COLOR STANDARDS FOR THE COLORIMETRIC 

 MEASUREMENT OF H-ION CONCENTRATION 



LOUIS J. GILLESPIE 



Contribution from the Research Laboratory of Physical Chemistry of the 

 Massachusetts Institute of Technology, No. 135 



Received for publication December 18, 1920 



In an article of the above title recently pubUshed in this jour- 

 nal, Medalia (1920) presents a system of color standards some- 

 what sunilar to one published by me a httle before (Gillespie, 

 1920). The work is evidently independent of mine, but the 

 proposed tables are in serious disagreement with the results of 

 my work. 



The cause of the disagreement apparently does not lie in a con- 

 flict of observations, but in the plan followed by Medalia in pre- 

 paring the tables. 



It is stated that a test of this plan with the indicator, brom- 

 thymol blue, "succeeded perfectly, i.e., the green color was found 

 at (pair no. 4) pH 7; or slightly yellowish green at (pair no. 3) 

 pH 6.8 according to this range. (The change of color of this 

 indicator was found by the writer to start with pH 6.2 instead of 

 pH 6 as given by Clark and Lubs.)" 



Unfortunately, this test is not sufficient to afford evidence in 

 favor of the plan as against the method used by me to ''smooth 

 out" experimental errors for the preparation of tables, because 

 the mass action equation used for this smoothing requnes that 

 such a limited test of the plan shall succeed perfectly, the error 

 involved being only 0.02 pH, well within the experimental error. 



In fact, the mass action equation requhes that, if one is able 

 to determine both Umits^ equally distant from the half-transfor- 



1 Although there are practical limits to the useful range, there is of course no 

 real point of pH where the indicator "starts in" to change color, but only a sub- 

 jective point "over the threshold" where it may appear to do so. 



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