Sodium rhodizonate test

J. H. Dillon, Sodium Rhodizonate Test A Chemically Specific Test for Lead in Gunshot Residues, AFTE Journal 22, no. 3 (October 1990) 251. 

Sodium rhodizonate test Barium salts yield a reddish-brown precipitate with sodium rhodizonate (see under Barium, Section III.31, reaction 7). Sulphates and sulphuric acid cause immediate decolourization because of the formation of insoluble barium sulphate. This test is specific for sulphates. 

Figure 10.11 The sodium rhodizonate test An example is shown in the color insert. 

Analysis. The bright scarlet flame color of Sr indicates that atomic emission and absorption methods wiU be good for its analysis. Sr is quantitatively determined by colorimetry down to 200 ppm using chloranrlic acid, by atomic absorption spectroscopy (AAS) to 100 ppb, to 1 ppb by electrothermal absorption spectroscopy (ETAS), and to 0.1 ppb by inductively-coupled plasma emission spectroscopy (ICPES) and inductively-coupled plasma mass spectroscopy (ICPMS). A spot test for Sr which extends to 40 ppm is provided by K2Cr04 and sodium rhodizonate. 

Sodium rhodizonate reagent reddish-brown precipitate of strontium rhodi-zonate in neutral solution. The test is applied to the elements of Group IV. Barium reacts similarly and a method for the detection of barium in the presence of strontium has already been described (Section III.31, reaction 7). To detect strontium in the presence of barium, the latter is converted into the insoluble barium chromate. Barium chromate does not react with sodium rhodizonate, but the more soluble strontium chromate reacts normally. 

Place a drop of barium chloride solution upon filter or drop-reaction paper, followed by a drop of a freshly prepared 0-5 per cent aqueous solution of sodium rhodizonate. Treat the reddish-brown spot with a drop of the acid or alkaline test solution. The coloured spot disappears. 

The precipitation capacity of sodium rhodizonate is particularly interesting within the group of the alkaline earth metals. Barium gives a red-brown precipitate of barium rhodizonate. Strontium salts react in the same way with rhodizonates, but calcium salts do not. Under suitable conditions, however, barium may be identified in the presence of strontium. The rhodizonate test is thus recommended for the detection of barium within the ammonium carbonate (alkaline earth) group, or for its detection when bivalent heavy metals are known to be absent. 

Procedure. A drop of the neutral or slightly acid test solution is placed on filter paper and then a drop of an aqueous 0.2 % solution of sodium rhodizonate. According to the amount of barium present, a more or less intense red-brown stain is formed. 

Strontium salts react similarly to barium salts with sodium rhodizonate. However, strontium rhodizonate is easily soluble in dilute hydrochloric acid in the cold, whereas the barium compound is converted into an insoluble bright red acid salt. It must be noted that small amounts of barium rhodizonate prepared in a test tube in neutral solution, and then treated with hydrochloric acid, seem to dissolve, because the red barium salt is formed in a fine state of division and is hard to see. If, however, the reaction is carried out on paper, the brown-red stain of barium rhodizonate is turned to a... 

If no brown fleck is formed on treating a drop of the test solution with a drop of sodium rhodizonate on filter paper, the absence of both barium and strontium is indicated a brown fleck may mean that either ion is present, or both. If the fleck disappears on treatment with dilute hydrochloric acid, only strontium is present if it turns red, barium (or barium and strontium) is present. 

If neutral or weakly acid solutions are to be examined, the following procedure may be used for the detection of barium and strontium in a micro drop. One drop of 0.5 % sodium rhodizonate solution is placed on Whatman filter paper No. 1. After the liquid has soaked in, the center of the yellow spot is treated with a drop of the test solution. A brownish or reddish-brown stain appears at once if barium or strontium is present. The fleck is then spotted with 1 or 2 drops of a mixture of equal parts of a saturated water solution of dimethylamine hydrochloride and 95 % ethanol. 

Test with sodium rhodizonate and alkali hydroxide ... 

Procedure, The test is conducted in a depression of a spot plate or on filter paper (S S No. 601). One drop of the neutral or weakly acid test solution is treated with a drop of freshly prepared 0.2 % sodium rhodizonate solution and one drop of 0.6 iST sodium hydroxide. The solution is mixed by blowing briefly... 

Test for calcium sulfate with sodium rhodizonate... 

Barium salts give a red-brown precipitate with rhodizonic acid or sodium rhodizonate (see page 119). The precipitate, produced on paper, is stable toward dilute hydrochloric acid, whereas sulfates and sulfuric acid cause immediate decolorization owing to the formation of insoluble barium sulfate. This fact can be applied to give a sensitive and specific test for sulfates. 

The test can also be made in a depression of a spot plate. A suspension of colored barium rhodizonate is prepared by bringing together one drop each of dilute barium chloride and sodium rhodizonate solution. The color is discharged when a drop of a solution containing sulfate is added. A blank test is necessary if small quantities of sulfate are being sought. 

If witherite and strontianite are to be differentiated, milligram quantities of the powdered carbonates are mixed in micro crucibles with about 0.5 g of ammonium sulfate and carefully heated until no more fumes of sulfur trioxide appear. The carbonates are transformed to sulfates by this treatment. The cold residues are spotted with a 0.2 % solution of sodium rhodizonate. A red color is formed with the portion originally containing strontianite. This test succeeds in the presence of calcite also, because, like barium sulfate, the calcium sulfate remaining after the fuming with ammonium sulfate does not show any reaction with sodium rhodizonate. 

Procedure. A micro crucible is used. A few grains of the solid (or ignition residue) is mixed with about 0.5 g of ammonium chloride and heated over a bare flame until fumes are no longer evolved. The residue is transferred to a small test tube and treated with a drop or two of freshly prepared 0.2 % sodium rhodizonate solution. The formation of a red to red-brown precipitate indicates the presence of barium sulfate in the sample. 

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. 

As outlined in the previous chapter, calcium sulfate can be detected by its reaction with sodium rhodizonate (see page 160) or sodium selenosulfate (see page 164). These tests can be applied in mineral investigations to identify gypsum and anhydrite. 

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. 

The rhodizonate test described on page 284 has an identification limit of 0.1 y and consequently only succeeds with one drop of a solution at a maximal dilution of 1 500,000. Lead can be detected in solutions of far greater dilutions if 10 ml of the sample is treated with sodium mercurisulfide... 

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. 

Procedure. One drop of the buffer solution (page 568) and a drop of 0.2 % sodium rhodizonate solution are placed on a flat surface of the sample. No immediate reaction is observed when the solutions are stirred, but after 3-4 minutes, the spotted area becomes slightly gray. The color intensity increases and becomes dark violet after about 6 minutes. If the surface of the metal is scratched with a steel needle and then spot-tested with the two solutions, the furrows turn dark violet within 3 minutes. It is also feasible to treat 1-3 mg of cuttings with the solutions. The surface of the particles turns dark violet after a few minutes. 

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