Ch. 30] NEED FOR FURTHER RESEARCH 549 



C olorimetric analysis. Colorimetric methods of analysis have had 

 wide application for some time in determining traces of metals in or- 

 ganic and inorganic materials (Sandell, 1944) . Such methods are based 

 on the conversion of the metal to be determined into a substance whose 

 solution (or suspension) is strongly colored. 



Colorimetric analysis was first applied to geochemical prospecting 

 on a large scale by the U. S. Geological Survey (Lovering, Sokoloff, 

 and Morris, 1948; Huff, 1948; Lakin et al, 1949; Reichen and 

 Lakin, 1949) . A complete analytical kit for determinations of zinc in 

 waters, soils, and vegetation can be packed in an easily portable tool 

 box, which can be replenished when necessary from stock solutions 

 carried by truck. Under normal working conditions in a temporary 

 shelter, one man can run 30 zinc determinations per day. The accuracy 

 is about ±30 percent, and the sensitivity is adequate to detect and 

 measure the zinc content of all normal samples. The advantages of 

 colorimetric methods are the portability, low initial cost, ease of opera- 

 tion, and sensitivity. 



Spot tests. Spot tests were first successfully applied in geochemical 

 prospecting by Soviet geologists (Sergeev, 1941, pp. 40-44). The basic 

 principle is much the same as for colorimetric analysis except that the 

 sensitive reagent is impregnated in filter paper and the estimation made 

 by observing the color of a spot produced by a drop of the solution 

 to be tested (Feigl, 1946). Russian workers report satisfactory re- 

 sults with copper, nickel, cobalt, and silver, with a productivity of 40 

 samples per element per man-day. Within the last year the U. S. Geo- 

 logical Survey has been conducting developmental work on spot tests 

 for copper and nickel. The advantages and disadvantages of spot 

 tests are much the same as for colorimetric analysis. Outstanding 

 features of spot tests, however, are ease of estimation and the fact that 

 a permanent record is obtained. 



Other methods. Other analytical methods that have been tried with 

 varying degrees of success in geochemical prospecting include polaro- 

 graphic analysis (Kolthoff and Lingane, 1941; Sergeev, 1941, p. 44), 

 nephelometry (Sandell, 1944, pp. 69-71), electrode polarization (Ser- 

 geev and Solovov, 1937) and microchemical methods (Short, 1940) . 



NEED FOR FURTHER RESEARCH 



In spite of the considerable volume of work that has already been 

 done, both on the principles and on the applications of geochemical 

 methods of prospecting for ores, the field is still in its infancy. Broadly 



