Permanent Color Standards 165 



ent in the two solutions are identical, if of the same composition, or 

 equivalent to each other, if of different compositions. Where frequent 

 determinations are to be made in this manner, it is very desirable to 

 have permanent standards for either of the cases mentioned. Loss of 

 time results if one must prepare a new set for each day or determination; 

 and one's work is uncertain if the colors of the standards change or 

 fade following some reaction affecting the color-forming constituent. 

 The synthetic organic chemist has supplied us with compounds of 

 every desirable shade of color which might be useful in the preparation 

 of such standards. Unfortunately, many of these compounds do not 

 maintain their color indefinitely when in solution and exposed to the 

 light. In some the color fades; in others the colored solute coagulates 

 and settles out, leaving the solution clear. 



In the effort to avoid the difficulty arising from such instability 

 of color with organic substances, occasional work has been done for a 

 number of years involving the use of solutions of inorganic salts. The 

 purpose of the present paper is to review the information available re- 

 garding the color of solutions of inorganic substances, to note the 

 reciuirements of a permanent color standard and the limitations of 

 inorganic substances for such purposes, to mention some of the possible 

 combinaticns for permanent standards, and to present a resume of 

 previous work upon the subject. Experimental work is under way 

 relating to certain colorimetric standards but sufficient data is not yet 

 available for any definite conclusions. 



The Color of Inorganic Substances in Solution. Compared with the 

 amount that has been wi-itten concerning the color of organic compounds, 

 there is a relatively small amount available regarding the color of in- 

 organic compounds. Particularly is this true both for the actual col- 

 ors of the solutions of the compounds and also for the permanency of 

 such colored solutions. 



Recently Bichowsky^ has discussed the color of inorganic compounds 

 from the standpoint of electronic structure and valence of the atoms. 

 The color exhibited by any compound is the result of its selective ab- 

 sorption of light, and only those substances are capable of such absorp- 

 tion which have an arrangement of electrons free to vibrate in the 

 proper manner. He points out that compounds of invariant valence 

 elements are colorless, due to their high electron stability and consequent 

 absorption of light of short wave lengths — the ultraviolet. According 

 to Bichowsky's table of valence colors for the elements of variable 

 valence, the chief elements we may expect to show color (omitting the 

 rare earth elements and the oxides and sulfides of the variable valence 

 elements) are copper, gold, (silver), titanium, vanadium, chromium, 

 molybdenum, tungsten, uranium, manganese, iron, cobalt, nickel, ruth- 

 enium, rhodium, palladium, osmium, iridium, and platinum. 



In figure 1 is given Mellor's adaptation of the periodic table as 

 arranged by Bayley, in which the atomic weights and the atomic num- 

 bers of the elements have been included. The elements of invariant 

 valence (forming colorless compounds) have been indicated with a cross. 



'Jr. Am. Chem. Soc. 40, 500 (1918). 



