16 QUANTITATIVE AGRICULTURAL ANALYSIS

The second form predominates in basic solutions and the group
[ = C6H4 =] is in some way responsible for the red color. The first
form is colorless and predominates in acid solutions.

Methyl Red. — This dye is p-dimethylaminoazobenzene-o
carboxylic acid:

The indicator solution is prepared by dissolving 1 gm of the solid
in 100 cc of 95-per cent alcohol. The solution is pale yellow ii^
basic solutions and violet red with acids. It is especially good
for the titration of ammonium hydroxide and the alkaloids, all
being weak bases. It cannot be used if much carbonic acid is
present, hence is useless for the titration of carbonates. The
color range includes PH = 4.4 to 6.0.

Methyl Orange. — The methyl orange of commerce is the
sodium salt of a sulphonic acid:

This is a yellow substance which forms a yellow solution in
water. In presence of acids the salt is decomposed and a red
form, previously existing in equilibrium, now predominates. The
color range includes PH = 2.9 to 4.0.
A water solution containing 0.5 gm in 1000 cc is used as
indicator in volumetric analysis. A single drop is usually
sufficient to give a perceptible color to 1000 cc of solution.
The three indicators described above practically cover the
range of hydrogen ion exponents from 2.9 to 10, with the excep-
tion of a gap between 6.0 and 8.3. This fact makes unnecessary
the employment of indicators other than these three for the
great majority of volumetric analyses, even when quite weakly
ionized acids or bases are being titrated. It is nearly always
possible to choose a strong electrolyte for the standard solution
and one of these indicators will then generally serve to cover the
portion of the curve that represents equivalent neutrality. The
number of indicators that have been proposed and used for
analytical purposes is very large. Many of these are useful for
the determination of existing hydrogen ion concentration and
these will be mentioned in a later chapter (page 139). l
* For an exhaustive discussion of the whole subject of indicators see
PRIDEATDC : "The Theory and Use of Indicators."