Page 237 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

SEPARATION OF CHLORIOE AN0 BROMIOE ON AN ANION EXCHANCER 7.1 1

The following results were obtained in a typical experiment:
Weight of zinc taken = 25.62 mg; found = 25.60 mg
Weight of magnesium taken = 14.95 mg; found = 14.89 mg
Magnesium may conveniently be determined by atomic absorption spectro-
scopy (Section 21.21) if a smaller amount (CU 4 mg) is used for the separation.
Collect the magnesium effluent in a 1 L graduated flask, dilute to the mark with
de-ionised water and aspirate the solution into the flame of an atomic absorption
spectrometer. Calibrate the instrument using standard magnesium solutions
covering the range 2 to 8 ppm.

7.11 SEPARATION OF CHLORIDE AND BROMIDE ON AN ANION EXCHANGER
Theory. The anion exchange resin, originally in the chloride form, is converted
into the nitrate form by washing with sodium nitrate solution. A concentrated
solution of the chloride and bromide mixture is introduced at the top of the
column. The halide ions exchange rapidly with the nitrate ions in the resin,
forming a band at the top of the column. Chloride ion is more rapidly eluted
from this band than bromide ion by sodium nitrate solution, so that a separation
is possible. The progress of elution of the halides is followed by titrating fractions
of the effluents with standard silver nitrate solution.
Procedure. Prepare an anion exchange column (Section 7.8) using about 40 g
of Duolite A113 (chloride form). The ion exchange tube may be 16 cm long and
about 12 mm interna1 diameter. Wash the column with 0.6M sodium nitrate
until the effluent contains no chloride ion (silver nitrate test) and then wash
with 50 mL of 0.3M sodium nitrate.
Weigh out accurately about 0.10 g of analytical grade sodium chloride and
about 0.20 g of potassium bromide, dissolve the mixture in about 2.0mL of
water and transfer quantitatively to the top of the column with the aid of
0.3 M sodium nitrate. Pass 0.3 M sodium nitrate through the column at a flow
rate of about 1 mL per minute and collect the effluent in lOmL fractions.
Transfer each fraction in turn to a conical flask, dilute with an equal volume
of water, add 2 drops of 0.2M potassium chromate solution and titrate with
standard 0.02 M silver nitrate.
Before commencing the elution titrate 10.0 mL of the 0.3 M sodium nitrate
with the standard silver nitrate solution, and retain the product of this blank
titration for comparing with the colour in the titrations of the eluates. When
the titre of the eluate falls almost to zero (i.e. nearly equal to the blank
titration) - ca 150 mL of effluent - elute the column with 0.6 M sodium nitrate.
Titrate as before until no more bromide is detected (titre almost zero). A new
blank titration must be made with 10.0 mL of the 0.6M sodium nitrate.
Plot a graph of the total effluent collected against the concentration of halide
in each fraction (millimoles per litre). The sum of the titres using 0.3 M sodium
nitrate eluant (less blank for each titration) corresponds to the chloride, and
the parallel figure with 0.6M sodium nitrate corresponds to the bromide
recovery.
A typical experiment gave the following results:
Weight of sodium chloride used = 0.101 2 g -= 61.37 mg Cl-
Weight of potassium bromide used = 0.1934 g -= 129.87 mg Br -