ANALYTICAL EDITION
283 --
Vol. 3, KO. 3
Notes on Use of Glass Color Standards for Determination of Phosphorus by Deniges Colorimetric Methodlp2 H. D. Chapman UNIVERSITY
OF
CALIFORNIA, RIVERSIDE, CALIF.
N UNSATISFACTORY feature of the blue colorimetric method for the determination of phosphorus is the rapidity with which the color fades necessitating the preparation of new standards frequently. Recently permanent glass color standards to be used in the Hellige comparator for the colorimetric determination of phosphorus by t h e above method have been placed on the market. A few notes regarding the accuracy of readings made with this equipment and sources of error to be guarded against are presented in this communication.
A
Experimental Procedure
The color standards, ten in number, are mounted on a circular frame, each one being designated by the Arabic numerals l , 2,3,. .lo, and when used in accordance with a given technic supposedly represent phosphorus concentrations increasing uniformly from 0.187 to 1.87 p. p. m. Pod. The construction of the Hellige comparator and the circular frame is such that upon rotation of the latter the different circular plates are brought into a field in juxtaposition with the unknown. Further informstion concerning the Hellige comparator and accessories may be found in chemical apparatus catalogs. Preliminary tests revealed that the color quality of the glass standards matched the actual blue color of phosphorus solutions very well, and that differences in color intensity between succeeding standards were sufficient to permit of several interpolated readings. Inasmuch as the color plates had been standardized by a method different in technic from the one in use in this laboratory, it first of all became necessary t o establish values for each by the particular method employed here. This method is essentially the same as that outlined by Truog and Meyer (1). The solution to be tested is combined with 4 cc. of reagent A (2.5 per cent ammonium molybdate in 10 N sulfuric acid), made up to a volume of 99.5 cc. with distilled water, and 10 drops (0.5 cc.) of 2.5 per cent stannous chloride reagent added. Solutions of known phosphate conteni were prepared and readings taken at exactly 2 minutes folloving the addition
