On the Determination of Cobalt with 1,2-Pyridylazo-2-naphthol
Abstract Generated abstract
This study evaluates the use of 1,2-pyridylazo-2-naphthol for photocolorimetric determination of small amounts of cobalt, with particular reference to potential analysis of copper nickel sulfide ores. The authors synthesized the reagent, prepared cobalt sulfate standards, extracted the cobalt complex into isoamyl alcohol at about pH 5, and measured optical density under specified colorimetric conditions using acetate or citrate phosphate buffers. Repeated determinations showed large variations in optical density for identical cobalt amounts, and both the cobalt complex and the blank solution exhibited decreasing color intensity on standing. The results indicate that the previously proposed colorimetric method is not sufficiently reliable, even for pure cobalt salt solutions.
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On the Determination of Cobalt with 1,2-Pyridylazo-2-naphthol
Yu. L. Lel’chuk, L. L. Skripova
(Presented by the scientific seminar of the Department of Analytical Chemistry)
1,2-Pyridylazo-2-naphthol was first synthesized by Chichibabin [1]. Liu [2], and later Cheng and Bray [3], investigated the possibility of using this compound in analytical chemistry. According to the latter authors, this reagent, which is yellow in water, carbon tetrachloride, and isoamyl alcohol, gives colored complex compounds with many heavy metals. The stability of these complexes depends to a considerable extent on the acidity of the solutions. Most metals form red complexes; cobalt and palladium form green complexes. The complex of divalent cobalt is stable in the pH interval 3.5–5.0, and that of trivalent cobalt in the interval 0.1–1.0. Ions of the alkali and alkaline-earth metals, aluminum, titanium, zirconium, ruthenium, rhodium, tellurium, iridium, monovalent mercury and ammonium, as well as the anions of sulfuric, nitric, hydrochloric, phosphoric, chromic, sulfurous, bromic, hydrobromic, hydroiodic, arsenic, acetic, and oxalic acids, do not give colored compounds or precipitates with this reagent.
The authors propose using 1,2-pyridylazo-2-naphthol as an indicator in the complexometric titration of zinc, copper, and cadmium, and as a specific reagent for the colorimetric determination of cobalt and palladium.
We carried out experiments to test the possibility of using this reagent for the photocolorimetric determination of small amounts of cobalt in copper–nickel sulfide ores.
1,2-Pyridylazo-2-naphthol was synthesized by the method described by Cheng and Bray [3]. To obtain the colored cobalt complex, we used a 0.1-percent solution of this reagent in twice-distilled methyl alcohol. Experiments were carried out with solutions containing 0.2, 4, 6, 8, and 10 γ of cobalt in the form of cobalt sulfate, which we obtained after two recrystallizations of a commercial chemically pure preparation. The cobalt concentration was determined gravimetrically with α-nitroso-β-naphthol according to the generally accepted procedure [4]. Cobalt was weighed as Co₃O₄.
To the solution containing the appropriate amount of cobalt sulfate, in a separatory funnel of 50 ml capacity, a buffer mixture was added to maintain the pH of the solution at 5, followed by 0.2 ml of a 0.1-percent
a solution of 1,2-pyridylazo-2-naphthol in methyl alcohol, a drop of saturated sodium periodate solution, and 10 ml of isoamyl alcohol measured from a burette. After vigorous shaking of the mixture for 5 minutes, separation of the phases, and isolation of the isoamyl layer, the optical density of the extract was measured on an FEK-M photoelectric colorimeter in a 20 mm cuvette with a red light filter and pure isoamyl alcohol as the reference solution. To maintain the pH of the solutions at approximately five, in the first series of experiments we each time added 1 ml of concentrated hydrochloric acid and 20 ml of a two-molar solution of sodium acetate; in the second series of experiments we used a buffer mixture consisting of citric acid and sodium hydrogen phosphate.
Our experiments showed that the optical density of solutions with one and the same amount of cobalt, in several repeated determinations, varies over wide limits. Thus, for example, for 6 γ of cobalt the optical density in five successive determinations was 0.289; 0.305; 0.319; 0.242; 0.253; for 10 γ of cobalt—0.359; 0.332; 0.442; 0.412.
In all our experiments, the maximum intensity of the color of the solutions was reached five minutes after mixing the reagents and vigorously shaking the mixture. However, upon further standing there is a gradual, noticeable decrease in the intensity of the color. Thus, for example, the optical density of a solution containing 4 γ of cobalt, after 25 minutes of standing, gradually decreased from 0.232 to 0.162. This fact undoubtedly indicates the instability of the colored cobalt complex under these conditions.
The color of the blank solution, prepared by adding all the reagents in the same sequence, only without cobalt, also proved unstable. In 20 minutes its optical density, when pure isoamyl alcohol was used as the reference solution, decreased from 0.072 to 0.018.
Thus, the method for the colorimetric determination of cobalt with 1,2-pyridylazo-2-naphthol proposed by Cheng and Bray does not give even moderately satisfactory results even in the case of pure cobalt salts.
LITERATURE
- A. E. Chichibabin, Journal of the Russian Physico-Chemical Society, 47, 1582, 1915.
- G. Ch. Liu, Ph. D. thesis. University of Illinois, 1951.
- Kuang Lu Cheng and Roger H. Bray, Anal. Chemistry 27, No. 5, 782, 1955.
- V. F. Hillebrand, G. E. Lundell, G. A. Bright, J. I. Hoffman, Practical Handbook of Inorganic Analysis, Goskhimizdat, Moscow, 1957.