–70° and, with stirring, 1 N \(NaNO_2\) or \(KNO_2\) solution is added dropwise until the pink color disappears, plus an additional 2–3 drops. The flask with the solution is again heated to boiling and boiled for 5 minutes to destroy the excess \(NaNO_2\); after this it is cooled, 5 ml of \(H_2SO_4\) (specific gravity 1.84) is added, the solution is cooled to room temperature, 0.25 ml of a 2.5-percent solution of \(AgNO_3\) is added, and it is titrated with 0.25 N \(H_2O_2\) solution to a pure green color. The excess \(H_2O_2\) is back-titrated with 0.1 N \(KMnO_4\) solution to a lilac color.

The total chromium, as \(CrO_3\), is calculated by the formula

\[ B_1\text{—total chromium as }CrO_3,\ \mathrm{g/l} = \frac{(a n_1-\mathrm{б} n_2)\cdot 33.3}{b}, \]

where \(a\) and \(n_1\) are the number of ml and the normality of the \(H_2O_2\) solution used for titration;

\(\mathrm{б}\) and \(n_2\) are the number of ml and the normality of the \(KMnO_4\) solution used for titration;

\(b\) is the amount, in ml, of electrolyte taken for analysis;

33.3 is the milligram-equivalent of \(CrO_3\).

In a second sample of the same electrolyte, hexavalent chromium is determined as follows. One ml of electrolyte is taken, with dilution, into a 500-ml conical flask; about 200 ml of water, 10 ml of \(H_2SO_4\) (sp. gr. 1.84), and 0.25 ml of a 2.5 percent \(AgNO_3\) solution are introduced into the flask, and the mixture is titrated from a burette with 0.25 N \(H_2O_2\) solution to a pure green color. The excess \(H_2O_2\) is back-titrated with 0.1 N \(KMnO_4\) solution to a lilac color. \(CrO_3\) is calculated by the formula

\[ B\text{—content of }CrO_3,\ \mathrm{g/l} = \frac{(a_2H_1-\mathrm{б}_2H_2)\cdot 33.3}{b}, \]

where \(a_2\) and \(H_1\) are the number of ml and the normality of the solution used for titration;

\(\mathrm{б}_2\) and \(H_2\) are the number of ml and the normality of the \(KMnO_4\) solution used for titration;

\(b\) is the amount, in ml, of electrolyte taken for analysis;

33.3 is the milligram-equivalent of \(CrO_3\).

Finally, the \(Cr^{3+}\) content is calculated by the formula

\[ Cr^{3+}\ \mathrm{g/l}=(B_1-B)\cdot 0.52, \]

where \(B_1\) is the content of total chromium as \(CrO_3\), in g/l,

\(B\) is the content of \(CrO_3\), in g/l,

0.52 is the coefficient for converting \(CrO_3\) to \(Cr^{3+}\).

For comparison of these two methods, an electrolyte was prepared containing total chromium 250 g/l, calculated as \(CrO_3\). Of this amount, 241 g/l was \(CrO_3\) and 4.68 g/l was \(Cr^{3+}\). The results obtained by these two methods are given in Tables 1 and 2.

Table 1

Table 1 contains the results of determining \(Cr^{3+}\) by the first method, and Table 2 by the method proposed by us. On comparing the data of these tables, it is evident that the proposed method gives more consistent and more...

Table 2

more accurate results. In addition, the method we propose is simpler and less expensive, since it does not require the use of ammonium persulfate or the Zimmermann–Reinhardt mixture. Silver is collected as waste for regeneration, by precipitation from the titrated solutions with sodium chloride or hydrochloric acid.

Conclusions

1. A new method has been developed for determining trivalent chromium in electrolytes of an acid chromium-plating galvanic bath; it is more accurate and more economical than existing methods.

2. It has been shown that, when Mohr’s salt is replaced by hydrogen peroxide in the determination of Cr⁶⁺ and Cr³⁺, the results obtained are more accurate, reproducibility increases, and the need to use the Zimmermann–Reinhardt mixture is eliminated; the latter considerably reduces the cost of the analysis.

Literature

1. E. G. Kruglova, P. M. Vyacheslavov. Control of Electroplating Baths and Coatings. Mashgiz, Moscow–Leningrad, pp. 46–47, 1961.