New Yorx AGRICULTURAL EXPERIMENT STATION. 279 
point is reached, because the addition of acid replaces rapidly con- 
ducting OH’ ions with more slowly conducting acid radicle anions, 
The addition of a given amount of acid always replaces the same 
number of OH’ ions, and decreases the conductivity by a given 
amount; therefore, if the conductivities are plotted as ordinates and 
the amounts of acid added, volume of solution being constant, as 
abscissae, the resulting curve will be a straight line sloping down- 
ward to the neutral point. At this point it breaks sharply. If the 
acid be a strong one, like sulphuric acid (Fig. 21) the curve will 
rise again rapidly in a nearly straight line. If the acid be a weak 
_polybasic one, like succinic, the curve may still continue falling 
slightly on the acid side of the neutral point, due to the fact that 
the acid-salt has somewhat lower conductivity than the neutral, but 
the slope is slight compared with that on the alkaline side of the 
nentral point, and the point is plainly marked by a sharp angle. If 
the acid be so weak, however, that it hydrolyzes, so that its normal 
salt reacts alkaline, as is the case with phosphoric acid, the curve 
does not fall straight to the neutral point and break there, but bends 
gradually (H,PO,, Fig. 22), and without breaking. The decrease 
in the number of OH’ ions is not a linear, but a complex function 
ot the amount of acid added, and they do not disappear until an 
, 
excess of acid has been added. In acids of this character, the 
neutral point can not be determined accurately by either conductivity 
or titration with phenolphthalein. Sjoqvist! working with hydro- 
lyzible hydrochloric acid salts of albumin employed the approximate 
method of determining the intersection of the asymptote of the 
curve on the acid side (corresponding to the alkaline in our case), 
with the horizontal line tangent to the curve at its minimum, 
assuming that the abscissa of the intersection denoted the equivalent 
weight of albumin. . 
The work of former investigators has shown that the addition of 
more casein to a sodium caseinate solution neutral to phenolph- 
thalein does not appreciably change the conductivity. Instead of 
the curve’s rising beyond the neutral point, as in the case of sul- 
phuric acid, or falling slightly, as in the case of succinic acid, it 
becomes horizontal. If an acid sodium salt of casein, analogous to 
"Skand, Arch. Physiol., 5:276. 1895. 
