Page 240 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

7 ION EXCHANCE

(b) Zinc ion eluate. Pipette 50.0 mL of the solution into a 250 mL conical
flask, neutralise with hydrochloric acid, and dilute to about 100 mL with water.
Add 2 mL of the buffer mixture, then a little solochrome black indicator powder,
and titrate with standard 0.01 M EDTA until the colour changes from red to
pure blue.
(c) Original cadmium ion solution. Dilute 2.00 mL (pipette) to 100mL in a
graduated flask. Pipette 10.0 mL of the diluted solution into a 250 mL conical
flask, add ca 40 mL of water, followed by solid hexamine and a few milligrams
of xylenol orange indicator. If the pH is correct (5-6) the solution will have a
pronounced red colour (see Section 10.59). Titrate with standard 0.01 M EDTA
until the colour changes from red to clear orange-yellow.
(d) Cadmium ion eluate. Pipette 50.0 mL of the solution into a conical flask,
and partially neutralise (to pH 3-4) with aqueous sodium hydroxide. Add solid
hexamine (to give a pH of 5-6) and a little xylenol orange indicator. Titrate
with standard 0.01 M EDTA to a colour change from red to clear orange-yellow.
Some typical results are given below.
0.20 mL of original Zn2+ solution required 17.50 mL of 0.010 38M EDTA
:. Weight of Zn2+ per mL = 17.50 x 5 x 0.010 38 x 65.38 = 59.35 mg
50.0mL of Zn2+ eluate -= 17.45 mL of 0.010 38 M EDTA.
:. Zn2+ recovered = 5 x 17.45 x 0.010 38 x 65.38 = 59.21 mg
0.200 mL of original Cd2+ solution required 19.27 mL of 0.010 38 M EDTA.
:. Weight of Cd2+ per mL = 5 x 19.27 x 0.010 38 x 112.4 = 112.4mg
50.0 mL of Cd2+ eluate -= 19.35 mL of 0.010 38 M EDTA.
:. Cd2+ recovered = 5 x 15.35 x 0.010 38 x 112.4 = 112.8 mg.

7.14 CONCENTRATION OF COPPER(I1) IONS FROM A BRlNE SOLUTION USlNG A
CHELATING ION EXCHANGE RESIN
Theory. Conventional anion and cation exchange resins appear to be of limited
use for concentrating trace metals from saline solutions such as sea water. The
introduction of chelating resins, particularly those based on iminodiacetic acid,
makes it possible to concentrate trace metals from brine solutions and separate
them from the major components of the solution. Thus the elements cadmium,
copper, cobalt, nickel and zinc are selectively retained by the resin Chelex-100
and can be recovered subsequently for determination by atomic absorption
spectroph~tometry.~~ enhance the sensitivity of the AAS procedure the eluate
To
is evaporated to dryness and the residue dissolved in 90 per cent aqueous
acetone.* The use of the chelating resin offers the advantage over concentration
by solvent extraction that, in principle, there is no limit to the volume of sample
which çan be used.
Reagents. Standard copper (II) solutions. Dissolve 100 mg of spectroscopically
pure copper metal in a slight excess of nitric acid and dilute to 1 L in a
graduated flask with de-ionised water. Pipette a 10 mL aliquot into a 100mL
graduated flask and make up to the mark with acetone (analytical grade); the
resultant solution contains 10 pg of copper per mL. Use this stock solution to

* In the illustrative experiment described here, copper(I1) ions in a brine solution are concentrated
from 0.1 to about 3.3 ppm prior to determination by atomic absorption spectrophotometry.