64 Conductivities and Viscosities in Pure and in Mixed Solvents. 



ciliated, was made under the most favorable conditions. These con- 

 stants, therefore, more likely represent the true constants of the 

 indicator. 



PHENOLPHTHALEIN. 



Regarding phenolphthalein as a monobasic acid, we have from 

 equation 2 



Q X H 

 LH 



Dividing this by the ion product of water, it follows that 



LH X OH K 

 Q+ " K,- 



Phenolphthalein has but one component of absorption, and the appli- 

 cation of Beer's law is much simpler than for methyl orange where 

 there are two components of absorption. A solution of phenolphthalein 

 in pure water is perfectly transparent. The addition of an excess of 

 sodium hydroxide converts all the colorless lactoid molecules into the 

 red quinoid salt, the concentration of which we will represent by c'. 

 Let c be the concentration of the quinoid salt in the phenolphthalein 

 solutions containing ammonium hydroxide and ammonium chloride. 

 Since the depth of solution was maintained constant, we have the two 

 fundamental equations given below: 



In (I/Io) = - Kc (22) 



ln(I/Io)'=-Kc' (23) 



The constant K is the same in both cases. Dividing 22 by 23 we 

 have 



In (I/Io) , , c' X In (I/Ip) 



m(I/I )' In (I/Io)' 



Where (I/Io)' is the percentage transmission of a solution of phenol- 

 phthalein containing an excess of alkali, for some given wave-length 

 of light; (I/Io) that for the solution in which the concentration of the 

 quinoid salt c, is to be determined for the same wave-length of light. 

 Knowing, then, the amount of phenolphthalein converted into the 

 quinoid salt, LH in equation 21 is given by T c, where T represents 

 the total amount of phenolphthalein. The hydroxyl ion concentration 

 was varied by the addition of ammonium chloride to ammonium 

 hydroxide. 



The value of OH to be substituted in the hydrolysis equation for 

 phenolphthalein was obtained from the following equation : 



NH 4 X OH 



= Iv, 



(NH 4 OH + NH 3 ) 



