Permanent Colo)' Standards 169 



Little can be stated at present regarding the colors of non-aqueous 

 solutions of salts of the color-forming elements. In some cases there 

 is a profound change in the color with the change from water as 

 the solvent, as, for example, the change from the red of cobaltous chloride 

 in aqueous solution to the deep blue with the salt dissolved in absolute 

 ethyl alcohol. A solution of the same salt in acetone is a deeper red 

 than an aqueous solution. A number of these solutions are being in- 

 vestigated. 



A very limited class of solutions consists of those in which the ele- 

 ments as such are dissolved. A saturated, aqueous solution of bromine 

 m water has a marked orange-red color. Iodine in carbon tetrachloride 

 gives a beautiful pink colored solution, while in ethyl alcohol it shows 

 a yellowish color. In certain other solvents these elements give some- 

 what different colors. 



Certain colloidal systems consisting of a solid dispersed through a 

 liquid show marked colors, such as the red of ferric hydroxide in water. 

 It is distinctly questionable as to whether such a system would be of 

 any value for permanent color standards. 



Previous WorJc. An attempt has been made to collect the refer- 

 ences relating to the use of solutions of inorganic salts as permanent 

 color standards. The following resume includes a statement of the 

 principles and methods involved in the work for which reports have been 

 located. 



Crookes, Odling and Tidy' devised an instrument for comparing 

 the colors of waters with solutions of inorganic salts. This instrument 

 consisted of a pair of 24-inch tubes, one of which contained the water 

 under examination, while the other remained empty. Behind the latter 

 were placed two hollow wedges, one containing a 1 per cent solution of 

 cupric sulfate, the other a solution of ferric and cobaltous chlorides con- 

 taining 0.7g. of iron and 0.3g. of cobalt per liter, with a slight excess 

 of free hydrochloric acid. These wedges were pushed over the empty 

 tube until their color matched that of the water under examination in 

 the other tube, and the color was recorded as equal to so many milli- 

 meters of blue and red solutions. 



Hazen'" proposed the use of a mixture of potassium chloroplatinate 

 and cobalt chloride as a color standard for natural waters. The stock 

 solution contained 1.246g. of potassium chloroplatinate (0.5g. Pt) and 

 Ig. of crystallized cobaltous chloride (0.25g. Co) and 100ml. of concen- 

 trated hydrochloric acid, the whole being diluted to one liter. Solutions 

 for comparison were made by diluting 1, 2, 3, . .ml. to 50. This solu- 

 tion was found to be unaltered after one year, even when standing in 

 the light. 



Jackson" reported the use of permanent color standards for the 

 colorimetric determination of several constituents in water. The various 

 standards used consisted of mixtures of solutions of the following salts : 



^ Chem. N. 43, 174 (1881). 

 '"Am. Chem. Jr. IJ,, 300 (1892). 



A.P.H.A.— Standard Methods of Water Analysis, p. 9 (1917). 

 "Tech. Quar. l.J, oM (1900). 



A.P.H.A.— Loi'. cit. p. 17 (1917). 



