Spot test for gold with solution of benzidine in acetic acid

Gold(III) ions, Au³⁺, in aqueous solution can be spot-tested with benzidine. However, the presence of various heavy-metal and alkali-metal salts interferes with this test. Further, colored gold hydrosols are produced. The spot test with p-dimethylaminobenzylidenerhodanine should be considered as an alternative.

The reagent solution is a 0.05 % solution of benzidine in 10 % acetic acid. When a drop of this solution is brought together with a drop of the test solution on a filter paper, the appearance of blue color indicates gold: Au³⁺ ions oxidize benzidine to benzidine blue, a quinoneimine type of dyestuff (see page 241 in [1]).

Limit of Identification: 0.02 γ gold in 0.001 mL of test solution.
Dilution Limit:  1 : 50,000.
Interference: various metal ions.

Note: γ equals 0.001 mg and refers to the quantity of gold dissolved in a given volume of test (sample) solution. The dilution limit is defined as (see page 4 in [1]):
10^-6 · Limit of Identification (in γ) / Volume of test solution (in mL).

Benzidine

Reagents
Name: Benzidine
IUPAC name: 4,4′-diaminobiphenyl
Empirical formula: C₁₂H₁₂N₂
Molar mass: 184.24 g/mol
CASRN: 92-87-5
SMILES: Nc1ccc(cc1)-c1ccc(cc1)N
- - -
Name: Acetic acid 
Empirical formula: C₂H₄O₂
Molar mass: 60.05 g/mol
CASRN: 64-19-7
SMILES: CC(=O)O

Spotted at Pinterest: Spot test reagents.

Reference
[1] Fritz Feigl and Vinzenz Anger: Spot Tests in Inorganic Analysis. Sixth Edition. Elsevier Publishing Company, Amsterdam, The Netherlands, 1972.