I. P. Onufrienok, L. L. Skripova

New Developments in the Control of the Oxalic-Acid Electrolyte of an Anodizing Galvanic Bath

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

Usually, as the electrolyte of a galvanic anodizing bath, a solution containing 30 g/L of oxalic acid is used [1].

During operation of the bath, aluminum, iron, and magnesium oxalate salts may accumulate in the oxalic-acid electrolyte. Some of these salts are present in solution (salts of Al³⁺, Fe³⁺ and, in part, Mg²⁺); another portion of the salts forms precipitates (salts of Fe²⁺, Mg²⁺, and other divalent cations).

If the oxalic-acid solution can easily be freed from precipitates of salts by periodic filtration, then it does not appear possible to free the electrolyte from salts present in solution by simple operations. Apparently, salts of trivalent iron and aluminum can accumulate in solution until all the free oxalic acid has been used up. As for magnesium salts, according to our experiments, no more than 0.6 g/L of them can accumulate in an oxalic-acid solution; larger amounts of magnesium already precipitate in the form of magnesium oxalate.

In factory laboratories, control of the oxalic-acid electrolyte is usually reduced to determining the total content of oxalate ion by permanganatometry and to determining aluminum. The latter is precipitated with o-oxyquinoline, with a gravimetric finish. On the basis of these two determinations, the presence of free oxalic acid is judged. If determination of the oxalate ion takes 15 minutes, the gravimetric determination of aluminum requires no less than four hours. Thus, an indication of the presence of free oxalic acid in the electrolyte can be obtained no earlier than after 4 hours.

Working on improving the control of the oxalic-acid electrolyte, we found that an indication of the presence of free oxalic acid, sufficient in accuracy, can be obtained with an expenditure of only 20–30 minutes; and a complete analysis of the electrolyte, with determinations of the oxalate ion, free oxalic acid, iron, aluminum, and magnesium, can be carried out in 1–1.5 hours.

A study of the behavior of oxalic-acid electrolytes composed of definite amounts of free oxalic acid and oxalate salts of aluminum, iron, and magnesium during analyses showed that: a) the total amount of oxalate ion is accurately determined by titration with permanganate in a sulfuric-acid medium; b) by titration with sodium hydroxide

in the presence of the phenol red indicator, the presence of free oxalic acid is accurately determined; c) titration with caustic soda in the presence of phenolphthalein as an indicator gives the sum of free oxalic acid and oxalic acid bound to iron and aluminum ions (the latter determination is accurate in the absence of magnesium; in the presence of magnesium, somewhat elevated results are obtained); d) iron, aluminum, and magnesium are determined with sufficient accuracy complexometrically. This enabled us to propose the following analytical procedures for the control of oxalic-acid electrolytes.

Determination of the total oxalate-ion content

Into a 100-ml conical flask, 5 ml of electrolyte is pipetted, 20 ml of 4 N sulfuric acid is added, the mixture is heated to 70–80°, and titrated with 0.1 N KMnO₄ solution to a stable pale-pink color. The results are calculated as H₂C₂O₄·2H₂O by the formula

\[ \text{total } \mathrm{H_2C_2O_4\cdot 2H_2O}\ \mathrm{g/l} = \frac{a n \cdot 63.03}{b}, \]

where \(a\) and \(n\) are the number of ml and the normality of the KMnO₄ solution consumed;

\(b\) is the number of ml of electrolyte taken for analysis;

63.03 is the milligram-equivalent of \(\mathrm{H_2C_2O_4\cdot 2H_2O}\).

Determination of free oxalic acid

Into a 100-ml conical flask, 5 ml of electrolyte is pipetted, 5 drops of a 0.1-percent solution of phenol red indicator are added, and the mixture is titrated with 0.1 N caustic soda solution until a clearly visible transition of the yellow color to red. The results are calculated by the formula

\[ \mathrm{H_2C_2O_4\cdot 2H_2O}_{\text{free}}\ \mathrm{g/l} = \frac{a_1 \cdot n \cdot 63.03}{b}, \]

where \(a_1\) and \(n\) are the number of ml and the normality of the caustic soda solution used for the titration; \(b\) is the number of ml of electrolyte taken for analysis; 63.03 is the milligram-equivalent of \(\mathrm{H_2C_2O_4\cdot 2H_2O}\).

Determination of oxalic acid bound to iron and aluminum

Into a 100-ml conical flask, 5 ml of electrolyte is pipetted, 5 drops of a 0.1-percent solution of phenolphthalein indicator are added, and the mixture is titrated with 0.1 N caustic soda solution to a clearly visible red-violet color. The calculation is made by the formula

\[ \text{bound to } \mathrm{Al^{3+}} \text{ and } \mathrm{Fe^{3+}}\ \mathrm{H_2C_2O_4\cdot 2H_2O}\ \mathrm{g/l} = \frac{(a_2-a_1)n \cdot 63.03}{b}, \]

where \(a_2\) is the number of ml of NaOH solution used for titration with the phenolphthalein indicator; \(a_1\) is the number of ml of NaOH solution used for titration with the phenol red indicator; \(n\) is the normality of the solution; 63.03 is the milligram-equivalent of \(\mathrm{H_2C_2O_4\cdot 2H_2O}\).

Determination of iron and aluminum

As the basis for developing a method for determining iron and aluminum, we used the work of A. A. Bashkirzev and E. M. Yakimets [2], who were

The conditions for determining these elements with Trilon B have been studied. The only difficulty was that oxalate ions interfere with the complexometric determination of iron and aluminum. It was necessary to select an oxidizing agent for destroying the oxalate ions such that the products of its interaction would not affect the complexometric determination of aluminum and iron; ammonium persulfate proved to be such an oxidizing agent.

Course of the analysis. Into a 100-ml heat-resistant chemical beaker, 20 ml of electrolyte, 5 ml of concentrated sulfuric acid are introduced with a pipette, and the mixture is evaporated on a hot plate until white fumes of sulfuric acid appear. The beaker is cooled, the walls of the beaker are carefully washed with a small amount of water (10–20 ml), and 1 g of ammonium persulfate is added; the beaker is covered with a watch glass and placed on the hot plate, heated until the excess ammonium persulfate is completely decomposed, then, after slightly shifting the glass, it is evaporated until dense white fumes appear and cooled. It is diluted with 10–20 ml of water, 1–2 ml of concentrated hydrochloric acid is added, and the mixture is heated until the salts are completely dissolved.

After dissolution, the solution is transferred to a 250-ml conical flask, where the solution is neutralized with concentrated ammonia, and toward the end with diluted (1:10) ammonia to pH = 1–2 (spot test on universal indicator paper).

Next the sample is heated to 50–60°, 1 ml of a 10% solution of sodium sulfosalicylate is added; if iron is present in the sample, the solution turns violet-cherry in color; then the sample is titrated with 0.1 N Trilon B solution until the violet-cherry coloration disappears. The color at the end of titration becomes almost yellow. The consumption, in ml, of Trilon B solution used for titrating the iron is recorded. Aluminum is then determined in the sample; for this purpose an excess of Trilon B solution (10 ml) is introduced into the sample, the solution is heated to the beginning of boiling, neutralized with diluted (1:10) ammonia to pH = 5 (spot test on universal indicator paper), after which 10 ml of acetate buffer solution with pH = 4.8 is added to the solution and it is cooled under the tap to room temperature. The cold solution is titrated with 0.05 N ferric chloride solution until the appearance of a stable pale cherry-violet coloration (the transition is rather sharp). The contents of iron and aluminum are calculated by the formulas:

\[ \mathrm{Fe}^{3+}\ \mathrm{g}/l=\frac{a\cdot n\cdot 27.925}{b}, \]

where \(a\) and \(n\) are the volume in ml and the normality of the Trilon B solution used for titrating the iron;

27.925 is the milligram-equivalent of iron;

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

\[ \mathrm{Al}^{3+}\ \mathrm{g}/l=\frac{(a_1 n-\delta n')\cdot 13.485}{b}, \]

where \(a_1\) and \(n\) are the volume in ml and the normality of the excess Trilon B solution taken for the determination of aluminum;

\(\delta\) and \(n'\) are the volume in ml and the normality of the ferric chloride solution used for titration;

13.485 is the milligram-equivalent of aluminum;

\(b\) is the volume of electrolyte, in ml, taken for analysis.

The acetate buffer solution is prepared by mixing 2 N sodium acetate solution and 2 N acetic acid solution (1:1).

Determination of magnesium

Into a 100 ml beaker, 10 ml of electrolyte is measured with a pipette; 20 ml of water, 5 ml of concentrated HCl, and 5 ml of concentrated ammonia are added; the mixture is heated to boiling, and the hydroxides of iron and aluminum are precipitated with ammonia until a perceptible excess odor of ammonia is present. After standing for 2–3 min, the precipitate is filtered off and washed 4–6 times with a 2-percent hot solution of ammonium chloride. The filtrate is collected in a 250 ml conical flask. After cooling, 5 ml of chloride-ammonia buffer solution and a pinch of the indicator Eriochrome Black T, ground with KCl in the ratio (1:100), are added to the filtrate, and it is then titrated with 0.1 N Trilon B solution until the cherry-red color changes to pure blue. The magnesium content is calculated from the formula

\[ \mathrm{Mg}^{2+}\, g/l=\frac{a\cdot n\cdot 12.16}{b}, \]

where \(a\) and \(n\) are the volume in ml and the normality of the Trilon B solution used for titrating magnesium;
\(12.16\) is the milligram-equivalent of magnesium;
\(b\) is the volume of electrolyte taken for the determination of magnesium.

The chloride-ammonia buffer solution is prepared by dissolving 20 g of NH\(_4\)Cl and 100 ml of concentrated ammonia in water, followed by making the volume of the solution up to 1 liter.

Applying the above methods of control to oxalic-acid electrolytes containing specified amounts of free oxalic acid and its salts, we obtained the data given in Tables 1 and 2. Table 1 contains data for an electrolyte containing oxalic acid and its salts with Fe\(^{3+}\) and Al\(^{3+}\).

Table 1

Table 2 gives the analysis of an electrolyte containing oxalic acid and its salts with Fe\(^{3+}\), Al\(^{3+}\), and Mg\(^{2+}\). From the data of Tables 1 and 2 it is seen that the content of free oxalic acid is determined quite accurately even in the presence of considerable amounts of magnesium in the electrolyte, and for in-shop control this one determination is sufficient. Titration of the sample with caustic soda using phenolphthalein as indicator and preliminary determination of free oxalic acid makes it possible to judge contamination of the bath with iron and alumi-

These two determinations can be carried out in 15–20 minutes and, consequently, there is full possibility of rapidly correcting the bath. A full analysis may be performed occasionally for control.

Table 2

Conclusions

1. Rapid methods have been developed for control of oxalic-acid electrolytes of anodizing galvanic baths, possessing sufficient accuracy.

2. For shop control it is sufficient to perform two titrations of the electrolyte with 0.1 N sodium hydroxide solution: with phenol red indicator (free oxalic acid is determined) and with phenolphthalein indicator (the sum of free oxalic acid and acid bound with iron and aluminum is determined).

LITERATURE

1. G. T. Bakhvalov, L. N. Birkgan, V. P. Laburin. Handbook of Electroplating. Metallurgizdat, Moscow, 214–219, 1954.

2. A. A. Bashkirtseva and E. M. Yakimen. On the complexometric determination of aluminum. Zh. zav. lab., No. 10, 1166, 1959.