Separation of chloride and bromide on an anion exchanger

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 spectroscopy (Section 21.21) if a smaller amount (ca 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. 

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 40g of Duolite A113 (chloride form). The ion exchange tube may be 16 cm long and about 12 mm internal 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 0.3 M sodium nitrate. 

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