Page 239 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

SEPARATION OF CAOMIUM AN0 ZINC ON AN ANION EXCHANCER 7.13
with 1 M nitric acid. The zinc and cadmium in their respective effluents may be
determined by titration with standard EDTA.
Elements such as Fe(III), Mn, Al, Bi, Ni, Co, Cr, Cu, Ti, the alkaline-earth
metals, and the lanthanides are not absorbed on the resin in the HCl-NaCl
medium.
Reagents. Anion exchange column. Prepare an anion exchange column using
25-30 g of Duolite Al 13 (chloride form) following the experimental details given
in Section 7.8. Allow the resin to settle in 0.5 M hydrochloric acid. Transfer the
resin slurry to the column: after settling, the resin column should be about
20 cm in length if a 50 mL burette is used.
Reagent I. This consists of 0.12M hydrochloric acid containing 100 g of
analytical-grade sodium chloride per litre.
Reagent II. This consists of 2 M sodium hydroxide containing 20 g of sodium
chloride per litre.
Zinc-ion solution. Dissolve about 7.0 g of analytical grade zinc sulphate
heptahydrate in 25 mL of Reagent 1.
Cadmium-ion solution. Dissolve about 6.0 g of analytical grade crystallised
cadmium sulphate in 25 mL of Reagent 1.
EDTA solution. 0.01 M. See Section 10.49.
Buffer solution, pH = 10. Dissolve 7.0 g of analytical grade ammonium
chloride and 57 mL of concentrated ammonia solution (sp. gr. 0.88) in water
and dilute to 100 mL.
Solochrome black indicator mixture. Triturate 0.20 g of the solid dyestuff with
50 g of potassium chloride.
Xylenol orange indicator. Finely grind (triturate) 0.20 g of the solid dyestuff
with 50 g of potassium chloride (or nitrate). This solid mixture is used because
solutions of xylenol orange are not very stable.
Nitric acid, ca 1 M.
Procedure. Wash the anion exchange column with two 20mL portions of
Reagent 1; drain the solution to about 0.5 cm above the top of the resin. Mix
thoroughly equal volumes (2.00mL each) of the zinc and cadmium ion
solutions and transfer by means of a pipette 2.00 mL of the mixed solution to
the top of the resin column. Allow the solution to drain to within about 0.5 cm
of the.top of the resin and wash down the tube above the resin with a little of
Reagent 1. Pass 150mL of Reagent II through the column at a flow rate of
about 4mLmin-' and collect the eluate (containing the zinc) in a 250mL
graduated flask; dilute to volume with water. Wash the resin with about 50 mL
of water to remove most of the sodium hydroxide solution. Now place a 250 mL
graduated flask in position as receiver and pass 150 mL of 1 M nitric acid through
the column at a rate of about 4 mL min-'; the cadmium will be eluted. Dilute
the effluent to 250 mL with distilled water.
The resin may be regenerated by passing Reagent 1 through the column, and
can then be used again for analysis of another Zn-Cd sample.
Analyses. (a) Original zinc-ion solution. Dilute 2.00 mL (pipette) to 100 mL in
a graduated flask. Pipette 10.0 mL of the diluted solution into a 250 mL conical
flask, add ca 90 mL of water, 2 mL of the buffer solution, and sufficient of the
solochrome black indicator mixture to impart a pronounced red colour to the
solution. Titrate with standard 0.01 M EDTA to a pure blue colour (see
Section 10.59).