Primary metal dithizonates

It functions as a monoprotic acid (pKa = 4.7) up to a pH of about 12 the acid proton is that of the thiol group in (C). Primary metal dithizonates are formed according to the reaction ... 

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. 

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. 

The extraction constant for primary silver(I) dithizonate, from aqueous solution to a range of organic solvents, has been found to he in the range log K = 5.8-8.94.442,444 It was concluded that changing the solvents did not achieve any worthwhile improvement in separability of silver from other metal ions compared to what could be achieved by addition of suitable masking agents. Carbon tetrachloride was the preferred extractive solvent (Amax = 463 nm).444... 

A primary or secondary dithizonate is obtained, depending on whether the dithizonate reacts as the anion of the monobasic acid (HDz ) or the dibasic acid (Dz ). Secondary dithizonates are formed by only a few metals. They are represented by such formulae as AgaDz and CuDz. Generally, acidic media and excess of dithizone favour the formation of primary dithizonates, whereas in alkaline media, and with insufficient amounts of the reagent, secondary dithizonates are formed. 

Investigations of the structures of primary dithizonates [89] have shown the metal to be bonded to the sulphur atoms by replacement of the hydrogen in the thiol group, and also co-ordinately bonded to a nitrogen atom, according to the formula 4.38 (for M " ). 

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