The antimony or the arsenic must be present as antimony(III) or arsenic(III). The reaction of arsenic(III) or antimony(III) with potassium bromate may be written:
2KBrO3
+ 3M2O3 + 2HC1 = 2KC1 + 3M2O5 +
2HBr (M = As or Sb)
The presence of tin and of considerable quantities of iron and copper interfere with the determinations.
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, 15mL of concentrated hydrochloric acid and then two drops of indicator solution (xylidine ponceau or naphthalene black 12B). Titrate slowly with the standard 0.1N potassium bromate with constant swirling of the solution. As the end point approaches, add the bromate solution dropwise with intervals of 2-3 seconds between the drops until the solution is colourless or very pale yellow. If the colour of the indicator fades, add another drop of indicator solution. (The immediate discharge of the colour indicates that the equivalence point has been passed and the titration is of little value.)
As an alternative, a reversible indicator may be employed, either (a) 1-naphthoflavone (0.5% solution in ethanol, which gives an orange-coloured solution at the end-point), or (b) p-ethoxychrysoidine (0.1% aqueous solution, colour change pink to pale yellow). Under these conditions, the measured 25 mL portion of the arsenic solution is treated with 10 mL of 10 per cent potassium bromide solution, 6 mL of concentrated hydrochloric acid, 10 mL of water and either 0.5 mL of indicator (a) or two drops of indicator (b).