PHYSICAL CHEMISTRY 27 



not get any of the base until the pH has been brought to 9 or 

 10. If one wishes to titrate a weak acid, such as an organic 

 acid, one will choose phenolphthalein as indicator, for the weak 

 acid gives so low a hydrogen-ion concentration that on the 

 addition of alkali the turning point of such an indicator as 

 methyl orange (pll 3 to 4) would be reached long before the 

 acid had been neutralized. On the other hand, if one wishes 

 to titrate a strong acid (HC1) in the presence of a weak one 

 (H 2 C0 3 or an organic acid) it will be possible to do so using 

 methyl orange, for even a small amount of HC1 gives a concen- 

 tration of H ions high enough to affect methyl orange, whereas, 

 when the strong acid is all neutralized, the methyl orange 

 changes color even though there is a weaker acid still present 

 untitrated. 



Of course such values are not strictly accurate, but they are 

 sufficiently so for many biological purposes. 



Also the hydrogen-ion concentration of a solution can be de- 

 termined by the indicator method. By making up a series of 

 solutions of graduated hydrogen-ion concentration and adding 

 certain indicators, one can get a series of solutions graded in 

 color or depth of hue. Making up an unknown solution under 

 similar conditions and adding the indicator, it is possible to 

 match the hue with that of one of the tubes in the standard 

 series, and thus determine with a fair degree of accuracy the pH 

 of the unknown. 



The student should be careful to get clearly in mind the 

 difference between hydrogen-ion concentration and titratable 

 acidity. The former is a measure of the amount of dissociated 

 or ionized hydrogen per liter; the latter is the amount of re- 

 placeable hydrogen, which need not necessarily be ionized. 



N 

 Whereas the titratable acidity of solutions of hydrochloric 



and acetic acid is the same, assuming of course the use of an 

 indicator sufficiently sensitive to H ions, the former has a far 

 greater H-ion concentration than the latter, because the former 

 is more highly dissociated. 



