Determination Of Nickel

Determination of Nickel By gravimetric method

Procedure  Weigh

out accurately 0.3-0.4 g of Nickel sample salt  Rinse and pour into a 500 mL beaker provided with a clock glass cover and stirring rod.  Add 5 mL of dilute hydrochloric acid (1: 1) and dilute to 200 mL.  Heat the solution to 70-80 OC ,

Add

a slight excess of the dimethylglyoxime reagent (30 -35 mL) and immediately add dilute ammonia solution drop wise, directly to the solution and with constant stirring until precipitation takes place, and then in slight excess.

Allow

to stand on the steam bath for 2030 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 O C and weighed after cooling in a desiccator

Wash the precipitate with cold water until free from chloride, and dry it at 110-120 OC 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 

Calculate

the percentage of nickel in the sample

Discussion Nickel

is precipitated by the addition of an ethanolic solution of dimethylglyoxime {CH.C(:NOH).C(:NOH).CH, 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 OC . With large precipitates, or in work of high accuracy, a temperature of 150 O C should be used: any reagent that may have been carried down by the precipitate is volatilised.  Ni2+(aq) +

2H2DMG(aq)

Ni(HDMG)2(s) + 2H+(aq)

Formula Structure of Ni(HDMG)2

N

N Ni

N

N

The precipitate is soluble in:

1.Free mineral acids (even as little as is liberated by reaction in neutral solution) 2.Solutions containing more than 50 per cent of ethanol by volume, 3.Hot water (0.6 mg per 100 mL), 4.Concentrated ammoniacal solutions of cobalt salts,

The precipitate is insoluble in:

1.Dillute ammonia solution, 2.Solutions of ammonium salts 3.Dillute acetic (ethanoic) acidsodium 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 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 (rectified spirit) 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.