Dithizone masking agents

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... 

Tungsten was separated on DEAE-Sephadex for determination of trace amounts of alkali and alkaline-earth metals by AAS without matrix interference Dithi-zone in o-dichlorobenzene was used as the stationary phase on a Chromosorb support for the enrichment and separation of Zn,Cd,Hg and Cu By adding selective masking agents to the sample solution the adsorption of the traces was prevented the matrix was adsorbed. The separation of mercury from large amounts of copper on columns coated with silver dithizonate for example was reported. 

Although the number of known specific reagents is rather small, there exist some methods based on the use of group reagents (that react with many elements), which in the presence of suitable masking agents and at proper pH have a specific action. Examples of such systems may be the determination of Zn with dithizone at pH 4-5 in the presence of... 

Selectivity in spectrophotometric methods for determining metals with dithizone is attained by controlling the acidity of the medium and using masking agents such as cyanide, EDTA, thiosulphate, or iodide. 

Extraction of bismuth with dithizone in CHCI3 or CCI4, in the presence of masking agents, is a selective method for separating traces of Bi. It is often connected with the determination of Bi as dithizonate (see below). 

When cyanide and tartrate are present as masking agents, only Pb, TI(I), and Sn(Il) dithizonates are co-extracted with bismuth from slightly alkaline medium (pH 8-9.5). Tin(IV) does not react with dithizone. Lead and thallium can be separated from bismuth since their dithizonates are unstable in slightly acidic medium (pH 3.0-3.5). After Bi, Pb, and T1 have been extracted into CCI4, the lead and thallium are stripped into an aqueous solution at pH 3.3. 

The principal masking agent used in the determination of Pb with dithizone is cyanide. 

The dithizone method is very sensitive. The molar absorptivity of zinc dithizonate in CCI4 solution (at Xm- 538 nm) is 9.26-10 (a = 1.42). If the correct pH and masking agents are used, the dithizone method is specific for zinc [34,35]. 

Add 6 drops of dithizone indicator to flask two and titrate it to a green end point. The fluoride acts as a masking agent to complex iron, titanium, silicon, calcium, and magnesium. 

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. 

Table 5 Use of Masking Agents in the Extraction of Metal Dithizonates - ...

Rather than adding masking agents or controlling the pH, the lead can be isolated beforehand by extracting lead iodide (ion association complex) or lead diethyldithiocarbamate. The lead iodide can be extracted with methylisobutyl ketone from HCl-acidified solution in the presence of KI (W2) while the diethyldithiocarbamate chelate can be extracted into carbon tetrachloride at pH 7 (L3). The lead can be back-extracted in the former case into an aqueous solution of sodium hydroxide and in the latter case into a solution of pH less than 7. Then it can be analyzed with dithizone. In this way, Zn, Cd, Hg, Ag, Al, and Sn do not interfere. 

Used as a 0.1 Af soln. in dil. aq. NaOH as masking agent in detn. of Zn with dithizone. Cryst. (H2O). Sol. hot H2O, EtOH, Me2CO, alkalis. 

Thiourea (Tu) and its derivatives (RR NCSNR R ) form strongly bound, water soluble complexes with Ag and the noble metals in acidic media. The Ag complex, for example, is [AgTua] , so that thiourea under these conditions serves as an excellent masking agent for Ag. As the pH is increased, however, the nature of the reaction changes, with the thiourea reacting similarly to dithizone or rhodanine, with release of a proton and forming a precipitate. 

Figure 11 shows such sigmoid curves for a hypothetical system where curve (a) is drawn for complex ML2 with [HL]org = 10-3 M and Kex = 102. Curves (b), (c) and (d) are drawn for [HL]org = 10-5 M, Ktx = 102 for complexes ML, ML2 and ML3 respectively. Curve (e) represents a complex ML2 for which [HL]org = 10-5 M but Kex = 10"2. The control that can be exercised by change of [HL]org and pH will be obvious and separations will always be possible if values of for two metals are different, for besides the variables mentioned above auxiliary complexing agents may be included in the aqueous phase to mask one or more of the unwanted cations (Section 10.3). Specific examples will be described for only one particular extractant (dithizone) to illustrate the general principle (Section 10.4.4.5). 

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