THEORY AND GENERAL PRINCIPLES 15

indicator changes color must include some portion of this steep
part of the curve and no other portion. The curves shown in
the figure for the "strong7' acids conform to condition (a).

In the curve for the neutralization of carbonic acid there is a
comparatively sudden break near the point of half neutraliza-
tion. This corresponds to the formation of sodium bicarbonate:

HjCOs + NaOH -> NaHC03 + H20.

Any indicator whose color change covers only this portion of the
curve will make a titration possible. Phenolphthalein, with a
Pu range of 8.3 to 10, will serve for this purpose if the first appear-
ance of pink (at the PH value of 8.3) is taken as the end point.
This point is, of course, not as sharp as could be desired. No
indicator can be found that will give a sharp change with equiva-
lent quantities of carbonic acid and a base, because of the gradual
slope of the neutralization curve at this point.

Examination of the curves for boric and phosphoric (weakly
ionized, polybasic) acids will show why these acids cannot be
accurately titrated. The curve for boric acid shows a faint
inflection at the point representing neutralization of the first of
the three hydrogen atoms, but this would involve only a very
gradual change in color of any indicator that would cover this
range of PH values. The case is quite similar for phosphoric acid.

Only a few of the common indicators are necessary for ordinary
titrations in analytical work and three of these will be described
briefly.

PhenolphthaJein.— This compound is a white crystalline
powder, almost insoluble in water but soluble in alcohol. For
use in volumetric analysis a solution of 5 gm in 1000 cc of 50-
per cent alcohol is suitable. One drop of this solution is sufficient
for 100 cc of solution being titrated. The range of color change
is PH = 8.3 to 10.0.

Phenolphthalein is a derivative of phthalic anhydride and the
solution contains two forms in equilibrium:

CO COO

<=» C6H/ +H

\C6H4OH