Permanent Color Standards 171 



low) and ammonio-cupric sulfate (blue)^\ When mixed in various pro- 

 portions, these salts were found to produce all hues except a few delicate 

 pinks and violets. The tints which could not be produced by this "Co- 

 Cro-Cu" series were obtained by means of mixtures of 0.001 N potas- 

 sium permanganate and 0.01 N potassium dichromate, known as the 

 "Cro-Mn" series. In the "Co-Cro-Cu" series the red and yellow solutions 

 were found to remain unchanged in color if kept well stoppered. The 

 blue solution, however, precipitates within a few weeks and must be 

 prepared frequently. In the "Cro-Mn" series each solution is stable 

 alone, but they begin changing an hour after mixing. 



Arny and Ring^' reported further use of the above mentioned solu- 

 tions in connection with certain pharmaceutical work. 



Arny and Ring"" reported the application of the three series of solu- 

 tions in the preparation of permanent standards for use in the colorimet- 

 ric determination of ammonia, nitrates, nitrites, vanillin, uric acid, 

 salicylic acid and phosphates. They state that practically every tint 

 desired in colorimetric work can be obtained with the three series of 

 colors. The original acidified "Co-Fe-Cu" solutions and their blends 

 neither fade nor precipitate until at least two years old; the ammoniacal 

 cobalt and chromate solutions show no fading after a year. The am- 

 moniacal copper solution precipitates and undergoes change of color 

 within a few weeks, but the blends keep satisfactorily when sealed in 

 ampules or kept in well filled, rubber stoppered bottles which are not 

 opened. The concentration recommended in this paper for the "Co- 

 Cro-Cu" series is 0.02 N. 



Arny, Kish and Newmark'^ discussed the unreliability of the Lovi- 

 bond tintometer for the testing of cottonseed oil, and proposed the use 

 of the "Co-Fe-Cu" and "Co-Cro-Cu" color standards. 



Kolthoff" was the first to propose the use of permanent color stand- 

 ards instead cf the usual buffer solutions and indicators in the colori- 

 metric determination of hydrogen ion concentration. By means of M/4 

 cobaltous nitrate and M/6 ferric chloride he prepared solutions which 

 match the colors of buffer solutions through the pH range of certain 

 indicators. Matches were made showing the change in color of neutral 

 red, methyl orange, tropaolin 00 and partially for methyl red. 



SnelP' recommends the use of Amy's "Co-Fe-Cu" series for the 

 preparation of permanent standards for the colorimetric determination 

 of carbon, lead, chlorine, and possibly for aluminium. He states fur- 

 ther, that for the determination of bismuth, permanent standards may 

 be made using the same reagents as in the case of the unknown sample. 

 Solutions of potassium chromate, potassium chloroplatinate and am- 

 moniacal cupric chloride may be used for permanent standards for the 

 determination of chromium, potassium, and oxygen, respectively. 



•'•Note — Salts of copper and cobalt are M 20 and the dichromate M 120. 

 'Mr. Am. Pharm. Assoc, i. 1294 (1915). 

 ".Jr. Ind. Eng. Chem. S. 309 (1916). 

 -' Ihid. 11, 950 (1919). 

 ■■-Pharm. Weekhlad .•;.''. 104 (1922). 

 -■' Colorimetric Ana'ys-is (1921 ) . 



