Q UAN TIT A TIVE DETERMINA TIONS

83

laboratory stock) of'solution, either fourth-normal or of such strength that
1 cc shall be equivalent to 0.005 gm of carbon dioxide, calculating equiva-
lent weights from the equations:

CO_2 + H_2O
Ba(OH)_2-fH_2CO_3-
Ba(OH)_2 + 2HC1-

+ H_2C0_3;
 >	BaCO_3 -f 2H_20;
 >	BaCl_2 + 2H_20.

(1)
(2)
(3)

Measure 2 per cent more than this amount, using a graduated cylinder.
Empty into a bottle of suitable capacity and add the necessary quantity of
water. Stopper and mix thoroughly. Since the solution has been warmed
by the reaction between acid and water it should stand until the tempera-
ture of the room is attained, before standardizing.
Standardization.—Calculate the weight of sodium carbonate required for
250 cc of a solution of strength equivalent to that of the desired acid.
Weigh this quantity of the prepared pure material on counterpoised glasses
and brush into a funnel which is placed in the neck of a 250-cc volumetric
flask. Rinse down with distilled water, remove the funnel and dilute to the
mark on the flask. Mix thoroughly, best by pouring into a dry beaker or
flask and back several times.
Fill a burette with the solution and another with the acid solution already
made and titrate as directed on page 58. Calculate the normality of the
solution, also the volume of water to be added to each 1000 cc to make
a solution of the exact concentration required. If water to be added is more
than 10 cc add nearly the required amount to each liter of the acid, mix,
and restandardize. If the quantity to be added is less than 10 cc the acid
is diluted as follows: Fill a dry 10DO-cc graduated flask to the mark with the
acid solution. This flask should be capable of holding the required amount
of water above the mark. From a burette add the calculated quantity of
water directly to the solution in the flask and mix thoroughly. Pour into a
dry bottle and make more diluted solution in the same manner, having
first rinsed and dried the graduated flask. Check the accuracy of the
dilution by restandardization.
A preliminary titration should be made to determine whether the barium
hydroxide solution is approximately equivalent to the standard acid. Meas-
ure 25 cc of the base into an Erlenmeyer flask, add a drop of phenolphthal-
ein and titrate rapidly to the disappearance of the pink color. Twenty-two
to 27 cc of acid should be required, although no special note is made of the
exact quantity, the value of the basic solution being determined accurately
by a blank titration, made at the time the carbonate analysis is carried out.
Water, Free from Carbon Dioxide.—All water that is to be used for dilutions
and for rinsing apparatus must be free from carbon dioxide and it must be
neutral to phenolphthalein. Carbon dioxide is removed by boiling the
water for 10 minutes, cooling and placing in a bottle provided with a guard
tube filled with soda lime. Boiling large quantities of water is often not
conveniently done in the laboratory. An equally satisfactory method is to
provide the bottle with a two-hole stopper through which pass two glass