<	82              QUANTITATIVE AGRICULTURAL ANALYSIS
i    '	'                                    removed by filtration "before titrating the unused base.     (3)
'    \	barium carbonate is filtered out, washed and dissolved   In   an
I   j	excess of standard acid, the unused acid being then titrated by
[	standard base.    In method (3) a more concentrated solution, of
1    '	barium hydroxide may be used and it need not be standardised.
j	Phenolphthaleiri is used as indicator in all three methods.   M^thod
j	(1) will be described.
,    <	Apparatus.—Parts A, B, C, D and J (including clamps   e, k
and /) of the apparatus shown in Fig. 20 are used.    If hyxlro-

fiq. 21.—The Meyer absorption tube.
chloric acid is used for decomposing the carbonate, tube
E, filled with pumice bearing silver sulphate in concentrated
sulphuric acid, is required also. Drying tubes F and O are
omitted. For the absorption the Meyer tube, shown in Fig;- 21,
is suitable.
Barium Hydroxide Solution.—A saturated solution of barium hydroxide
is prepared by warming and stirring the solid base with recently foiled
water, using a ratio of 70 to 100 gm of base to 1000 cc of water, accord-
ing to the purity of the barium hydroxide. Cool to room tempera-
ture and siphon into a bottle to be closed with a rubber stopper. Dilute
550 cc of this solution to 1000 cc with distilled water, mix and empty
into a bottle which is provided with a siphon or similar outlet, from, which
the solution may be drawn, and a guard tube of soda lime to remove c*xrr>on
dioxide from the air which is drawn in.
The last diluted solution should be about 0.25 normal. It is not adjusted
to any exact normality because its concentration is subject to change with
time. Instead, the standard acid to be prepared is taken as the primary
standard and "blank" titrations are made frequently.
Sodium Carbonate for Standardizing Acid.—Prepare as directed on pa/ge
57.
Hydrochloric Acid.—From the known per cent of hydrochloric acid a,nd
the specific gravity of the concentrated solution in the laboratory, calculate
the volume of solution necessary to make a suitable quantity (12OO cc to
10 liters, according to whether this is to be an individual preparation, or