Dithiocarbamate precipitation methods

Ellis and Teyden (private communication) used dithiocarbamate precipitation methods to determine between 2 mg/1 and 2 pg/1 of five elements. Table 11.11 shows the excellent agreement generally found between XRF results obtained using a Link XR 200/300 instrument and AAS techniques in the analysis of pre-concentrates. Agreement does not extend over the whole concentration range examined for manganese. Some disparity also occurs in zinc determinations and it is believed that the error is in the graphite furnace results. 

Ellis and Leyden (private communication) used dithiocarbamate precipitation methods to determine between 2 mg/1 and 2 pg/1 of five elements. Table 1.11 shows the excellent... 

Tris(pyrrolidine dithiocarbamato-)—cobalt(III) chelate, the precipitate formed by adding ammonium pyrrolidine dithiocarbamate to cobalt(II) solutions, has been found to be a good matrix for preconcentrating lead and several other metals by co-precipitation. Concentration factors of 40 to 400 are available by the method. The analyte is co-precipitated on the Co-APDC from a litre of sample. The precipitate is filtered on a fine porosity glass sinter and redissolved in a small volume of 6 M nitric acid. The solution is then used for atomic absorption analysis. 

The analysis of environmentally-relevant samples is a major field of application. Based on the work of Garbarino and Taylor [421], a method has been proposed by the US EPA (Environmental Protection Agency) [422] and later by DIN [423] for waste water analysis. The latter, standardized procedure describes the sample decomposition, the analytical range for 22 elements and frequent interferences of ICP-AES in waste water analysis. For the analysis of natural waters, hydride generation [424], preconcentration based on liquid-liquid extraction of the dithiocarbamate complexes [425], adsorption of trace elements onto activated carbon [426] and also co-precipitation [e.g. with In(OH)2] [427], etc. have been reported and special emphasis has been given to speciation (as given in the Refs, in [428]) and on-line preconcentration [134]. 

Zn by using the cobalt dithiocarbamate co-precipitation and back extraction (14) method. The blanks underwent a similar procedure. Both blanks and sample extracts were analyzed with a Hitachi 180-80 atomic absorption spectrophotometer, equipped with Zeeman background correction. The method of standard additions was used to correct matrix effects during analysis. The analytical conditions during the experiment were those recommended by the vendor (15). 

Several methods have been reported for concentrating lead, cobalt, and nickel in blood, urine, brines, and water prior to final determination by atomic absorption. Sprague and Slavin (6) described a procedure for determining these elements plus copper, cadmium, iron, and manganese in concentrated potassium chloride solutions. The metals were chelated with ammonium pyrrolidine dithiocarbamate (APDC) and extracted with methyl isobutyl ketone (MIBK). It was reported that the optimum pH for the extraction was approximately 2.8. Berman (J) described a similar chelation-extraction procedure for determining lead in urine and blood. Burrell (2) developed a procedure for determining cobalt and nickel in natural waters by atomic absorption in which both metals are first coprecipitated with ferric chloride from ten liters of water. The separated precipitate is subsequently dissolved and made up to 100 ml. volume with hydrochloric acid and water so that the final pH of the solution is 2.5. The nickel and cobalt are then chelated with APDC and extracted with three 10 ml. volumes of MIBK. Three extractions are necessary to achieve complete recovery of the chelated cations. A detection limit of 0.3 fig. of nickel per liter and 0.15 fig. of cobalt per liter was found. 

Another method that has been employed with some success to preconcentrate elements in natural water samples is co-precipitation. This relatively simple method offers the advantage of giving a fairly uniform deposit that can be collected easily. One of the earlier applications of this technique involved the use of iron hydroxide as a co-precipitant for the determination of Fe, Zn, and Pb in surface waters [56]. One of the more popular co-precipitants in use today is ammonium pynolidine dithiocarbamate (APDC). In the application of this method to the analysis of natural waters, detection limits in the range 0.4-1.2 ppb have been claimed for the elements V, Zn, As, Hg, and Pb [57]. Other co-precipitants have been described, including the use of iron dibenzyl dithiocarbamate for the determination of U at the ppb level in natural waters [58], zirconium dioxide for the determination of As in river water [59], and polyvinylpyrrolidone-thionalide for the determination of Fe, Cu, Zn, Se, Cd, Te, Hg, and Pb in waste and natural water samples [60]. The use of... 

A good quantitative method for the determination of dithiocarbamate involves its precipitation as the sparingly soluble zinc salt. The precipitate is then dried at about 60 and weighed. An alternative procedure involves ignition to zinc oxide. Miller used a volumetric method... 

Probably the most widely used procedure for trace metal analysis of seawater over the last 20 years has been preconcentration followed by ETAAS because of the wide availability, good sensitivity, and large range of elements that can be measured by this method. Preconcentration procedures have in general been one of three types. First, co-precipitation, with iron(III) hydroxide or cobalt pyrrolidinedithio-carbamate, being the most widely used co-precipit-ants. Second, cornplexation followed by solvent extraction with a number of different complexants, of which dithiocarbamates, 8-quinolinol, and dithi-zone are particularly popular, and with extraction into a range of solvents. The third approach is extraction on to a chelating column (usually Chelex-100 but recently also other complexants such... 

Most of the instrumental methods currently available do not provide the sensitivity or freedom from matrix interferences to determine trace elements in seawater at the picomolar and nanomolar levels directly. Therefore, in most cases a pre-concentration step (and separation from the matrix for some methods) is necessary before instrumental detection. These pre-concentration steps may be selected from a wide variety of selective techniques for example, liquid-liquid extraction with chelating agents (such as dithiocarbamates), chelating cation exchange with resins (such as Chelex-100), electrochemical pre-concentration in ASV and CSV, hydride generation for the measurement of metalloids or co-precipitation with Mg(OH)2 or selected metal chelates. Some of these techniques will be discussed in more detail in conjunction with the analytical procedures outlined in Sections 12.2-12.4. 

A more specific titration method than either of the above is based upon formation of a dithiocarbamic acid. The method given below follows that of Critchfield and Johnson for the titration of secondary aliphatic amines, but results may vary within a range of about 0-5 per cent. The precipitate which forms on addition of the carbon disulphide dissolves as the titration proceeds ... 

---