Spot test for platinum with 1,4-diphenyl-3-thiosemicarbazide

Platinum forms a green coordination compound, Pt(C₁₃H₁₁N₃S)₂, with the colorless compound 1,4-diphenyl-3-thiosemicarbazide, C₁₃H₁₃N₃S. The platinum complex is slightly soluble in water, but readily soluble in chloroform. The spot test handbook by Fritz Feigl and Vinzenz Anger gives the following procedure to test for a dissolved platinum salt in acid solution (see page 395 in [1]):

One drop of the test solution is mixed in a micro test tube with a few mg of solid 1,4-dipheny-3-thiosemicarbazide and then shaken with a little chloroform. A green color in the chloroform layer indicates the presence of platinum.

Limit of Identification: 0.5 γ platinum (see note below).
Dilution Limit:  1 : 100,000.
pH Dependency: independent over the range 2-7.

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).

1,4-diphenyl-3-thiosemicarbazide

Reagents
Name: 1,4-diphenyl-3-thiosemicarbazide
IUPAC Name: N,2-Diphenylhydrazinecarbothiomide
Empirical formula: C₁₃H₁₃N₃S
Molar mass: 243.33 g/mol
CASRN: 1768-59-8
ChemSpider ID: 677781
SMILES: c1ccccc1NNC(=S)Nc1ccccc1
- - -
Name: Chloroform
IUPAC Name: Trichloromethane
Empirical formula: CCl₃
Molar mass: 119.37 g/mol
CASRN: 67-66-3
ChemSpider ID: 5977
SMILES: ClC(Cl)Cl

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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 [https://books.google.com/books?id=II06Q8GGqNUC&pg=PA395&lpg=PA395&dq=1,4-diphenyl-3-thiosemicarbazide+complex+platinum&source=bl&ots=PlMv6thjFG&sig=emIhMuyKE01W-Z9APMgY2FzKQgc&hl=en&sa=X&ved=0CB4Q6AEwAGoVChMIn8Xb3cDZxgIVUBeSCh2vBAEu#v=onepage&q=1%2C4-diphenyl-3-thiosemicarbazide%20complex%20platinum&f=false].

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

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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.

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).