172 C. L. BRIGHTMAN, MEACHEM AND ACREE 



urements now in progress show that reliable data on hydrogen 

 ion concentrations will be secured only by using the isohydric 

 principle, conductivity methods, the hydrogen electrode and 

 the spectrophotometer with solutions of buffer salts and indi- 

 cators having varying concentrations and increasing per cent 

 of neutralization up to 100 per cent for both the indicator and 

 buffer salts. 



As such a study is very comprehensive and as other workers 

 may be interested in knowing whether, and to what extent, 

 phosphates show "salt effects" with phenolsulfonphthalein, we 

 shall present here a provisional graphical method for showing 

 the magnitude of the "salt effect" in a few cases and its influence 

 on the apparent hydrogen ion concentration and ionization con- 

 stant of this indicator. The application of these results to in- 

 vestigations of soils, wood extracts, biological fluids of all kinds, 

 foods, industrial liquids and solutions of every description, will 

 naturally occur to those interested. 



EXPERIMENTAL WORK 



When a tautomeric monobasic indicator is ionized 8 we have the 



following expression K = where K is the apparent ioniza- 



1 — a 



tion constant of the indicator acting as an acid, a is the con- 

 centration of the colored anions, H is the hydrogen ion concen- 

 tration and (1-a) is the concentration of the non-ionized molecules. 



If we assume that the color of the indicator solution is a meas- 

 ure of the amount of the indicator which has been ionized, then 

 it is possible to determine the value of (1-a) /a colorimetrically. 

 For example, if we use monosodium phenolsulfonphthalein as a 

 monobasic indicator, we find that it shows an absorption band 

 with a maximum for light of wave length y = 0.5557 n. In the 

 future we shall refer to this band as the green band. 



If light of intensity I falls upon a cell containing a solution of 

 the indicator, and the intensity of the transmitted light is I, then 



8 For fuller discussion of the quinone-phenolate theory applying to the sul- 

 fonphthaleins see references 6 and 7. 



