Spot testing

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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