The determination of arsenic in arsenic(III) compounds is based upon the following reaction:
IO3-
+ 2H3AsO3 + 2H+ +C1- = IC1 + 2H3AsO4
+ H2O
A similar reaction occurs with antimony(III) compounds. The determination of antimony (III) in the presence of tartrate is not very satisfactory with an immiscible solvent to assist in indicating the end point; amaranth, however, gives excellent results.
IO3-
+2[SbCl4]- +6H+ + 5Cl- = IC1 +
2[SbCl6]- + 3H2O
To assay a sample of arsenic(III) oxide the following procedure
may be used.
Dry some potassium iodate at 120°C for 1 hour and allow it to cool in a covered vessel in a desiccator. Weigh out exactly 5.350 g of the finely powdered potassium iodate on a watchglass, and transfer it by means of a clean camel-hair brush directly into a dry 1 L graduated flask. Add about 400-500 mL of water, and gently rotate the flask until the salt is completely dissolved. Make up to the mark with distilled water. Shake well. The solution will keep indefinitely. It must be emphasised that this 0.025M solution is intended for the reaction:
IO3-
+6H+ + Cl- +4e-= ICl + 3H2O
but when used in solutions of moderate acidity leading to the liberation of free iodine, ideally the solution then requires 4.28 g L-1 potassium iodate; the method of preparation will be as described above with suitable adjustment of the weight of salt taken.
Weigh out accurately about 1.1 g of the oxide sample, dissolve in a small quantity of warm 10 per cent sodium hydroxide solution, and make up to 250 mL in a graduated flask. Measure 25.0 mL of this solution into a stoppered reagent bottle of about 250 mL capacity, add 25 mL water, 60 mL concentrated hydrochloric acid and about 5 mL carbon tetrachloride or chloroform. Cool to room temperature. Run in the standard 0.025M potassium iodate from a burette until the solution, which at first is strongly coloured with iodine, becomes pale brown. The bottle is then stoppered and vigorously shaken, and the organic solvent layer acquires the purple colour due to iodine. Continue to add small volumes of the iodate solution, shaking vigorously after each addition, until the organic layer is only very faintly violet. Continue the addition dropwise, with shaking after each drop, until the solvent loses the last trace of violet and has only a very pale yellow colour (due to iodine chloride). The end point is very sharp and, after a little experience, is rarely overshot. If this should occur, a small volume of the oxide solution is added from a graduated pipette, and the end point re-determined. Allow to stand for ten minutes and observe whether the organic layer shows any purple colour; the absence of colour confirms that the titration is complete.
The acidity of the mixture at the end of the titration should be not less than 3M and not more than 5M; if the acidity is too high the reaction takes place slowly.