Dithizone complexes with metals

The nature of the donor atoms in the chelating agent. Ligands which contain donor atoms of the soft-base type form their most stable complexes with the relatively small group of Class B metal ions (i.e. soft acids) and are thus more selective reagents. This is illustrated by the reagent diphenylthiocarbazone (dithizone) used for the solvent extraction of metal ions such as Pd2+, Ag+, Hg2+, Cu2+, Bi3+, Pb2+, and Zn2 +. ... 

Multi-element analyses involving solvent extraction and high performance liquid chromatography (HPLC) have also been described. The extracts, containing metal-chelate complexes with sulphur-containing reagents, such as dithizone and diethyldithiocarbamate, were used directly for determination of the metals by HPLC.14... 

The cobalt complex is usually formed in a hot acetate-acetic acid medium. After the formation of the cobalt colour, hydrochloric acid or nitric acid is added to decompose the complexes of most of the other heavy metals present. Iron, copper, cerium(IV), chromium(III and VI), nickel, vanadyl vanadium, and copper interfere when present in appreciable quantities. Excess of the reagent minimises the interference of iron(II) iron(III) can be removed by diethyl ether extraction from a hydrochloric acid solution. Most of the interferences can be eliminated by treatment with potassium bromate, followed by the addition of an alkali fluoride. Cobalt may also be isolated by dithizone extraction from a basic medium after copper has been removed (if necessary) from acidic solution. An alumina column may also be used to adsorb the cobalt nitroso-R-chelate anion in the presence of perchloric acid, the other elements are eluted with warm 1M nitric acid, and finally the cobalt complex with 1M sulphuric acid, and the absorbance measured at 500 nm. 

Keeping in mind the above work, experiments were carried out to examine the effects of ultrasound, on the dissolution of zinc metal in an alkaline medium and the decomposition of zinc-dithizone complex in the presence of an ultrasonic field. To examine the effect of power ultrasound on the dissolution of zinc metal in alkaline media, 0.0480 g zinc metal was treated with 10 ml of 5 M NaOH solution. Two samples of this solution were exposed to ultrasound for 15 and 30 min, while, control samples were also kept in the similar condition and for the same duration. To compare their spectra and concentration of dissolved zinc in sonicated and control conditions, zinc-dithizone complex was formed by adding 0.5 ml of 0.005% dithizone solution. The red coloured complex, thus obtained, was extracted in chloroform and made upto to the mark in 25 ml volumetric flask with chloroform. 

Zn-dithizone complex gradually decreased (Fig. 9.2). Mechanism for the decomposition of this complex could thus be explained that the complex was broken into zinc ions and dithizone before dithizone molecules were oxidised to S=C (N=NC6H5)2, which, however, did not react further with metal ions [114] and therefore, the Zn-dithizone complex was decomposed. The decomposition of Zn-dithizone complex could thus be attributed to the oxidation of dithizone molecule in ultrasonic field. 

In actual practice, an excess of the standard solution of disodium edetate is added to the sample, pH is adequately adjusted for the residual titration with a metal-ion solution e.g., ZnS04 and employing an appropriate indicator which is sensitive enough to the respective titrant. However, the metal ion under estimation remains firmly complexed with the EDTA and offers little interference with the Zn-EDTA complex formed. It has been established experimentally that bismuth readily yields a highly stable complex which may be titrated conveniently between pH 1 and 2. Bismuth forms a stable complex by reacting with EDTA quantitatively at pH 4.0 and, therefore, dithizone is employed as an indicator to detect the end-point for it has a transition state of colour at pH 4.6. 

Dithizone (diphenylthiocarbazone) is a green compound that is soluble in nonpolar organic solvents and insoluble in water below pH 7. It forms red, hydrophobic complexes with most di- and trivalent metal ions. 

The crystal structures of methyl- and phenyl-mercury(II) dithizonates have been determined as part of a study on the photochroism of such compounds.149 In both structures the chelate is planar, with an irregular three-coordination at the metal including N and S of the ligand. The article provides a short structural review of dithizone complexes. 

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,...

The most widely applied reagents have been chelating agents which will complex with many metals, e.g. dithizone and the various thiocarbamate derivatives such as diethyldithiocarbamate and pyrrolidine dithiocarbamate. The latter agent as the ammonium salt (APDC) has been shown to complex some thirty elements [19] most of which can be readily extracted into various solvents. 4-Methylpentane-2-one (methyl isobutyl ketone or MIBK) is usually the favoured solvent because of its excellent compatibility with flames. The solubility of MIBK in water is not negligible and this limits the available concentration factor to ten higher molecular weight ketones (e.g. decan-2-one) offer better concentration factors and chloroform up to fifty times, but this latter solvent is only really suitable for electrothermal atomisation. 

Dithizone test Dithizone (diphenyl thiocarbazone) forms complexes with a number of metal ions, which can be extracted with carbon tetrachloride. The zinc complex, formed in neutral, alkaline or acetic acid solutions, is red ... 

Diphenylthiocarbazone ( dithizone ) forms intensely colored complexes with, for example, Mn, Fe, Co, Ni, Cu, Zn, Pd, Ag, Cd, Pt, Au, and Hg, which are soluble in CCLi and CHCI3, a property which facilitates then-separation and individual determination.Solutions ofmost metal dithizonates in organic solvents have the interesting property of being photochromic (see Photochromism), and the relatively long half-life of the photoexcited mercury(II) dithizonate complexes (ti/2 ca. 1 min) has facilitated then-investigation by conventional specttoscopic techniques. ... 

Other extraction agents such as dithizone, diethyl dithiocarbamates, and 8-hydroxy-quinoline (oxine) are also useful as chelating agents and extract several metals including transition metals, alkali and alkaline metals. The dithizone can extract up to 20 elements and these complexes behave the same and there is no additional advantage of using one over the other. The compound 8-hydroxyquinoline will form stable complexes with Al, Ca, Sr and Mg at pH 8.0 these metals form unstable complexes with APDC. At carefully controlled pH the oxidation states of metals may also separated, e.g. Fe[II] and Fe[III], The stmcture of 8-hydroxyquinoline is ... 

Lead on the surfaces of leaves is dissolved by shaking with nitric acid solution. The lead is extracted as the dithizone complex into methylene chloride at pH above 9. The intensity of the color of the complex is measured spectrophotometrically and compared to a calibration curve prepared similarly from lead standards to calculate the amount of lead. Cyanide and sulfite may be added as masking agents to eliminate most interference from other metals. 

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