Dithizone trace metal complexes

The reactions of silver(I) with 1,5-diphenyl thiocarbazone (56 dithizone = HaDz) by comparison to thiosemicarbazones have been thoroughly investigated.442,444 Dithizone has for many years been used as a colorimetric reagent for trace metal ions. Its metal complexes are of two types, the so-called primary and secondary dithizonates. Primary dithizonates are generally formed at low pHs where the ligand becomes mono depronated (pA = 4.5) but retains the NH proton. Secondary dithizonates are formed in the presence of an excess of metal and/or at higher pH values where the ligand becomes fully deprotonated. Since it has been estimated that for free dithizone pK2 > 15, the second proton obviously becomes labilized in the presence of metal ions. 

Fig. 4.13 Separation of a standard trace metal sample (extracted with dithizone solution). Divisions on baseline 1 min per division peaks, dithizone complexes of (a) Hg, (b) Cu, (c) Ni,...

Metal chelate complexes Kuz min summarized the developments in liquid-liquid extraction of metals and groups of metals with dithizone, dithiocarbonates, and 8-hydroxyquinolates, as well as other reagents and mixtures of reagents. He pointed out that extractions of traces of several elements can be carried out to advantage in several stages by use of pH control. 

It has been reported that y3-CD could improve the selectivity of the color reactions of various metal ions with triphenylmethane, xanthene acid dyes and some other coloring reagents. The effect of fi-CD on the association compound system of metal (Mo, Zn, Co)-thiocyanate basic dyes such as malachite green, crystal violet, rhodamine B, rhodamine 6G and butyhhodamine B, has been investigated and the result shows that /3-CD could contribute to a more sensitive and stable system which improve the solubility of the basic dyes and produce a favorable microenviromnent for the color reactions [63]. /3-CD could be employed to solubilize the 1,2-amino anthraquinone in water due to the formation of inclusion complex which acts as a ligand for metal ions could be used for the determination of palladium at trace levels by spectrophotometry. In the spectrophotometric determination of microamounts of Zn based on the Zn-dithizone color reaction, -CD could increase the apparent molar absorptivity at 538 nm by 8.37 times. In the presence of cr-CD, the determination sensitivity of copper in leaves based on the color reaction of Cu(II) and mesotetrakis (4-methoxy-3-sulfophenyl) porphyrin was enhanced by 50% in the spectrophotometric analysis [64,65]. 

The solvent extraction of metal-chelate complexes for trace analysis of water samples has been briefly reviewed (473). Concentration factors of less than 1000 are to be expected because of the need to handle reasonable quantities of organic solvent. However, since not all of the organic solvent sample is used in the analysis, this results in a less than optimal use of the system. An excellent batch-extraction apparatus recently has been described (474) for the extraction of organic substances from water. Concentration factors of more than 10,000 were easily obtained if organic volumes of 100 /w, or less, which were used to extract 1 liter of solution, could be fully analyzed (eg., by AAS). A useful system has been described (475) which combines three chelating agents (dithizone, 8-hydroxyquinoline, and acetylacetone) and quantitatively collects Al, Be, Cd, Co, Cu, Fe, Pb, Ni, Ag, and Zn in one extraction at a pH of 6. 

Precipitation of Ag as chloride, or better as bromide, with a slight excess of precipitant may be a useful method of separation. Traces of Ag have been separated by thallium iodide as the collector. The precipitation can be dissolved in diluted H2SO4-H2O2 solution. A widely applicable method for the isolation of traces of Ag consists in co-precipitation with elemental tellurium formed by adding tin(II) chloride to 2 mol 1 HCl solution of the sample containing Te(IV) (see above). The extraction constant of primary Ag dithizonate is such that Ag can be extracted quantitatively from 6 mol 1 H solution by the use of 0.001%(w/v) dithizone solution in carbon tetrachloride. Anions such as halides and thiocyanates that form slightly soluble or complex Ag salts must be absent when extraction is made in mineral acid medium. Other metals extracted at high acidity are Pd, Au, and Hg. Copper will also be extracted. 

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