Using dithizone

Chlorine at the percentage level at which it occurs in sea water is usually determined by classical procedures using standard silver nitrate as the titrant and potassium chromate indicator, or alternatively by the mercuric thiocyanate procedure using dithizone as indicator. As large dilutions of the original sample are involved in these analyses, it is essential to use grade A glassware and take all other suitable precautions, such as temperature control. 

Analysis. Colorimetric analysis of Pb with dithizone can be performed down to 200 ppb, with ETAAS and ICPAES going down to 1 ppb, and ICPMS to 0.1 ppb. Spot testing to a detection limit of almost 1 ppm can be carried out using dithizone and KCN. 

Workers at PerkinElmer (Perkin Elmer Ltd., Beaconsfield, UK, Private Communication) claim that sensitivity and precision in lead determinations in spinach were both improved by using the L vov platforms as opposed to normal off the wall techniques. This is because dependence on temperature is reduced. Johns et al. [26] has described a quantitative thin-layer chromatographic procedure for the determination of lead in plant tissues. The method is based on the use of ammonium pyrrolidinedithiocarbamate for the extraction and enrichment of lead. Instead of the previously reported conversion and visualisation of the lead complex in the short wavelength region using dithizone, the lead carbamate was converted to lead sulfide with the aid of a 6% solution of sodium sulfide in methanol/water (3 1) on silica plates, after development of the plates with toluene. See also Sects. 7.34.1,7.34.4 and 7.34.5... 

Jones et al. [280] used dithizone for post column derivativisation of cadmium, cobalt, copper, lead, nickel and zinc. The separation was achieved in aqueous media on a sulphonated 10% cross-linked polystyrene resin. Cadmium(II), cobalt(II), copper(II), lead(II), nickel(II) and zinc ions were detected down to nanogram levels. 

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. 

Reimann, K. Quick identification of heavy metal ions in water and sludge by means of thin-layer chromatography using dithizone. Z. Wasser Abwasser Forsch. 5, 3 (1972) C. A. 77. 9 470a (1972)... 

Earlier methods used to determine mercury in biological tissue and fluids were mainly colorimetric, using dithizone as the com-plexing agent. However, during the past two to three decades, AAS methods - predominantly the cold vapor principle with atomic absorption or atomic fluorescence detection - have become widely used due to their simplicity, sensitivity, and relatively low price. Neutron activation analysis (NAA), either in the instrumental or radiochemical mode, is still frequently used where nuclear reactors are available. Inductively coupled plasma mass spectrometry (ICP-MS) has become a valuable tool in mercury speciation. Gas and liquid chromatography, coupled with various detectors have also gained much importance for separa-tion/detection of mercury compounds (Table 17.1). 

Burguera and Burguera [16] developed a FI liquid-liquid extraction spectrophotometric method for the determination of cadmium in urine using dithizone. Original urine samples were directly injected into a buffered earner (pH 10.5) containing tartrate, and... 

Koch [1] has described a direct quantitative determination of di- -octyltin compounds in fat simulant HB 307 using dithizone reagent. This method has a lower detection limit of 0.75 ppm of the organotin compoimd. If the migration tests are carried out at a ratio of plastics surface (cm ) to amoimt of fat simulant (g) of 5 1, then the detection limit for organotin stabilisers is 15 pg/dm (Figure 7.1). 

Aldridge, W. N., Street, B. W., Spectrophotometric and Fluorimetric Determination of Tri- and Di-organotin and -organolead Compounds Using Dithizone and 3-Hydroxyflavone, Analyst [London] 106 [1981] 60/8. 

Sodium sulfate is the main inorganic salt expected in sulfonated surfactants. It may also be deliberately added to formulations as a filler or processing aid. Classical methods for sulfate determination are gravimetric or based on BaCl2 titration. It has been found that non-aqueous acid-base titration permits sulfate to be differentiated from sulfonates (18). Sulfate ion has been titrated with lead chloride solution using dithizone as indicator and acetone/water as solvent (19). Alternatively, it may be titrated with barium perchlorate with Sulfonazo III as indicator (9). 

Using Figure 7.27, explain how an aqueous mixture of Cu +, Pb +, and Cd + can be separated by extraction with dithizone in CCI4. 

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 +. ... 

The use of dithizone in combination with various organic bases for synergistic extraction has already been indicated (Section 6.4). 

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... 

Only one example of the use of dithizone in solvent extraction will be given in order to illustrate the general technique involved. 

Procedure. To 10 mL of a solution (Note 2) containing from 1 to 10 pg of nickel(II) add 5mL of a phthalate or acetate (ethanoate) buffer of pH 6.0 or, if the sample solution is acidic, use dilute ammonia to adjust the pH. To this solution now add 15mL of a chloroform solution of dithizone (7 x 10-5M) and 1,10-phenanthroline (3 x 10-5M). Shake the phases for five minutes in a... 

Notes. (1) The reagent solution should be freshly prepared using analytical-grade dithizone and 1,10-phenanthroline, preferably taken from new or recently opened reagent bottles. 

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. 

Discussion. For the determination of small amounts of lead (0.005-0.25 mg) advantage is taken of the fact that when a sulphide is added to a solution containing lead ions a brown colour, due to the formation of colloidal lead sulphide, is produced. However, for general use the dithizone method (see Section 6.13) is to be preferred and this will be described. 

Procedure. Place 10.0 mL of the working lead solution in a 250 mL separatory funnel, add 75 mL of the ammonia-cyanide-sulphite solution and then by the cautious addition of dilute hydrochloric acid adjust the pH of the solution to 9.5 (pH-meter). This operation must be carried out slowly if the pH of the solution falls even temporarily below 9.5, HCN may be liberated and so use of a fume cupboard is necessary. Now add 7.5 mL of the dithizone reagent to the separatory funnel, followed by a further 17.5 mL of chloroform. Shake for 1 minute, allow the layers to separate, then remove the chloroform layer. Measure the absorbance of this against a blank solution, using a 1 cm cell and a wavelength of 510 nm (green filter). 

MDHS 8 Lead and inorganic compounds of lead in air Colorimetric field method using sym-diphenylthio-carbazone (dithizone)... 

TLC has been used for the identification of heavy metals in aquatic plants. For this purpose, plants were mineralized with cone. H2SO4, HNO3, and HjOj, extracted with water, derivatized with dithizone, and chromatographed. The identified metals were zinc, copper, mercury, and lead [31]. 

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