Page 236 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

7 ION EXCHANCE

The reaction may be represented as:

and proceeds to completion because of the large excess and large volume of
sodium sulphate solution passed through the column.
The capacity of the resin in millimoles per gram is given by au/W
where a is the molarity of the sodium hydroxide solution, u is the volume in
mL, and W is the weight (g) of the resin.
Anion exchange resin. Proceed as in the previous experiment using 1.0 g,
accurately weighed, of the air-dried strongly basic anion exchanger (e.g. Duolite
A1 13, chloride form). Fil1 the 250 mL separatory funnel with ca 0.25M sodium
nitrate solution, and allow this solution to drop into the column at the rate of
about 2 mL per minute. Collect the effluent in a 500 mL conical flask, and titrate
with standard 0.1 M silver nitrate using potassium chromate as indicator.
The reaction which occurs may be written as:

The capacity of the resin expressed as millimoles per gram is given by
bu/W where u mL of bM AgNO, are required by W g of the resin.

7.10 SEPARATION OF ZINC AND MAGNESIUM ON AN ANION EXCHANGER
Theory. Several metal ions (e.g. those of Fe, Al, Zn, Co, Mn, etc.) can be
absorbed from hydrochloric acid solutions on anion exchange resins owing to
the formation of negatively charged chloro-complexes. Each metal is absorbed
over a well-defined range of pH, and this property can be used as the basis of
a method of separation. Zinc is absorbed from 2M acid, while magnesium (and
aluminium) are not; thus by passing a mixture of zinc and magnesium through
a column of anion exchange resin a separation is effected. The zinc is subsequently
eluted with dilute nitric acid.
Procedure. Prepare a column of the anion exchange resin using about 15 g of
Duolite A113 in the chloride form (Section 7.9). The column should be made
up in 2 M hydrochloric acid.
Prepare separate standard zinc (about 2.5 mg Zn mL- ' ) and standard
magnesium (about 1.5 mg MgmL-') ion solutions by dissolving accurately
weighed quantities of zinc shot and magnesium (for Grignard reaction) in
2 M hydrochloric acid and diluting each to volume in a 250 mL graduated flask.
Pipette 10.0mL of the zinc ion solution and 10.0 mL of the magnesium ion
solution into a small separatory funnel supported in the top of the ion exchange
column, and mix the solutions. Allow the mixed solution to flow through the
column at a rate of about 5 mL per minute. Wash the funnel and column with
50 mL of 2M hydrochloric acid: do not permit the level of the liquid to fa11
below the top of the resin column. Collect al1 the effluent in a conical flask; this
contains al1 the magnesium. Now change the receiver. Elute the zinc with 30 mL
of water, followed by 80 mL of ca 0.25 M nitric acid. Determine the magnesium
and the zinc in the respective eluates by neutralisation with sodium hydroxide
solution, followed by titration with standard EDTA solution using a buffer
solution of pH = 10 and solochrome black indicator (Section 10.62).