PAPERS ON GEOLOGY AND CHEMISTRY 327 



isfactory quality. The third or (1917) edition of Stand- 

 ard Methods of Water Analysis, A. P. H. A. allows the 

 use of Methyl Orange. 



It will be seen from the above that Methyl Orange is 

 the more popular indicator for this titration. The dis- 

 advantages of the others are so marked that it is hardly 

 necessary to refer to them. The necessity of shaking 

 the Erythrosine with chloroform makes its use tedious 

 and slow. Lacmoid must be used in a boiling solution. 

 This is a decided disadvantage. Methyl Orange can be 

 used in a cold solution, works very satisfactorily in all 

 cases excepting in water wdth an excess of alum. Methyl 

 Orange indicates a slight residual alkalinity in a water 

 in which all of the bicarbonate alkalinity has been used 

 up by the addition of alum and even where there is a 

 slight excess of alum in the Avater. Larger excesses of 

 alum give an acid reaction. If such a case is suspected 

 it is absolutely necessary to use some other indicator, 

 such as Erythrosine or Lacmoid, which reacts acid to a 

 dilute solution of alum. 



While the end point change of Methyl Orange is very 

 sharp and satisfactory to most chemists, quite often one 

 finds chemists to whom the color change is very indefi- 

 nite and difficult to distinguish. This is probably due to 

 a slight color blindness but is nevertheless a real disad- 

 vantage of this indicator. 



In 1916 Clark and Lubs^ described a new series of in- 

 dicators which they studied with reference to their use 

 in the colorimetric determination of the hydrogen ion 

 concentration. Acree and co-workers have studied the 

 chemical structure and mechanism of color change of 

 these indicators. The entire series Avithout exception are 

 brilliant colors and show a very marked color change at 

 their neutrality point. One of these indicators tetra- 

 brom phenol-sulphonpthalein changes color at a hydrogen 

 ion concentration almost identical with that of methyl 

 orange. The color change is from blue in alkaline to 

 yellow in acid solution. This color change is much dif- 

 ferent from that of Methyl Orange and we feel that many 



iW. M.Clark and H. A. Lubs, J of Bact. 2, 1-3, 109-136, 191-236 (1917). 



