Nickel is precipitated by the addition of an ethanolic solution of dimethylglyoxime (CH3-C(:NOH)-C(:NOH)-CH3, referred to in what follows as H2DMG} to a hot, faintly acid solution of the nickel salt, and then adding a slight excess of aqueous ammonia solution (free from carbonate). The precipitate is washed with cold water and then weighed as nickel dimethylglyoximate after drying at 110-120 °C. With large precipitates, or in work of high accuracy, a temperature of 150 °C should be used: any reagent that may have been carried down by the precipitate is volatilised.
Ni2+ + 2H2DMG =
Ni(HDMG)2 + 2H +
The precipitate is soluble in free mineral acids (even as little as is liberated by reaction in neutral solution), in solutions containing more than 50 per cent of ethanol by volume, in hot water (0.6mg per 100mL), and in concentrated ammoniacal solutions of cobalt salts, but is insoluble in dilute ammonia solution, in solutions of ammonium salts, and in dilute acetic acid-sodium acetate solutions. Large amounts of aqueous ammonia and of cobalt, zinc, or copper retard the precipitation; extra reagent must be added, for these elements consume dimethylglyoxime to form various soluble compounds. Better results are obtained in the presence of cobalt, manganese, or zinc by adding sodium or ammonium acetate to precipitate the complex; iron(III), aluminium, and chromium (III) must, however, be absent.
Dimethylglyoxime forms sparingly soluble compounds with palladium, platinum, and bismuth. Palladium and gold are partially precipitated in weakly ammoniacal solution; in weakly acid solution palladium is quantitatively precipitated and gold partially. Bismuth is precipitated in strongly basic solution. These elements, and indeed all the elements of the hydrogen sulphide group, should be absent. Iron (II) yields a red-coloured soluble complex in ammoniacal solution and leads to high results if much of it is present. Silicon and tungsten interfere only when present in amounts of more than a few milligrams. Iron(III), aluminium, and chromium (III) are rendered inactive by the addition of a soluble tartrate or citrate, with which these elements form complex ions.
Dimethylglyoxime is almost insoluble in water, and is added in the form of a 1 per cent solution in 90% ethanol or absolute ethanol; 1 mL of this solution is sufficient for the precipitation of 0.0025 g of nickel. As already pointed out, the reagent is added to a hot feebly acid solution of a nickel salt, and the solution is then rendered faintly ammoniacal. This procedure gives a more easily filterable precipitate than does direct precipitation from cold or from ammoniacal solutions. Only a slight excess of the reagent should be used, since dimethylglyoxime is not very soluble in water or in very dilute ethanol and may precipitate; if a very large excess is added (such that the alcohol content of the solution exceeds 50 per cent), some of the precipitate may dissolve.
A. Nickel in a nickel salt. Weigh out accurately 0.3-0.4 g of pure ammonium nickel sulphate (NH4)2SO4-NiSO4,6H2O into a 500mL beaker provided with a clockglass cover and stirring rod. Dissolve it in water, add 5 mL of dilute hydrochloric acid (1:1) and dilute to 200 mL. Heat to 70-80 °C, add a slight excess of the dimethylglyoxime reagent (at least 5 mL for every 10mg of Ni present), and immediately add dilute ammonia solution dropwise, directly to the solution and not down the beaker wall, and with constant stirring until precipitation takes place, and then in slight excess. Allow to stand on the steam bath for 20-30 minutes, and test the solution for complete precipitation when the red precipitate has settled out. Allow the precipitate to stand for 1 hour, cooling at the same time. Filter the cold solution through a sintered-glass or porcelain filtering crucible, previously heated to 110-120°C and weighed after cooling in a desiccator. Wash the precipitate with cold water until free from chloride, and dry it at 110-120 °C for 45-50 minutes. Allow to cool in a desiccator and weigh. Repeat the drying until constant weight is attained. Weigh as Ni(C4H7O2N2)2, which contains 20.32 per cent of Ni.
B. Nickel in nickel steel. Weigh out accurately about 1 g of the drillings or borings of the nickel steel (or sufficient of the sample to contain 0.03-0.04 g of nickel) into a 100-150 mL beaker or porcelain basin, dissolve it in the minimum volume of concentrated hydrochloric acid (about 20 mL should suffice), and boil with successive additions of concentrated nitric acid (ca 5 mL) to ensure complete oxidation of the iron to the iron(III) state. Dilute somewhat, filter, if necessary, from any solid material, and wash the paper with hot water; dilute the filtrate (or solution) to 250 mL in a 400 mL beaker. Add 5 g of citric or tartaric acid, neutralise the solution with dilute aqueous ammonia solution, and then barely acidify (litmus) with dilute hydrochloric acid. If a precipitate appears or if the solution is not clear when it is rendered ammoniacal, more tartaric or citric acid must be added until a perfectly clear solution is obtained upon adding dilute ammonia solution. Any insoluble matter should be filtered off and washed with hot water containing a little ammonia solution. Warm the solution to 60-80 °C, add a slight excess of a 1 per cent ethanolic solution of dimethylglyoxime (20-25 mL), immediately followed by dilute ammonia solution dropwise until the liquid is slightly ammoniacal, stir well, and allow to stand on the steam bath for 20-30 minutes. Allow the solution to stand at least 1 hour and cool to room temperature during this time. Filter off the precipitate through a weighed filtering crucible; test the filtrate for complete precipitation with a little dimethylglyoxime solution, and wash the precipitate with cold water until free from chloride. Dry the precipitate at 100-120 °C for 45-60 minutes, and weigh as Ni(C4H7O2N2)2.