Lead rhodizonate spot test

Feigl et al. (1972) describe several spot tests for lead, including using sodium rhodizonate, dithizone, and potassium iodide. Sulfide solutions have been used to detect lead in paint (Vind and Matthews, 1976 Sayre, 1970). Proprietary kits for detecting lead in paint based on sodium sulfide or rhodizonate are being marketed, and other kits are available for detecting lead in other species (Luk et al., 1990). Results from spot test kits are discussed in the paragraphs that follow. 

Preliminary laboratory studies to evaluate the characteristics of proprietary spot test kits based on sodium rhodizonate have been completed by Lin-Fu (1985). These studies showed that a color change indicative of the presence of lead occurred in laboratory-prepared paint films at an area concentration of about 1 mg/cm a color change was not observed for films with lead concentrations less than 0.1 mg/cm. Studies also reported that these kits could detect about 0.1 pg of lead in a water-soluble salt. This implies that lead ions from the lead compound in the paint films were not being readily leached by the solution used in the kit. Further, it was reported that detection level was lower for lead nitrate than lead chloride. 

Procedure. A drop of the test solution is placed on filter paper and spotted with a drop of freshly prepared 0.2 % solution of sodium rhodizonate. A blue fleck or ring is formed if lead is present. When an intense reaction occurs, the blue fleck can be transformed to scarlet by spotting with a drop of buffer solution (pH = 2.8) containing 1.9 g sodium bitartrate and 1.5 g tartatic acid per 100 ml. 

Procedure. The test is conducted in a small quartz crucible. One or two mg of the finely ground sample are mixed with several mg of zinc chloride and a drop of water and taken to dryness. The evaporation residue is then heated to faint redness in the covered crucible. The cold residue is stirred with 1 or 2 drops of hot water and the suspension is transferred to a conical tube and centrifuged. A drop of the clear liquid is treated on a spot plate with a freshly prepared 0.2 % solution of sodium rhodizonate and three drops of concentrated ammonia. The mixing is accomplished by blowing on the liquid. A violet precipitate or color appears if calcium is present. A comparison test is required when tiny quantities of calcium are suspected. The same amount of zinc chloride should be used in both cases since it often gives a slight self-reaction due to slight contamination with calcium and lead. 

Crude and pure metals (Zn, Mg, Al, etc.), that are soluble in sulfuric acid, can be tested for lead as follows A little of the metal is dissolved in dilute sulfuric acid and the solution evaporated to fumes (SO3). The residue is taken up in hot water and filtered through a small paper. After washing free of acid, the paper is spread on a porcelain or glass plate, and spotted with buffer solution and sodium rhodizonate. A red fleck indicates the presence of lead sulfate and, consequently of lead. 

Zinc oxide or zinc dross can be tested for lead by direct spotting of the sample with sodium rhodizonate and buffer solution. A rapid method of detecting lead in zinc oxide is especidly useful if the material is to be used in the preparation of glass, because lead often enters glass in this way. A little (0.5 --1 mg) of the sample is fumed with sulfuric acid the residue is extracted with water filtered, and the undissolved material on the paper tested with the reagent and buffer solutions. 

Slightly colored or colorless materials are spotted at the impact area with a drop of the buffer solution followed by a drop of the reagent solution. If lead is present, a red fleck appears precisely on the spot where traces of lead have been retained. When traces of lead are to be identified or detected on intensely colored substrates, a piece of filter paper wetted with the buffer solution is pressed for several minutes against the surface being tested. A drop of sodium rhodizonate solution is then placed on the filter paper. When lead is present, a red fleck is formed on the paper and indicates exactly the location of the traces of lead on the substrate. 

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