Tech Data

Characteristics of Gold

The element gold AU
Atomic Number 79
Atomic Weight 197
Specific gravity 19.3
Valence states Au+, Au+++
Electrochemical 7.356g Au+ / amp hr.
equivalent 2.450g Au+++ / amp hr.

The element gold, symbol Au, has an atomic number 79, an atomic weight of 197 and a specific gravity of 19.3. It exists in two valences, plus one and plus three. Gold has an electrochemical equivalent of 7.356 grams of plus one gold per ampere hour, or 2.450 grams of plus three gold per ampere hour.

The electrochemical equivalent of the monovalent gold ion (Au¹+) is 7.356 g / A-hr. this equivalent to 122.5 mg / A -min. Further calculations show that 0.372 A -min / SF are required to deposit a thickness of 1 micro inch (1 millionth of an inch) _ at 100% cathode efficiency.

Chemistry of Gold

K Au (CN)² → K+ + Au (CN)² ¯

The salt of gold ionizes as shown. The complex gold cyanide ion Au (CN)² ¯ does not ionize further to give Au+¹ and CN¯¹ ions. During plating, the electrons neutralize the charge on the gold (+1) ion directly from the complex ion.

You will note later that acid gold baths containing this gold salt are operated at a pH of 3 to 5. The covalent bond between the cyanide and the gold is very strong which accounts for the high stability even under these acidic conditions. Normally, cyanide baths are unstable at a pH of less than 10 and liberate HCN into the atmosphere.

Gold Plating Baths