AN ACCURATE TITRIMETRIC METHOD FOR THE DETERMINATION OF SMALL AMOUNTS OF COPPER IN NON-FERROUS METAL ORES

V. B. SOKOLOVICH, Yu. L. LELCHUK, B. N. BESPROZVANNYKH

(Presented by the scientific seminar of the Department of Analytical Chemistry)

Earlier [1] we showed that the determination of copper in non-ferrous metal ores, with assurance to the second decimal place when its content is within tenths of a percent, can be achieved by a gravimetric method based on the precipitation of copper by Reinecke’s salt in the form of the diaminotetrapyridinochromate complex from a two-gram ore sample and weighing the complex after drying it at 110°C.

The principal advantages of this method are the selective action of Reinecke’s salt on copper, which makes it possible to determine copper in the presence of almost all other metals usually encountered in ores, and the comparatively small conversion factor from the weighed form to copper [2].

A disadvantage of the method, which could limit its use in routine analyses, is the relative length of the determination, characteristic of all gravimetric methods of analysis. In this connection we continued our investigation with the aim of replacing the gravimetric finish by a more convenient and rapid titrimetric finish.

Dymov and Shchelkunova [3] determined copper in alloyed steels with Reinecke’s salt using both a gravimetric and a volumetric finish. The volumetric finish consisted in oxidizing trivalent chromium in the diaminotetrapyridinochromate copper complex to the chromate ion and then determining it permanganatometrically. The method guaranteed only tenths of a percent of copper.

Bagbanly [4], determining copper in steels, precipitated it with a 2.5% solution of Reinecke’s salt, dissolved the copper chromate complex in a 5% alkaline solution, and, in hydrochloric-acid medium, titrated monovalent copper and rhodanide ion with potassium iodate.

Our experiments showed that the iodometric finish is laborious, and determination of the equivalence point is often accompanied by a considerable error.

By replacing the iodometric finish with a bromatometric one, we succeeded in making this method more rapid and accurate, guaranteeing the determination of hundredths of a percent of copper in ores.

Our method is based on reducing copper from a gram sample of ore to the monovalent state by means of \(K_2SnCl_4 \cdot 2H_2O\), precipitating it with Reinecke’s salt, decomposing the complex

\[ \mathrm{Cu[Cr(NH_3)_2(CNS)_4] \cdot H_2O} \]

with a two-percent solution of sodium hydroxide upon heating, and titrating monovalent copper and the rhodanide ion with potassium bromate in a hydrochloric-acid medium in the presence of methyl orange.

Experimental Part

To a conical flask with a stopper, 0.5 g of ammonium fluoride and 10 ml of hydrochloric acid, sp. gr. 1.19, were added to 1 g of finely ground ore. After boiling for five minutes, 5 ml of nitric acid, sp. gr. 1.40, was added to the same flask, and boiling was continued for 25–30 min. The stopper was then removed, 10 ml of sulfuric acid (1:1) was introduced into the flask, and the whole was evaporated until dense fumes of sulfuric anhydride appeared. After the walls of the flask had been rinsed, fuming was continued for several more minutes, until the nitrogen oxides had been completely removed. To the decomposed and cooled sample were added 50 ml of water and 10 ml of hydrochloric acid (1:1), and heating was continued until the sulfates dissolved. The insoluble residue was filtered off and washed with hot water until the reaction for iron with yellow prussiate of potash was negative. The filtrate was diluted with water to a volume of 120–150 ml, heated almost to boiling, and, with stirring, a 25% solution of \(K_2SnCl_4 \cdot 2H_2O\) was added dropwise until the reaction mixture was completely decolorized, followed by a subsequent excess of 2 ml [5].

To the hot reduced solution, freshly prepared 1% Reinecke’s salt solution was added in small portions with continuous shaking until the solution above the precipitate became reddish. To coagulate the precipitate, the reaction mixture was kept for about 15 min on a boiling water bath, cooled to 40–30°, and the precipitate was filtered through an asbestos plug, washed three times with 5% hydrochloric acid solution, five times with warm water, and transferred together with the plug into the flask in which the precipitation had been carried out. The funnel was washed over the flask with hot 2% sodium hydroxide solution, which was added to the precipitate in a volume of 20 ml. Then, carefully, with periodic shaking, the reaction mixture was heated on a boiling water bath for about 10 min, until the precipitate dissolved and the pink coloration of the solution disappeared completely. The walls of the flask were wiped and washed with small portions of hot water.

Our observations showed that the accuracy of the entire determination depends to a considerable extent on the care with which the process of treating the precipitate with alkali is carried out. Heating must not be stopped before the pink coloration of the solution disappears. Nor should heating be continued after the pink coloration has disappeared. In the first case, high results were obtained; in the second, the solution became pinkish and the results of the determination were low.

The colorless or slightly grayish solution obtained was rapidly cooled under the tap, acidified with 15 ml of hydrochloric acid, sp. gr. 1.19, cooled again, and, after the addition of 2–3 drops of methyl orange, slowly (at a rate of 1–2 drops per second), with vigorous—

with shaking was titrated with 0.1 N potassium bromate solution until the pink coloration of methyl orange disappeared.

If the color of the indicator weakened during the titration, another 1–2 drops of methyl orange were added to the reaction mixture. The end of the titration was checked by adding one additional drop of indicator, which was decolorized on shaking.

The chemistry of the processes of decomposition of copper diamminetetrarhodanochromate by alkali and oxidation of the decomposition products by potassium bromate in a hydrochloric-acid medium can be expressed by the following equations:

\[ \mathrm{Cu[Cr(NH_3)_2(CNS)_4] + 4NaOH = CuCNS + 2NH_3 + NaCrO_2 +} \]

\[ \mathrm{+\,3NaCNS + 2H_2O} \]

\[ \mathrm{6CuCNS + 18NaCNS + 25KBrO_3 + 18HCl + 21H_2O =} \]

\[ \mathrm{= 6CuSO_4 + 18H_2SO_4 + 24HCN + 18NaCl + 25KBr.} \]

In accordance with the structure of the molecules HCNS \((\mathrm{C}\rightleftarrows \mathrm{N}\!-\!\mathrm{H}\) with \(\mathrm{S}\) bonded to \(\mathrm{C})\) and HCN \((\mathrm{C}\!-\!\mathrm{H}\rightleftarrows \mathrm{N})\), the following electronic equations of the process may be written:

\[ \begin{aligned} \mathrm{Cu}^{+} - \bar e &= \mathrm{Cu}^{2+} \\ 4\mathrm{S}^{2-} - 32\bar e &= 4\mathrm{S}^{6+} \\ 4\mathrm{C}^{4+} + 8\bar e &= 4\mathrm{C}^{2+} \end{aligned} \left\} 25\bar e\right. \]

\[ \mathrm{Br}^{5+} + 6\bar e = \mathrm{Br}^{1-}. \]

It follows from these equations that one molecule of copper diamminetetrarhodanochromate loses 25 electrons, and the reducing equivalent of copper in this reaction is only \(1/25\) of its atomic weight. This circumstance is chiefly responsible for the high accuracy of the method and its suitability for determining small amounts of copper.

Table 1

Determination of copper in solutions of pure copper salts

Table 2

Determination of copper in copper–nickel sulfide ores

Table 3

Checking the accuracy of the method by the addition method

Reinecke salt and potassium chlorostannite were synthesized by us according to the procedure described by Feiberg [2].

Table 1 gives the results of some of our determinations of copper by the above-described procedure in solutions of pure copper salts; Table 2 gives those for two samples of copper–nickel sulfide ores; and Table 3 gives the results of checking the accuracy of the method by the addition method. In the latter case, corresponding amounts of copper, in the form of a titrated solution of a copper salt obtained from electrolytic copper, were added to accurately weighed ore samples.

Conclusions

An accurate titrimetric method is proposed for the determination of small amounts of copper in ores. The method is based on the reduction of copper to the mon—

of the monovalent state with potassium chlorostannite, precipitation of it with Reinecke salt, destruction of the complex with alkali, and titration of monovalent copper and of the rhodanide ion with potassium bromate in a hydrochloric-acid medium in the presence of methyl orange.

The maximum deviation from the mean value in 22 determinations of copper in copper–nickel sulfide ores by this method is \(\pm 0.005\%\). The time required for four parallel determinations after decomposition of the ore is about two hours.

REFERENCES

1. Sokolovich V. B., Onufrienok I. I., Besprozvannykh B. N., Izv. TPI, 111, 11, 1961.
2. Feinberg S. Yu. Analysis of Non-Ferrous Metal Ores. Metallurgizdat, Moscow, 1953.
3. Dynob A. I. and Shchelkunova A. N. Zavodskaya laboratoriya, 20, 270, 1954.
4. Bagbanly I. L. Dokl. AN Azerb. SSR, 12, 639, 1956.
5. Analysis of Mineral Raw Materials. Goskhimizdat, Leningrad, 1956.