Arsenates (and arsenites, after oxidation by treatment with nitric acid) in solution are precipitated as silver arsenate - Ag3AsO4, by the addition of neutral silver nitrate solution. The silver arsenate is dissolved in dilute nitric acid, and the silver titrated with standard thiocyanate solution. Small amounts of antimony and tin do not interfere, but chromates, phosphates, molybdates, tungstates, and vanadates, which precipitate as the silver salts, should be absent. An excessive amount of ammonium salts has a solvent action on the silver arsenate.
25 mL of the arsenate solution is diluted with an equal volume of distilled water in a 250 mL beaker, add and a few drops of phenolphthalein solution. Add sufficient sodium hydroxide solution (ca 1M) to give an alkaline reaction, and then discharge the red colour from the solution by just acidifying with acetic acid. Add a slight excess of silver nitrate solution with vigorous stirring, and allow the precipitate to settle in the dark. Pour off the supernatant liquid through a sintered-glass crucible, wash the precipitate by decantation with cold distilled water, transfer the precipitate to the crucible, and wash it free from silver nitrate solution. Wash out the receiver thoroughly. Dissolve the silver arsenate in dilute nitric acid (ca 1M) (which leaves any silver chloride undissolved), wash with very dilute nitric acid, and make up the filtrate and washings to 250 mL in a graduated flask. Titrate a convenient aliquot portion with standard ammonium (or potassium) thiocyanate solution in the presence of iron(III) nitrate as indicator.