Page 211 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

OETERMINATION OF NICKEL BV SVNERCISTIC EXTRACTION 6.17

Nitrobenzene, analytical grade.
Sodium hydroxide, analytical grade pellets.
Procedure. Calibration. Pipette successively 1,2,3,4 and 5 mL of 10 -4 M silver
nitrate solution, 1 mL of 20 per cent ammonium acetate solution, 5mL of
10-,M 1,lO-phenanthroline solution, 1 mL of 10-'M EDTA solution and 1 mL
of 1 M sodium nitrate solution into five 100 mL separatory funnels. Add sufficient
distilled water to give the same volume of solution in each funnel, then add
20 mL of nitrobenzene and shake by continuous inversion for one minute. Allow
about 10 minutes for the layers to separate, then transfer the lower organic
layers to different 100 mL separatory funnels and add to the latter 25 mL of
M bromopyrogallol red solution. Again shake by continuous inversion for
one minute and allow about 30 minutes for the layers to separate. Run the
lower nitrobenzene layers into clean, dry 100 mL beakers and swirl each beaker
until al1 cloudiness disappears (Note 1). Finally transfer the solutions to 1 cm
cells and measure the absorbance at 590 nm against a blank carried through
the same procedure, but containing no silver. Plot a calibration curve of
absorbance against silver content (pg).

Determination. To an aliquot of the silver(1) solution containing between 10
and 50 pg of silver, add sufficient EDTA to complex al1 those cations present
which form an EDTA complex. If gold is present ( k250 pg) it is masked by
adding sufficient bromide ion to form the AuBr; complex. Cyanide, thiocyanate
or iodide ions are masked by adding sufficient mercury(I1) ions to complex
these anions followed by sufficient EDTA to complex any excess mercury(I1).
Add 1 mL of 20 per cent ammonium acetate solution, etc., and proceed as
described under Calibration.
Note. (1) More rapid clarification of the nitrobenzene extract is obtained if the beakers
contain about five pellets of sodium hydroxide. The latter is, however, a source of
instability of the colour system and its use is, therefore, not recommended.

6.17 DETERMINATION OF NICKEL BY SYNERGlSTlC EXTRACTION*
Discussion. Using dithizone and 1,lO-phenanthroline (Note l), nickel is rapidly
and quantitatively extracted over a broad pH range (from 5.5 to at least 11.0)
to give a highly coloured mixed ligand complex having an absorption band
centred at 520 nm. The complex is sufficiently stable to permit the removal
of excess dithizone by back-extraction with 0.1 M sodium hydroxide, so that a
'monocolour' method is applicable. The molar absorptivity of this complex is
4.91 x 104 mol - ' L cm - ', which makes the method significantly more sensitive
than any other method for determination of nickel.,
Procedure. To lOmL of a solution (Note 2) containing from 1 to 10pg of
nickel(I1) add 5 mL of a phthalate or acetate (ethanoate) buffer of pH 6.0 or,
if the sample solution is acidic, use dilute ammonia to adjust the pH. To this
solution now add 15 mL of a chloroform solution of dithizone (7 x 10-5M)
and 1,lO-phenanthroline (3 x 10-5M). Shake the phases for five minutes in a

*This experiment is not recommended for students having little experience of analytical work.