Spot-test

To form a colored compound in a spot test with a paint film, lead ions must be leached or separated from the lead-containing species. Dissolution of lead ions depends on the type, concentration, and dispersion of lead-containing compounds in the film and the binder, and the position of the lead-containing layer(s) in the composite film. Lead pigments used in relatively high concentrations provide hiding or control corrosion and in lower concentrations, they provide color in still lower concentrations, they facilitate drying. 

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. 

The paint and substrate on each test specimen were analyzed for lead using a PXRF detection unit. The PXRF unit was calibrated daily and the presence of lead was reported in units of mg/cm. The actual painted materials used were either flat or round. A general discussion of the measurement for lead in these materials and other shapes of painted materials follows. 

Many studies have been carried out on the use of spot tests based on 

The incidence of false positive results was not reported in the Dewberry Davis study. However, several elements sometimes found in paint films can be expected to contribute to false positive results if present in sufficient concentrations and suitable chemical form these include copper, cobalt, mercury, manganese, and iron. Vind et al. (1976) reported that positive responses were not obtained under typical testing conditions for iron oxide pigments. In contrast, they found that neat biocides (containing mercury or copper) and driers (containing cobalt or manganese) did turn black in the presence of sulfide ions. However, they concluded that given the usually low concentrations of these materials in paints, interference due to these elements would be minimal. 

If ammonium molybdate is added to an acidified orthophosphate solution, followed by a reducing agent such as stannous chloride or ferrous sulphate, an intense blue colour ( molybdenum blue ) will develop. A more satisfactory and sensitive version of this test is to use benzidine as the reducing agent in which case an intense blue colour arises from both the molybdenum blue and a reduced product from the benzidine which is also blue. 

The benzidine reaction forms the basis of a very sensitive spot test. A spot of acidified phosphate solution is placed on a filter paper followed by a drop of ammonium molybdate solution and then the benzidine solution. The filter paper is held over ammonia to neutralise the free acid and a blue stain appears. Under these conditions there is no interference from silicate or arsenate if they happen to be present. The method is sensitive to about 1 part in 50,000. 

Another test for phosphorus (as phosphate) is based on the fact that the phosphomolybdates (and phosphotungstates) form deeply coloured insoluble lakes with triphenylmethane dyes such as methyl violet or malachite green (Chapter 12.8). The procedure is as follows a drop of test solution is placed on a filter paper which is then sprayed with a 1% solution of methyl violet. After about half a minute the paper is then sprayed with a solution of acidified ammonium molybdate and a blue spot develops. This method is sensitive to 1 part in 500,000. 

Para-phenylenediamine is an excellent reagent for detection of an aldehyde group in a hchen substance. Its disadvantages are instability and suspected carcinogenic activity. To overcome the rapid oxidation of an alcoholic solution of p-phenylenediamine a method for stabilizing the solution for at least one year by adding disodium edetate (ethylenediamine tetraacetic disodium salt) has been described 441). 

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Jungreis, E. Spot Test Analysis, 2nd ed. Wiley New York, 1997. 

E. Eeigl, Spot Tests in OrganicMnalyses, 7th ed., Elsevier Scientific Publishing Co., New York, 1966, p. 211. 

Qualitative. The classic method for the quaUtative determination of silver ia solution is precipitation as silver chloride with dilute nitric acid and chloride ion. The silver chloride can be differentiated from lead or mercurous chlorides, which also may precipitate, by the fact that lead chloride is soluble ia hot water but not ia ammonium hydroxide, whereas mercurous chloride turns black ia ammonium hydroxide. Silver chloride dissolves ia ammonium hydroxide because of the formation of soluble silver—ammonia complexes. A number of selective spot tests (24) iaclude reactions with /)-dimethy1amino-henz1idenerhodanine, ceric ammonium nitrate, or bromopyrogaHol red [16574-43-9]. Silver is detected by x-ray fluorescence and arc-emission spectrometry. Two sensitive arc-emission lines for silver occur at 328.1 and 338.3 nm. 

Analysis of zinc solutions at the purification stage before electrolysis is critical and several metals present in low concentrations are monitored carefully. Methods vary from plant to plant but are highly specific and usually capable of detecting 0.1 ppm or less. Colorimetric process-control methods are used for cobalt, antimony, and germanium, turbidimetric methods for cadmium and copper. Alternatively, cadmium, cobalt, and copper are determined polarographicaHy, arsenic and antimony by a modified Gutzeit test, and nickel with a dimethylglyoxime spot test. 

Spot Test (AASHTO T102). The test distinguishes asphalts that contain bodies poorly tolerated by the asphalt system. 

Spot tests for determining chlorosulfuric acid are based on the use of powdered tellurium, which gives a cherry-red color, and powdered selenium, which gives a moss-green color in the presence of the acid. 

Testing and Control. Analysis and testing are required whenever a new plating solution is made up, and thereafter at periodic intervals. The analyses are relatively simple and require Httie equipment (78—80). Trace metal contaminants can be analy2ed using spot tests, colorimetricaHy, and with atomic absorption spectrophotometry (see Trace and residue analysis). Additives, chemical balance, impurity effects, and many other variables are tested with small plating cells, such as the Hull cell developed in 1937 (81,82). 

Chemical analysis of surfaces within crevices often reveals high concentrations of chloride. Chemical spot tests can indicate acidic pH. 

Corrosion products and deposits. All sulfate reducers produce metal sulfides as corrosion products. Sulfide usually lines pits or is entrapped in material just above the pit surface. When freshly corroded surfaces are exposed to hydrochloric acid, the rotten-egg odor of hydrogen sulfide is easily detected. Rapid, spontaneous decomposition of metal sulfides occurs after sample removal, as water vapor in the air adsorbs onto metal surfaces and reacts with the metal sulfide. The metal sulfides are slowly converted to hydrogen sulfide gas, eventually removing all traces of sulfide (Fig. 6.11). Therefore, only freshly corroded surfaces contain appreciable sulfide. More sensitive spot tests using sodium azide are often successful at detecting metal sulfides at very low concentrations on surfaces. 

Some cracks were clustered into small areas along one side of the tube, but close examination revealed numerous cracks scattered over the tube surface. Chemical spot tests revealed the presence of sulfur-containing compounds on the external surface. 

F. Fiegl, Spot Tests in Organic Analysis, 7th Edn, Elsevier Science, New York, 1989. ISBN 0444402098. 

It is also desirable to spot test the instrument s response between calibrations. For this purpose, several suppliers of compressed gas prepare cylinders containing almost any desired concentration of the gas or vapor of interest. If it is not practical... 

F. Feigl, Spot Tests, Organic Applications. Vol. II, p. 296. Elsevier, Amsterdam, 1954. 

Tiipfel-probe,/. spot test, drop test, -reaktion, /. spot reaction, drop reaction, -zelle, /. pitted cell. 

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