Carbon dioxide may exist in water in three forms—free carbon dioxide,
bicarbonate (pp. 37–38), and carbonate (p. 38). One-half the carbon
dioxide as bicarbonate is known as the half-bound carbon dioxide. The
carbon dioxide as carbonate plus one-half that as bicarbonate is known
as the bound carbon dioxide.
_Procedure._—Pour 100 cc. of the sample into a tall narrow vessel,
preferably a 100 cc. Nessler tube. Add 10 drops of phenolphthalein
indicator, and titrate rapidly with N/22 sodium carbonate, stirring
gently, until a faint but permanent pink color is produced. The free
carbon dioxide (CO_{2}) in parts per million is equal to 10 times the
number of cubic centimeters of N/22 sodium carbonate used.
Because of the ease with which free carbon dioxide escapes from water,
particularly when the gas is present in large amount, a special sample
should be collected for this determination, which should preferably be
made at the time of collection. If the analysis cannot be made at the
time of collection approximate results with water not too high in free
carbon dioxide may be obtained on samples collected in bottles
completely filled so as to leave no air space under the stopper. Bottled
samples should be kept, until tested, at a temperature lower than that
of the water when collected. If mineral acids or certain salts are
present correction must be made. At best, the results of the titration
are uncertain because the proper end-point for correct results differs
in color with different types of water.