62 
BULLETIN OF THE BUREAU OF FISHERIES. 
If the sample contains a very large amount of free C0 2 even in the flask, it is 
better to carry out a second titration, adding almost the amount required in the 
first titration and shaking well before opening the bottle to add more. By using 
this method the N/44 Na 2 C0 3 will neutralize exactly one-half as much N/44 HC1 
with phenolphthalein as with methyl orange. 
DETERMINATION OF FIXED CARBON DIOXIDE. 
This is determined in a separate sample of 100 cc. and may be titrated in a 
beaker or graduated flask as above. Two drops of methyl orange are added to 
the sample and N/44 HC1 is added until a faint permanent pink is obtained. The 
detection of this end point seems to be difficult for some eyes. It is not definite 
with artificial light or very long after sunset. Samples of methyl orange have 
been found that give a poor color. This is easily detected, especially if the colors 
given by several samples are compared. This has also been noted by the American 
Public Health Association. They recommend Erythrosine as more reliable than 
methyl orange. 
Seyler states that the volumetric methods are based on the following facts: 
1. Carbonates are alkaline to phenolphthalein, bicarbonates are neutral, and 
free carbon dioxide is acid. 
2. Methyl orange is unaffected by carbon dioxide, hence the bases present 
as carbonates and bicarbonates can be titrated at once with standard acid 
(Na 2 C0 3 + 2HCl = 2NaCl + H 2 0 + C0 2 ). The bicarbonates of calcium and mag- 
nesium are neutral to phenolphthalein. From this fact the following conclusions 
are drawn: 
(а) If a water is neutral or acid to phenolphthalein, the "half-bound” carbon 
dioxide is equal to the "fixed.” 
(б) If a water is alkaline to phenolphthalein, it can contain no free carbon 
dioxide, and the "half-bound” will be less than the "fixed” by an amount deter- 
mined by titration with acid until it is neutral to phenolphthalein. 
CALCULATIONS OF CARBON DIOXIDE. 
The following formulae are based on N/44 solutions and 100 cc. samples of 
water. 
Acid water . — Number of cc. of Na 2 C0 3 (with phenolphthalein) X 2.528 equals 
cc. of free carbon dioxide per liter of water. Number of cc. of HC1 (with methyl 
orange) X 2.528 equals cc. of fixed carbon dioxide per liter of water. Acid or 
half-bound carbon dioxide is equal to the fixed. 
Alkaline water . — Fixed carbon dioxide is the same as in acid water. Half- 
bound is equal to the fixed less two times the number of cc. HC1 required to dis- 
charge phenolphthalein pink X 2.528. 
Keiser and Leavitt (1908) and Keiser and McMaster (1908) state that the 
acid carbonate of calcium seems to be CaC0 3 1.75 to 1.8 (H 2 C0 3 ). Their work 
was repeated during the winter of 1912-13 at the New Mexico School of Mines, 
and at this altitude (4,616 feet) the compound formed was so unstable that it 
decomposed before it could be analyzed. 
