Cobalt compounds have been used for coloring glass for at least four thousand years. Cobalt is a silvery white metal. Like iron and nickel, cobalt is ferromagnetic. It is the only element capable of increasing the magnetization of iron (i.e., 65% Fe and 35% Co). It has an outstanding magnetic property and has the highest known curie temperature of 1121˚C. 1 Widia metal, a cobalt-containing alloy comprising 13% Co, 84% W and 3% carbon is the hardest alloy known. Magnetic alloys consume 20% of cobalt product, and a large number of such alloys are available for specific purposes. Among the permanentmagnetic materials are high-cobalt (30 -50%) steels and Alnico-type ferrous alloys containing 12% Al, 20% Ni and 5 -25% cobalt. Cobalt-60 is an important source of radiaoactivity, which has been widely used in biological work in tracer studies, in metal plating, corrosion, catalysis and in medicine in the distant therapy treatment of Malignancies. 3,4 Cobalt may be determined gravimetrically by precipitation with pyridine in a thiocyanate media and weighing as Co(C5H5N)4(SCN)2.Volumetrically, Co(II) salts may be determined by potentiometric titration with standard K4Fe(CN)6 in the presence of a high concentration of ammonium citrate and ammonia. Colorimetric method for determining cobalt uses the blue color developed by chloride ion or thiocyanate ion when small amounts of cobalt are present. The method reported in this paper is the first of its kind in the literature for the determination of Co(II) present in small quantities using a masking technique. Experimental Reagents and solutionsAll of the chemicals used were of analytical reagent or pure grade. A Co(II) standard solution was prepared by dissolving cobaltous chloride in distilled water. An EDTA solution (0.02 M) was prepared in distilled water. A lead nitrate solution (0.02 M) was as prepared by dissolving Pb(NO3)2 in distilled water and was standardized by the chromate method.5 Xylenol Orange indicator was made by using ground potassium nitrate crystals (1:100); a 1% solution of 1,10-phenanthroline was made up in alcohol. ProcedureTo a solution containing 0.5 -12.0 mg of Co(II), a 0.02 M EDTA solution was added in excess. The solution was diluted to about 80 ml and the pH was adjusted to 5.0 -6.0 with hexamine. The unreacted EDTA was titrated with a lead nitrate solution using Xylenol Orange as an indicator. A 1,10-phenanthroline solution in alcohol (2.0±0.2 ml for every 2 mg of Co) was then added in excess. The liberated EDTA was titrated against the lead nitrate solution to the same end point as before. The results of seven determination of Co(II) are presented in Table 1. Results and DiscussionThe log β value of the Co(II)-EDTA complex is reported to be 15.2.Cobalt(II) forms a stable complex with 1,10-phenanthroline with a log β value of 19.9. 6 The results show that Co(II) forms a stable water-soluble complex at pH 5.0 -6.0. The amount of reagent required to decompose the Co(II)-EDTA complex was established by adding different volumes of 1% 1,10-phenanthrol...

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Abstract.A simple and accurate complexometric method is proposed for the determination of TI(III) using semicarbazide hydrochloride as a releasing agent. In the presence of diverse metal ions, thallium is complexed first with a known excess of EDTA, and the surplus EDTA is then titrated with standard zinc sulfate at pH 5.0-6.0 (hexamine) using xylenol orange indicator. An excess of 5% aqueous neutral solution of semicarbazide hydrochloride is then added and the released EDTA is titrated against standard zinc sulfate solution. The method works well in the range 2-50 mg of TI(III) with relative errors < 0.5%, standard deviations <~ 0.05 mg and coefficient of variation ~< 0.4%. The method is applied for the determination of thallium content in complexes and alloy compositions Key words: thallium determination, complexometry, masking, semicarbazide hydrochloride.Thallium is widely distributed on the earth's crust with very low concentrations. Thallium compounds are highly toxic, but have wide applications. Thallium forms alloys with Pd, Pt, Ti, Zr, La and Bi and these have very important uses. The most unusual alloy of T1 is the Hg-T1 alloy which forms a eutectic at 8.7% T1 and freezes at -59 ~ about 20 ~ below the freezing point of mercury. Some thallium alloys and compounds are used as catalysts [-1] and as rodenticides. Thallium has also been used for heart imaging [-2] and thallous acetate is used in medicine for the treatment of tuberculosis and skin diseases. Because of the toxic nature and wide applications of thallium and its compounds, a selective complexometric method for the determination of thallium is highly useful.

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