Cobalt blue fluoride

The procedure utilises eriochrome blue black RC (also called pontachrome blue black R Colour Index No. 15705) at a pH of 4,8 in a buffer solution. Beryllium gives no fluorescence and does not interfere iron, chromium, copper, nickel, and cobalt mask the fluorescence fluoride must be removed if present. The method may be adapted for the determination of aluminium in steel. 

Ammonium tetrathiocyanatomercurate(II)-cobalt sulphate test This test is similar to that described under reaction 8, except that a minute amount of a dilute solution of a cobalt salt (nitrate, sulphate or acetate) is added. Coprecipitation of the cobalt tetrathiocyanatomercurate(II) yields a blue precipitate composed of mixed crystals of Zn[Hg(SCN)4] + Co[Hg(SCN)4], Iron(II) salts give a red colouration but this can be eliminated by the addition of a little alkali fluoride (colourless [FeF6]3- ions are formed). Copper salts should be absent. 

Cobalt(III) forms few simple salts, but the green hydrated fluoride CoF3-3.5H20 and the blue hydrated sulfate Co2(S04)3-18H20 separate on electrolytic oxidation... 

Procedure. One or two drops of the sHghtly acid test solution is mixed with a few milligrams of ammonium fluoride on a spot plate then 5 drops of 10 % ammonium thiocyanate solution in acetone are added. In this way, 1 y cobalt in 3 drops of a cobalt solution (1 60,000) can be detected by the blue color in the presence of 1000 times the amount of iron. 

By adding thiocyanate anions, cobaltous cations Co + yield a blue complex, the tetrathiocyanocobaltate(II) complex, [Co(SCN)4]. Conversely, this reaction permits the detection of Co + by adding potassium or sodium thiocyanate. This reaction is disturbed by Fe + since the complex ([Fe(SCN)] +) it forms in the same conditions exhibits an intense red color that may mask that given by [Co(CN)4] . Hence, in order to detect Co + in the presence of Fe +, the latter must be complexed with fluoride ions (added as sodium fluoride) or with sodium and potassium tartrate. The stability constants of the complexes [Fe(SCN)] + and [FeF] + are, respectively, and K = 10 -, and that of the tartrate complex K= 10 -. This process is quite exemplary of the analytical possibilities offered by the formation of complexes. 

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