Pe)-))iane)it Color Standards 167 



Limitations of Inorganic Compounds. When one attempts to select 

 a set of solutions of inorganic materials whose colors include the seven 

 primary, visible colors of the solar spectrum, he is at once confronted 

 with an inherent scarcity of individual colors of a wide variety. The 

 range is fairly well covered from the red of an aqueous solution of 

 cobaltous chloride to the blue of an aqueous solution of cupric sulfate. 

 These, however, do not include deep reds on the one end nor deep blues, 

 indigos nor violets on the other. As will be noted later, deeper reds 

 r.nd blues may be obtained under certain conditions but the stability 

 of the solutions has not been found entirely ."-atisfactory. Some solu- 

 tions have good colors but cannot be mixed with others, which makes 

 their use limited. 



While at certain times one may obtain a closely agreeing match 

 between an unknown, colored solution and an inorganic permanent stand- 

 ard, at other times such a match may be difficult to accomplish, as in 

 tests on such complex systems as the usual surface water. On the one 

 hand, pure or mixed solutions show a clearness and brilliancy of color 

 which many complex samples do not possess. On the other hand, the 

 -color produced in the solution to be tested may depend upon a variety 

 of conditions, such as temperature, quantity of reagent added, manner of 

 making reagent, variable quantity of accompanying substances, time 

 elapsing after the preparation of the sample before comparison of the 

 color, and others practically impossible to control clo.^ely. Uniformity 

 in preparation and handling both of sample and standard are necessary 

 in such cases. 



Elevation of the temperature often changes the color of absorbing- 

 media''. It is well known, for example, that the red color of an aqueous 

 solution of cobaltous chloride turns blue if the temperature is raised 

 sufficiently. Another red solution, matching the color of the first but 

 having a different composition, probably would not show the same change 

 of color on passing through the same changes of temperature. When 

 such a change of temperature causes a chemical modification, changes 

 in the absorption spectrum occur; but in chemically stable systems, such 

 as potassium bichromate, the alteration is small or nil.\ 



Another difficulty piesents itself. It seems probable that, if one 

 matched the green of a solution of nickelous sulfate with a green pro- 

 duced by blending solutions of cobaltous chloride, ferric chloride and 

 cupric sulfate, the mechanism giving rise to the same color sensation in 

 the two cases for one individual would not do so for another'. Arny" 

 has found, for the conditions he studied, that several individuals obtained 

 the same Lovibond readings on a number of different solutions; but 

 Watson' states that the color sensation produced in diflrerent persons by 

 the same quality of light may vary considerably. 



Besides the effect of a change of temperature just mentioned, several 



= Wood— Physical Optics, p. 441 (1911). 

 ' Sheppard — Loc. cit., p. 16o. 



■"'Richards and Ellms— Jr. Am. Chem. Soc. IS, 75 (1896). 

 « Arny— Report 8th. Intr. Congr. Appl. Chem. 2(1. 319 (1912). 

 ' Watson— Textbook of Physics, p. 559 (1919). 

 Sheppard — Loc. cit. p. 150. 



