Radiometric Measurements of Constants of Indicators. 65 



If ammonium chloride and ammonium hydroxide are present in the 

 same solution, the concentration of the NH4 ions is furnished almost 

 entirely by the ammonium chloride. Let S equal the concentration of 

 the salt and B the concentration of the base. We then have for dilute 

 solutions 



= K, (25) 



or 



OH = ^-|P (26) 



The ionization constant for ammonium hydroxide 1 at 25 is given as 

 18X10" 6 . All of the work on phenolphthalein was done at 20, and 

 reducing the above value to 20, we obtain from equation 20, q being 

 equal to 1,400 calories, K 6 = 17.4 X 10~ 6 . 



A pure sample of phenolphthalein was recrystallized from absolute 

 methyl alcohol. A weighed amount of purified phenolphthalein was 

 dissolved in 50 c.c. of absolute ethyl alcohol. By means of a small pipette, 

 carefully calibrated, a small volume of this alcoholic solution, say 1.25 

 c.c., could be quite accurately measured. This volume was diluted 

 with conductivity water to 2,000 c.c. Experiments were made which 

 showed that the effect of this small trace of alcohol was negligible. 

 (See McCoy.) The ammonia employed for the solution of ammonium 

 hydroxide was distilled from barium hydroxide to eliminate carbon 

 dioxide. All solutions were prepared with conductivity water at 20. 

 Special care was taken to prevent carbon dioxide from coming in 

 contact with any of the solutions. 



It is well known that an excess of alkali causes a rather rapid fading 

 of solutions of phenolphthalein. Experiments were made which showed 

 that a very slight error would be introduced if the solutions of phenol- 

 phthalein containing an excess of alkali were used directly after being 

 prepared. Filling the cells with the solution, and taking radiometric 

 measurements for 5 wave-lengths of light, required about 8 minutes, 

 and during this short space of time the change in transparency was 

 found to be very slight. 



A few of the experiments made with solutions of phenolphthalein 

 containing an excess of alkali are recorded in table 38. All solutions 

 were diluted to 100 c.c. and each contained equal amounts of phenol- 

 phthalein. A normal solution of sodium hydroxide was used for these 

 experiments. The percentage transmissions given by solutions I and 

 II are for 0.8 c.c. and 1 c.c., respectively, of the alkali. The interval 

 which elapsed between the time the solutions were prepared and the 

 last measurement made was between 7 and 8 minutes. Measurements 



Carnegie Inst. Wash. Pub. No. 63, 298 (1907). 



