12 ^TAM/TATirE AGRICi'LTUKAL AXALYSIS
Twenty-five cubic eeatimeters of the oxalate solution contained
X gin and 1 cc of permanganate solution is
equivalent to gin of sodium oxalate. This
weight, multiplied by the ratio of the equivalent weight of iron
or of calcium to that of sodium oxalate, will give the weights
of these substances that are equivalent to 1 cc of the standard
solution. Then
^ I.L025 X 2.5340 _XJ>o!8<8== Q 0021g m Fe
IT solution =0= ....... 94.25'X.....67.005
or
°-%X^ X 20ffl6 = Q 0(X)78 Ca.
24.2,3 X b/.OQo
Indicators.—Any substance that is used to show the end
point of a definite reaction is an "indicator/7 The indicator
may do this by a change of color in solution or by the appearance
of a precipitate. In some cases the standard solution itself or
the substance titrated may act as indicator. A familiar example
of this is the oxidation of iron by potassium permanganate. As
long as any ferrous iron is present the intensely colored per-
manganate is reduced to practically colorless manganese salts
but the least drop of permanganate in excess colors the solution
and indicates the complete oxidation of all iron present. In
this case, as with other color changes and precipitations of in-
organic compounds, the reaction at the end is definite and well
understood.
^Neutrality1' Indicators.—The indicators that are used to show
neutrality points in reactions of acids and bases with each other
are usually organic and their color changes are reversible as the
point of neutrality is passed in either direction. The color
change is due to a change in molecular structure which, in turn,
is in equilibrium with hydrogen or hydroxyl ions present in the
solution.
Hydrogen Ion Concentration.—The volumetric titration of
acids with bases, or conversely, is a process of neutralization.
This is the production of a condition where neither hydrogen nor
hydroxyl ions are present in more than very slight and negligible
excess. Neither of these ions can be absolutely eliminated from
any aqueous solution. Both must be present and in such proper-