Spot test for gold with p-dimethylaminobenzylidenerhodanine

Gold(III) ions, Au³⁺, in aqueous solution can be spot-tested by using a reagent paper that is prepared by impregnating a spot test paper with a saturated alcoholic solution of  p-dimethylaminobenzylidenerhodanine. When a drop of the neutral or weakly acid test solution is placed on the dried reagent paper, a violet fleck or ring forms in the presence of gold ions.

Limit of Identification: 0.1 γ gold (see note below).
Dilution Limit:  1 : 500,000.
Interference: Ag, Hg, Pd.

The spot test handbook by Fritz Feigl and Vinzenz Anger (translated by Ralph E. Oesper) gives the following details (see page 240 in [1]):

Water-soluble gold^III salts give a red-violet precipitate with p-dimethylaminobenzylidenerhodanine in neutral and weakly acid solutions. It is likely that this reaction is not due to the formation of a gold^III salt of p-dimethylaminobenzylidenerhodanine in neutral and weakly acid solutions. The latter probably reduces Au^III to Au^I, and a gold^I salt of the reagent is produced.

Silver, mercury and palladium salts must be absent as they also react with p-dimethylaminobenzylidenerhodanine to give colored insoluble products.

The interference of silver in this test for gold can be avoided by precipitation and removal of silver chloride. The concentration of silver ions in a saturated solution of AgCl in 0.1 N hydrochloroc acid is so low that in one milliliter the reaction with the reagent is no stronger than that of 0.1 γ gold. In the presence of chloride, the reactivity of mercury is also decreased and even completely inhibited for small amounts. Palladium salts can be removed by precipitation in acid solution with dimethylglyoxime; the yellow crystalline inner complex Pd-dimethylglyoxime is formed.

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).


Reagent
Name: p-Dimethylaminobenzylidenerhodanine
Synonym: 5-(4-Dimethylaminobenzylidene)rhodanine
Empirical formula: C₁₂H₁₂N₂OS₂
Molar mass: 264.37 g/mol
CASRN: 536-17-4
EC-Number: 208-625-2
SMILES: S=C1NC(=O)C(S1)=Cc1cccc(cc1)N(C)C

p-Dimethylaminobenzylidenerhodanine

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Reference
[1] Fritz Feigl and Vinzenz Anger: Spot Tests in Inorganic Analysis. Sixth Edition. Elsevier Publishing Company, Amsterdam, The Netherlands, 1972 (page 240: https://books.google.com/books?id=II06Q8GGqNUC&pg=PA240&lpg=PA240&dq=Spot+test+for+gold+with+p-dimethylaminobenzylidenerhodanine&source=bl&ots=PlMv4theHI&sig=Cdw-P4TZGne2Y0Vvb23V4i8B4lw&hl=en&sa=X&ei=fluhVYuSEYKpogSm34CQAQ&ved=0CCAQ6AEwAA#v=onepage&q=Spot%20test%20for%20gold%20with%20p-dimethylaminobenzylidenerhodanine&f=false).