Formic acid

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Formic acid (HCOOH) is a colorless liquid that is miscible in water.

In refining it can be used to reduce some metal compounds back into metal.

CAS number : 64-18-6


Physical properties

Molecular weight : 46.03
Melting point : 8.4 °C (47.1 °F)
Boiling point : 100.8 °C (213.4 °F)
Density : 1.22 g/cm3

Reducing PGM salts

(Main article : Reducing PGM salts with formic acid

Formic acid is used in refining to reduce the colored salts of palladium or platinum back into metal. The resulting metal is in a fine black powder form.

The reaction can be slow at start but when it get going it can easily lead to a boil over. In the reduction gases are created so care is advised.

Reducing silver

(Main article : Reducing silver nitrate with formic acid)

Formic acid can be used to selectively reduce silver nitrate into silver.

The pH of the silver nitrate solution is adjusted to 1.5, then sodium formate at a pH of 4 is added while the solution is heated. The sodium formate is made by mixing formic acid with sodium hydroxide solution until pH 4 is reached. The silver precipitates quite rapidly.

The method is capable of producing 999/1000 fine silver or better from solutions without PGM:s.

If there is palladium present it has to be removed before the silver is precipitated, for example with DMG. Any platinum will remain in solution and can be precipitated after the silver by raising the pH to 6 and increase the heat until almost boiling.

Controlling the pH is vital for this method.


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