The salts of arsenic acid, termed arsenates, are isomorphous with the phosphates, and in general character and reactions resemble the phosphates very closely; thus both series of salts give similar precipitates with "magnesia mixture" and with ammonium molybdate solution, but they can be distinguished by their behaviour with silver nitrate solution, arsenates giving a reddish-brown precipitate,whilst phosphates give a yellow precipitate.
There are several volumetric methods for assaying lead ores, but the best known is that based on the precipitation of lead by ammonium molybdate in an acetic acid solution.
A standardized solution of ammonium molybdate is then added from a burette.
As long as the solution contains lead, the addition of the molybdate solution causes a precipitation of white lead molybdate.
The molybdate solution should be of such a strength that i cc. will precipitate o oi gramme of lead.