Gold is generally determined by spectrophotometric procedures using complexing and chromogenic agents such as Methyl Violet 1,2 , Methylene Blue 3-7 , Brilliant Green 8,9 , Methyl Green 10 , Rhodamine B 11,12 and Malachite Green. 13 These methods are based on ionassociation complexes of AuX 4 -(where X=Cl -, Br -, SCN -, CN -etc.) with various basic dyes, but the complexes are water-insoluble compounds and require extraction with organic solvents. In addition, most of those methods are unsatisfactory in sensitivity. On the other hand, few methods 14 have been reported based on an ion-association complex of AuL n m+ (where L n =lig-and) with an acidic dye.We have already reported a few simple and sensitive spectrophotometric methods 15,16 , which do not require any process of solvent extraction, for the determination of metal ions with an acidic dye and a ligand. Therefore, we speculated that a simple, sensitive and selective determination of gold would be established by selecting a suitable acidic dye and/or ligand.In this paper, a color reaction of gold(III) was studied by utilizing an association complex with an acidic dye and a ligand, and suitable conditions were established for the spectrophotometric determination of gold(III). The proposed method was applied to the assay of gold in commercial pharmaceutical preparations.
Experimental
Reagents and solutionsStandard gold(III) solution (1.0×10 -2 M, 1 M=1 mol dm -3 ) was prepared by dissolving the metal (99.999%) in aqua regia, evaporating nearly to dryness and dissolving in 1 M hydrochloric acid. A working solution (1.0×10 -4 M) was made by suitable dilution of this stock solution as required. A thiamine solution (1.0×10 -2 M) was prepared by dissolving a proper quantity of thiamine nitrate in water. A solution (1.0×10 -3 M) of Phloxine was prepared by dissolving Phloxine B in water. A solution of methylcellulose (denoted by MC, 25 cps) was prepared in 0.5% aqueous solution. A buffer solution(pH 4.6) was prepared by mixing 0.05 M trisodium ethylenediaminetetraacetate (denoted by EDTA·3Na) and 0.1 M citric acid solutions. All other reagents and materials were of analytical grade and were used without further purification. Distilled, deionized water was further purified using a Millipore-Q water purification system.
ApparatusA Shimadzu Model UV-160 recording spectrophotometer with 1.0-cm silica cells was used for conventional measurements. The pH measurements were made with a Horiba Model F-11 pH meter combined with a calomel electrode. A Shimadzu Model ICPS-1000V inductively coupled plasma emission spectrometer was used.
Standard procedure for determination of gold(III)The following components were mixed in a 10-cm 3 volumetric flask: a solution containing 0.2 -8 µg of gold(III), 0.5 cm 3 of a 0.5% MC solution, 1.0 cm 3 of a 1.0×10 -2 M thiamine solution, 2.0 cm 3 of the buffer solution and 1.0 cm 3 of a 1.0×10 -3 M Phloxine solution. The mixture was then made up to volume with water, transferred into a test tube, mixed well and kept at 40˚C for 20 min. After...
