A New Method for Determining Hexavalent Chromium in the Electrolyte of an Acid Chromium Plating Bath
Abstract Generated abstract
A titrimetric method is proposed for determining hexavalent chromium in acid chromium plating electrolytes, intended as a more accurate alternative to the conventional reduction with Mohr’s salt followed by permanganate back titration. The procedure reduces chromium(VI) with standardized hydrogen peroxide in sulfuric acid in the presence of a small amount of silver nitrate, then back titrates excess hydrogen peroxide with potassium permanganate and calculates chromium trioxide content from the reagent consumption. Comparative tests on an electrolyte containing 250 g/l chromium trioxide showed smaller average error and better reproducibility than the conventional method, while omission of silver nitrate produced strongly inflated results. The authors attribute the improvement to inhibition by silver ions of hydrogen peroxide decomposition induced by perchromic acid, suggesting wider use of hydrogen peroxide in chromatometric determinations.
Full Text
I. P. Onufrienok, A. G. Borodulina, N. I. Gavryusheva
A NEW METHOD FOR DETERMINING HEXAVALENT CHROMIUM IN THE ELECTROLYTE OF AN ACID CHROMIUM PLATING BATH
(Presented by the scientific seminar of the Faculty of Chemical Technology)
Usually \(\mathrm{Cr}^{6+}\) in the electrolyte of acid chromium plating is determined according to the following procedure: 1 ml of electrolyte (taken with dilution) is placed in a 500 ml conical flask, diluted with water to 200 ml, 10 ml of \(\mathrm{H_2SO_4}\) (sp. gr. 1.84) and an excess of 0.1 N Mohr’s salt solution are added (until the color changes from yellow to green). The excess Mohr’s salt is back-titrated with 0.1 N \(\mathrm{KMnO_4}\) solution until a lilac coloration appears [1].
We have found that \(\mathrm{Cr}^{6+}\) is determined considerably more accurately in acid chromium-plating electrolytes by the method we propose, which is as follows.
To 1 ml of electrolyte (taken with dilution) in a 500 ml conical flask are added about 200 ml of water, 10 ml of \(\mathrm{H_2SO_4}\) (sp. gr. 1.84), 0.25 ml of a 2.5 percent \(\mathrm{AgNO_3}\) solution, and the mixture is titrated from a burette with 0.25 N \(\mathrm{H_2O_2}\) solution until a pure green color is obtained. The excess \(\mathrm{H_2O_2}\) is back-titrated with 0.1 N \(\mathrm{KMnO_4}\) solution until a lilac color appears. The content of \(\mathrm{CrO_3}\) in the electrolyte is calculated by the formula
\[ \mathrm{CrO_3}\ \mathrm{g/l}=\frac{(a n_1-\delta n_2)\cdot 33.3}{b}, \]
where \(a\) and \(n_1\) are the number of ml and the normality of the \(\mathrm{H_2O_2}\) solution used for titration;
\(\delta\) and \(n_2\) are the number of ml and the normality of the \(\mathrm{KMnO_4}\) solution used for titration;
33.3 is the milligram-equivalent of \(\mathrm{CrO_3}\);
\(b\) is the number of ml of electrolyte taken for analysis.
Silver wastes after titration of the sample are collected as a precipitate by adding \(\mathrm{NaCl}\) or \(\mathrm{HCl}\).
To compare these two methods, a chromium electrolyte containing 250 g of \(\mathrm{CrO_3}\) per liter was prepared. Table 1 gives the results obtained by the generally accepted first method.
Table 2 gives the results of analysis of the same electrolyte by the method we propose. From the data of this table it is evident what a great influence the addition of \(\mathrm{AgNO_3}\) has on the results of the analysis. Earlier, in one of our works [2], we observed that when a hydrogen peroxide solution is rapidly added to the titrated solution containing \(\mathrm{Cr}^{6+}\), inflated values are observed (relative to the theoretical ...
quantities) the consumption of hydrogen peroxide solution. We explained this by the fact that hydrogen peroxide and the perchromic acid formed induce each other’s decomposition. Therefore, for the broad use of hydrogen peroxide, instead of Mohr’s salt, it was necessary to find a suitable inhibitor protecting hydrogen peroxide from premature decomposition.
Table 1
| No. of experiments | Consumption of 0.1 N Mohr’s salt solution, ml | Consumption of 0.1 N KMnO₄ solution, ml | Titration time, min | Found CrO₃, g/l | Absolute error, g/l |
|---|---|---|---|---|---|
| 1 | 77,00 | 2,60 | 25 | 247,75 | −2,25 |
| 2 | 76,00 | 1,00 | 20 | 249,75 | −0,25 |
| 3 | 77,20 | 3,30 | 20 | 246,09 | −3,90 |
| 4 | 77,00 | 0,50 | 20 | 250,13 | +0,13 |
| Average | −1,57 |
Table 2
| No. of experiments | Addition of 2.5-percent AgNO₃ solution, ml | Consumption of 0.2700 N H₂O₂ solution, ml | Consumption of 0.1 N KMnO₄ solution, ml | Titration time, minutes | Found CrO₃, g/l | Absolute error, g/l |
|---|---|---|---|---|---|---|
| 1 | 0,25 | 28,00 | 0,60 | 17 | 249,75 | −0,25 |
| 2 | 0,25 | 28,00 | 0,65 | 16 | 249,58 | −0,42 |
| 3 | 0,50 | 28,00 | 0,70 | 14 | 249,42 | −0,58 |
| 4 | 0,50 | 28,00 | 0,80 | 14 | 249,08 | −0,92 |
| 5 | 1,00 | 28,00 | 0,70 | 17 | 249,42 | −0,58 |
| 6 | 1,00 | 28,00 | 0,80 | 17 | 249,08 | −0,92 |
| 7 | 1,00 | 28,33 | 1,55 | 5 | 250,18 | +0,18 |
| 8 | 1,00 | 33,40 | 6,90 | 4 | 250,35 | +0,35 |
| Average | −0,39 | |||||
| 9 | Without addition of AgNO₃ | 31,80 | 0,35 | 10 | 278,14 | +28,14 |
| 10 | Without addition of AgNO₃ | 31,20 | 0,30 | 5 | 272,97 | +22,97 |
| 11 | Without addition of AgNO₃ | 32,00 | 1,25 | 15 | 283,55 | +33,55 |
| 12 | Without addition of AgNO₃ | 31,20 | 0,55 | 18 | 278,68 | +28,68 |
| 13 | Without addition of AgNO₃ | 42,00 | 21,55 | 15 | 306,02 | +56,02 |
Such an inhibitor was found by us—it is silver ions; as can be seen from the tabular data, even a small amount of silver ions causes hydrogen peroxide to react in stoichiometric ratios. Table 2 also shows that the results for the determination of CrO₃ by the proposed method are more reproducible and more accurate.
Conclusions
- A new method has been developed for determining hexavalent chromium in electrolytes of an acid chromium-plating bath, possessing greater accuracy and reproducibility than the known methods.
- The inhibitor \(AgNO_3\) has been found, which protects hydrogen peroxide from decomposition induced by perchromic acid; the latter provides broad possibilities for replacing Mohr’s salt in chromatometry.
REFERENCES
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E. G. Kruglova, P. M. Vyacheslavov. Control of Electroplating Baths and Coatings. Mashgiz, Moscow–Leningrad, p. 46, 1961.
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I. P. Onufrienok, V. M. Aksenenko. Determination of tellurium in lead–antimony–tellurium alloys. Zh. “Analit. khimii”, vol. XIV, no. 5, 637–638, 1959.