Preparation of Potassium Bis biureto nickelate III

Dissolve 3.3 g biuret (NH2CONHCONH2) in a solution of 4.5 g KOH in 23 cm water. Add to this solution, dropwise and with constant stirring, a solution of 3.95 g NiS04.6H20 in 23 cm water. Then add, with stirring, a solution of 8.25 g K2S2OS in 450 cm water. Filter the blue-black precipitate formed at the pump, wash with 2 M H2S04(3 X 15 cm ) and water at 70 C (5 x 15 cm ) and dry at 110 C. 

Weigh the product K[Ni(NHCONHCONH)2] and calculate the percentage yield based on NiS04.6H20. 

Ascorbic acid solution, freshly prepared and standardised, will reduce Ni(III) to Ni(II), the excess ascorbic acid can be back titrated with any suitable oxidant eg Mn(III) sulphate. 

Weigh out accurately about 0.4 g of your preparation, previously finely ground. Add a known excess of standard ascorbic acid solution (SO cm of 0.028 M solution, fireshly prepared in boiled-out distilled water). The solid should dissolve completely, when Ni(II) is formed in solution. Titrate the excess ascorbic acid against standardised Mn2(S04)3 solution (Sec. 12.4.1) using N-phenylanthranilic acid as indicator. The end point is indicated by a sudden change to violet colour. Repeat the determination and calculate the percentage Ni in your sample and hence the percentage purity of your preparation. 

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