With the development of the atomic absorption technique for rapid processing of gold content of low value solutions, some methods have fallen in disuse in the extraction plants on modern gold operations. Nevertheless, it is a regular means of assaying precipitated solution on site and can be performed rapidly by the plant operator. In solution-assaying the gold and silver are separated from the liquid, either by precipitation or by deposition on lead, and six methods of assaying are available,
- Evaporation on litharge in plastic lined porcelain basins, followed by fluxing and fusion
- Precipitation by copper sulphate, then filtering, fluxing and fusion
- Precipitation by cuprous chloride, followed by filtration, fusion and cupellation
- Precipitation by zinc-lead couple, employing lead acetate and zinc dust. After the gold is precipitated, surplus zinc is dissolved with boiling hydrochloric acid and removed by decantation. The residual lead sponge is cupelled, and the resultant gold bead parted, annealed and its mass determined
- Deposition on lead foil. Corrugated lead foil is introduced into the solution at boiling point, together with potassium permanganate and hydrochloric acid. The solution, after 30 minutes boiling are decanted and the lead foil cupelled
- Electrolytic deposition. This is conducted in glass jars with platinum anodes and lead foil cathodes. After the gold has been deposited on the cathodes, these are removed and cupelled, followed by parting and annealing. Unprecipitated solutions have to be accurately assayed for previous methods. For all other solutions, such as filter wash and return dam, the cuprous chloride is employed.