Dithizone, reactions

When the zinc-dithizone reaction is carried out on filter paper or on a spot plate, 0.4 y and 0.05 y zinc, respectively, can be detected by a red fleck or coloration, if a solution of 10 mg dithizone in 100 ml carbon tetrachloride is used as reagent. 

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 ... 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

The extraction system which was measured by the HSS method for the first time was the extraction kinetics of Ni(II) and Zn(II) with -alkyl substituted dithizone (HL) [14]. The observed extraction rate constants linearly depended on both concentrations of the metal ion [M j and the dissociated form of the ligand [L j. This seemed to suggest that the rate determining reaction was the aqueous phase complexation which formed a 1 1 complex. However, the observed extraction rate constant k was not decreased with the distribution constant Kj of the ligands as expected from the aqueous phase mechanism. 

These experimental results strongly suggested a significant contribution of other processes than just the aqueous reaction. On this issue, the HSS method revealed that the dissociated form of the -alkyl-dithizone did adsorb at the interface generated by vigorous agitation [5]. Therefore, the extraction rate was analyzed by introducing the interfacial reaction... 

Noticeably, the reaction of Ni2+ with dithizone is quite slow and sluggish. Nevertheless, this slow reaction is significantly accelerated by the addition of nitrogen-containing bases like 1, 10-phenanthroline. The resulting complex may be represented by the following equation ... 

Strong nucleophilic character of dithizone could assist in the generation of such dipolar intermediates (422, 423, and 424), and there is plenty of general analogy for such intermediates. Particular reference may be made to the closely related cycloaddition reactions of meso-ionic 1,2-diazoles 373 and 382. The transformation 424 - 421 could well involve a pentacovalent phosphorane intermediate as well. 

The reaction of 2,6-dichloro-3-methyl-5-nitropyrimidine 163 with dithizone 164 resulted in the formation of the 6-chloropyrimido[4,5-< ]-l,3,4-thiadiazine 43 shown in Equation (26) which, as discussed in Section 10.20.5.4 (Equation 7), is a useful substrate for subsequent nucleophilic substitution <2005PS2477, 2004HC0335, 1997IJG223>. 

Disulphides, 1078 dialkyl, 496, 498 Dithizone, 955 Di-p-tolyl ketone, 930, 931 Dixon gauze rings, 97 n-Dodecane, 237 n-Dodecyl bromide, 282 n-Dodecyl chloride, 275 n-Dodecyl p-toluenesuiphonate, 825 Doebner reaction, 463, 465, 710, 711, 719... 

We see that the distribution coefficient for metal ion extraction depends on pH and ligand concentration. It is often possible to select a pH where D is large for one metal and small for another. For example. Figure 23-4 shows that Cu2+ could be separated from Pb2+ and Zn2+ by extraction with dithizone at pH 5. Demonstration 23-1 illustrates the pH dependence of an extraction with dithizone. Box 23-1 describes crown ethers that are used to extract polar reagents into nonpolar solvents for chemical reactions. 

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. 

Reaction of 2-chloro-3-nitropyridine (316) with TV -phenylbenzothiohydrazide and Et3N gave 1,3-diphenyl-17/-pyrido[2,3-e]-l,3,4-thiadiazine (317) (70%). The pyridine (316) reacted with dithizone under identical conditions to deliver the pyridothiadiazine (52) (75%). Compounds (317) and (52) are believed to be formed via a Smiles rearrangement <80JOC3677>. 

Honaker and Preiser reported the first fundamental kinetic mechanism of chelate extraction in 1962 [1]. They elucidated that the rate-determining step for the extraction of divalent metal ions with dithizone was the formation of their 1 1 complexes in the aqueous phase. They proposed that a simple batch extraction method could be used as an alternative method of the complicated stopped-flow method, which was the only method available at the time, to measure such a fast reaction rate. Since the 1970s, hydrometallurgy has been developed in many countries, and extensive kinetic studies on the metal extraction have been conducted in an effort to improve the extraction rate as well as to develop effective and reusable extractants. The extractants used in hydrometallurgy are required to be highly hydrophobic and readily coordinative with various metal ions. On the basis of the interfacial adsorptivity of the extractant, Flett et al. [2] expected an interfacial reaction mechanism in the chelate extraction process. There was, however, no experimental evidence to prove the interfacial mechanism directly [3]. 

An equation of the form of (23-32) has been derived and verified experimentally for the dithizone extraction of zinc. This equation may be derived by writing the equilibrium constant as the extraction constant of the reaction... 

Nevertheless, when Friedeberg carried out an extraction of silver using dithizone in carbon tetrachloride at pH 2, he found that copper was left in solution. Evidently, this separation is based on a slow rate of attainment of equilibrium of the reaction with copper in the presence of EDTA, for he noted that at higher concentrations the copper is extracted slowly. Friedeberg also found that EDTA prevents the extraction by dithizone of lead, zinc, bismuth, cadmium, nickel, cobalt, and thallium at any pH value. 

Extraction techniques that involve a chemical reaction can be classified as nonselective extraction or concentration, when more than one analyte is extracted from the solution by using the organic collectors (e.g., 8-hydroxyquinoline and dithizone derivatives) and selective extraction or separation. The first step in such an extraction technique is the formation of the complex by adding the reagent(s) to the solution of analyte, and after the extraction of the complex in an organic solvent. The problem that can arise in a SIA system with these types of extraction is the precipitate that is formed, and this can contaminate the other sample and also can block the tubing. To avoid these problems, it is necessary either to dilute the sample in such a way that the precipitation equihbria will not be reached and that all the complex will remain dissolved in the solution, or by derivatization of the hgands to... 

The selectivity of spectrophotometric methods has been greatly increased by the development of derivative spectrophotometry (see Chapter 1.5). Derivative spectrophotometry enables one to single out, by means of various mathematical algorithms of data processing, a separate signal due to a selected component, from the sum of absorbances of the analysed mixture. This technique was successfully applied in determinations of a number of elements in mixtures such as Pd, Pt and Au [37], Pd and Pt in iodide solutions [38], Au, Pd and Pt in bromide solutions [39], Ru(IIl) and Rh(IIl) in the form of octadecyldithiocarbamate complexes [40], Ru and Os in chloride solutions [41], Cu, Hg and Pb as dithizonates [42], complexes of various metals with 4-(2-pyridylazo)resorcinol [43], Fe(IIl) with EDTA in the presence of Cr(III), A1 and Mn [44], Cr(III) and Cu(II) with EDTA [45], and Cu and Co in a flow system [46]. Derivative spectrophotometry was also used in the study of Sr- complexing reactions with various crown ethers [47]. 

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