Silver thiocyanate absorption

After 2xh hours the gas flow is terminated and the power supply is disconnected. The layer of brown oil formed in the bottom of the vessel contains no indium and can be rejected. The supernatant is carefully removed to a 100-mL round-bottomed flask, and the solvent is removed by pumping in vacuo at room temperature until the volume is reduced to about 15 mL. The resultant white solid is collected and washed quickly with several small volumes of diethyl ether although indium(III) chloride is soluble in diethyl ether, the rate of dissolution is fairly slow, and the losses in this washing are tolerable. The collection and washing operations are carried out under dry nitrogen. Yield 0.23 g (26%, based on indium consumed). Anal. Calcd. for InCU In, 51.9 Cl 48.1. Found In (atomic absorption), 51.9 Cl (silver nitrate-potassium thiocyanate titration) 48.8. 

The absorption is complete in about one hour. The pyridine and its methiodide are then washed out with water, acidified with nitric acid, a known volume of silver nitrate added and the excess of the latter estimated by thiocyanate, using ferric alum as indicator. 

Sensitive extraction-spectrophotometric methods are based on the extractable (into CHCI3, 1,2-diehloroethane, benzene, or toluene) ion-associates of basic dyes and anionic Ag complexes with cyanide [35,36], iodide [37,38], and bromide [39]. In these methods, use has been made of such dyes as Crystal Violet [35,39], Brilliant Green [38,39], Malachite Green [39], Methylene Blue [36], and Nile Blue A [37]. In some of these methods the molar absorptivities are elose to MO [36,39]. A flotation method has been proposed, based on the addition compound [R6G ][Ag(SCN )2] [R6G ][SCN ] which is formed by silver ions (at pH 2-5) in the presence of thiocyanate and Rhodamine 6G (flotation with DIPE, the precipitated compound is washed and dissolved in acetone, e = 1.5-10 ) [40]. The complex Ag(CN)2 , associated with Crystal Violet, has been utilized in another flotation-spectrophotometric method of determining silver [41]. Silver has been determined also in a system comprising thiocyanate and Rhodamine B, as an aqueous pseudo-solution, in the presence of poly(vinyl alcohol) [42]. 

The nature of thiocyanate complexes of silver in non aqueous solvents has recently been reassessed. IR and Raman spectra were used to show that when Ag was complexed by SCN , the species present and the equilibrium steps involved were solvent specific. Characteristic absorption bands are summarized in Table 24. In pyridine, complexation passed through [Ag2(SCN)] and AgSCN to [Ag(SCN)2], such that even when the [SCN ]T/[Ag ]r ratio was 0.059, 50% was present as the hnear [Ag(SCN)2] ion. 

The dithizone method for the determination of silver is used if atomic absorption is not available [38]. The method is empirical and very sensitive to other metal contamination. The problems of interference can be overcome using ammonium thiocyanate solution to remove the silver. The spectrophotometric measurement is carried out at 620 or 460 nm. 

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