590 LEON S. MEDALIA 



acid and an alkaline tube at either end of each set of the color 

 standards. This would add two more colors to the range of 

 each indicator and thus greatly enhance the usefulness of each 

 set of color standards, more especially as regards the two end 

 pairs. 



Such tubes would contain each 10 cc. of the acid and alkaline 

 solutions, respectively, as used for the particular indicator, to 

 each of which would be added in turn 0.8 cc. of the "indicator 

 watery solution." They would give the full acid and full alkaline 

 colors of the particular indicator and would thus be of value in 

 determining whether a given unknown falls within the visual 

 or "virage" range of the particular indicator or not. 



There were two questions, however, that had to be deter- 

 mined experimentally with reference to this : 



1. Whether there would be a difference in color sufficiently 

 marked to distinguish between such a tube, on the acid side, 

 when compared with the first pair of the "color standards;" 

 and the tube, on the alkaline end, when compared with the 

 seventh pair of the set. 



2. Whether any definite pH could be attributed to such tubes 

 when studied by calibrating them with buffer mixtures of known 

 H-ion concentration. 



The first question can be answered in the affirmative: There 

 is, according to the findings, a definite and distinct difference 

 between the tubes on either side of the scale as compared with 

 the respective pairs nearest to them. The solution in the tube 

 on the acid end of the scale is the same as that described for the 

 acid tubes of the pairs in each of the sets of "color standards" 

 for each indicator. Similarl}^ the tube on the alkahne end of 

 the scale contains the solution used for the alkaline tubes of the 

 pairs in each set. 



Thus with thymol blue (acid range) , the acid tube of this indi- 

 cator is made with 0.5 volume per cent of concentrated HCl, 

 the alkaline tube of tliis indicator contains 0.001 per cent con- 

 centrated HCl and to each of these is added 0.8 cc. of 0.02 per 

 cent of the indicator watery solution. 



