•so/ix

Potassium Thiocyanate Method.—\Vi * - *
of calcium or magnesium carbo^i' ^ J ^ }
iron present combine with any fiv«. ftl < 1 i i i t
these metals. If an alcohol Voiat'\»" «,'
is added to such a soil the solution n;
intensity of which has been shown to
tional to the acidity of the soil Al^
alcoholic solution of logwood he :uM* <]
the intensity of which is again propt it
of both aluminium and acidity in tia M >•'

As an explanation of this color format:,
that, in an acid soil, iron and aluiiiir^ti:
partly hj^drolyzed, largely colloidal MI^^ 1
weakly ionised soil acid. These sa!t> an* i
such salts as potassium thiocyanaU, wrj,*
insoluble oxides or silicates, such :i*» would
or basic soil. This might be expressed thus:

0\

M\»

V "1» <

< V

x prt ^on1" in a neutral

+ H2O <=± FeOH-.4, + HI,

FeOH.A2 + 3KCNS + H.4. *=* FeiCXS.h + 3K.4 ~t-

i I -2

where A represents any acid radical The ferric t
thus produced colors the solution somewhat in pioportluii to the
amount of acid which made iron available for this reaction.
The addition of a standard solution of a
ferric thiocyanate and destroys the color.
It has been noted in making soil acidity by
method that certain soils cause the supernatant to assume a
green color, as the red color of the ferric thiocyanate
Analysis has shown that this color is doe to the formation of a
manganese compound, which is produced after the solution
been made basic. This green color develops if the contains
as little as 0.008 per cent of soluble manganese. In
cases it has been found that it starts to develop as as the red
color entirely disappears, but its intensity is increased if o ce more
of base be added than that required to titrate to the disappear-
ance of red. This red color disappears when Pa equals 5.5
while manganese does not start to precipitate as hydroxide until
J. Ayr. AV/., 10,420 (1920:'.