224 REPORT OF THE CHEMIST OF THE 
The titration results, on the other hand, as will be shown pres- 
ently, may indicate only a small proportion of acid taken up, or 
even less than none. We made determinations of acid in the fil- 
trates obtained from mixtures of casein and acid in numerous ex- 
periments in which we used concentrations varying from N-125 
to N-500 HCl, employing both the titration and conductivity 
meshods as checks upon each other. We found that whenever the 
filtrate gave a pronounced proteid reaction, the titration results fell 
below the conductivity results and that the more pronounced the 
proteid reaction, the greater was the difference between the two 
sets of results. 
In some cases, when the decrease in conductivity showed that 
a large part of the hydrochloric acid had been removed from solu- 
tion or combined with casein in a soluble form, titration with alkali 
showed that the filtrate had actually a greater acidity than could be 
accounted for by the total amount of acid originally used in the 
experiment. The titration caused a voluminous precipitate and the 
filtrate showed an intense xanthoproteic reaction in such cases. 
The increased acidity was due to the acidity of the dissolved proteid 
itself, 
There are, then, two reasons why the determinations of acidity 
by titration and by conductivity do not give concordant results 
when the solution contains dissolved proteid, the titration results 
being. in error. First, the dissolved casein-acid compound decom- 
poses at once in the presence of alkali, freeing the acid previously 
combined. Second, the dissolved proteid also combines with and 
neutralizes part of the alkali. The acidity of the filtrate, instead 
of representing only the amount of acid not taken up by the casein, 
equals the acid not so taken up, plus the acid that has combined 
with casein to form a soluble casein-acid compound, plus the acidity 
of the dissolved proteid. Experiment shows that the solution of 
a gram of casein in a given volume of acid increases by about 
8% cc. the volume of N-10 NaOH required to neutralize the acid. 
This also is the amount of alkali required to dissolve and neutralize 
to phenolphthalein one gram of free casein. The acidity of the 
casein itself appears to be unchanged by solution in dilute acid, 
whether or not the nature of the casein complex is changed. 
