Page 210 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

6 SOLVENT EXTRACTION

Procedure. Weigh out 0.135 g of pure ammonium nickel sulphate
(NiSO4,(NH4),SO4,6H20) and dissolve it in 1 L of water in a graduated flask.
Transfer 10.0mL of this solution (Ni content about 200pg) to a beaker
containing 90 mL of water, add 5.0 g of citric acid, and then dilute ammonia
solution until the pH is 7.5. Cool and transfer to a separatory funnel, add 20 mL
of dimethylglyoxime solution (Note 1) and, after standing for a minute or two,
12 mL of chloroform. Shake for 1 minute, allow the phases to settle out, separate
the red chloroform layer, and determine the absorbance at 366 nm in a 1.0 cm
absorption ce11 against a blank. Extract with a further 12 mL of chloroform and
measure the absorbance of the extract at 366 nm; very little nickel will be found.
Repeat the experiment in the presence of 500 pg of iron(II1) and 500 pg of
aluminium ions; no interference will be detected, but cobalt may interfere
(Note 2).
Notes. (1) The dimethylglyoxime reagent is prepared by dissolving 0.50g of
dimethylglyoxime in 250 mL of ammonia solution and diluting to 500 mL with water.
(2) Cobalt forms a brown soluble dimethylglyoxime complex which is very slightly
extracted by chloroform; the amount is only significant if large amounts of Co ( > 2-3 mg)
are present. If Co is suspected it is best to wash the organic extract with ca 0.5M ammonia
solution: enough reagent must be added to react with the Co and leave an excess for
the Ni. Large amounts of cobalt may be removed by oxidising with hydrogen peroxide,
complexing with ammonium thiocyanate (as a 60 per cent aqueous solution), and
extracting the compound with a pentyl alcohol-diethyl ether (3: 1) mixture. Copper(I1)
is extracted to a small extent, and is removed from the extract by shaking with
0.5M ammonia solution. Copper in considerable amounts is not extracted if it is
complexed with thiosulphate at pH 6.5. Much Mn tends to inhibit the extraction of Ni;
this difficulty is overcome by the addition of hydroxylammonium chloride. Iron(II1)
does not interfere.

6.16 DETERMINATION OF SILVER BY EXTRACTION AS ITS ION ASSOCIATION
COMPLEX WlTH 1.10-PHENANTHROLINE AND BROMOPYROGALLOL RED
Discussion. Silver can be extracted from a nearly neutral aqueous solution into
nitrobenzene as a blue ternary ion association complex formed between
silver(1) ions, 1,lO-phenanthroline and bromopyrogallol red. The method is
highly selective in the presence of EDTA, bromide and mercury(I1) ions as
masking agents and only thiosulphate appears to interfere.8
Reagents. Silver nitrate solution, 10-4M. Prepare by dilution of a standard
0.1 M silver nitrate solution.
1,lO-Phenanthroline solution. Dissolve 49.60 mg of analytical grade 1,lO-
phenanthroline in distilled water and dilute to 250 mL.
Ammonium acetate (ethanoate) solution, 20 per cent. Dissolve 20g of the
analytical grade salt in distilled water and dilute to 100mL.
Bromopyrogallol red solution, 10-4M. Dissolve 14.0 mg of bromopyrogallol red
and 2.5 g of ammonium acetate in distilled water and dilute to 250 mL. This
solution should be discarded after five days.
EDTA solution, IO-'M. Dissolve 3.7225 g of analytical grade disodium salt in
distilled water and dilute to 100 mL.
Sodium nitrate solution, 1 M. Dissolve 8.5 g of analytical grade sodium nitrate
in distilled water and dilute to 100 mL.