ABSTRACT
The method described involves extraction of the dissolved gas from an acidified sample in a modified van Slyke apparatus, and measurement of the electrical. resistance of a standard sodium hydroxide solution before and after equilibration with the extracted gas.
The original sample must be not less than about 30 ml. for which a 10 ml., sample is drawn into the extraction apparatus.
There is a linear relation between the resistance finally measured and the total carbon dioxide content of the original sample up to a maximum determined by the strength of the standard hydroxide solution, which is thus adjusted to the desired range of carbon dioxide concentration, e.g. 0 ·005 N-NaOH for 0 –50 mg./L and 0 ·02 N for 0 –200 mg./l. CO2.
The method is easily sensitive to 0 ·3 mg./l. in the range 0 –50 mg./l. total carbon dioxide and about 1 mg./l. in the range 0 –200 mg./l. With a rigidly standardized procedure increased accuracy was obtained so that replicate determinations, on standard solutions in the 0 –50 mg./l. range did not differ by more than 0 ·16 mg./l.
Precautions for handling samples containing carbon dioxide in solution prior to determination have been suggested.
About 15 –20 min. are required for each determination and to Mrs D. E..
Prepared by aerating conductivity water for several hours. Complete removal of the carbon dioxide by this means is not possible and water treated in this way contained 0·8 mg./l. (Calculated -value is 0·3 mg./l. at 24° C. and 660 mm. barometric pressure.)
The carbonate concentration was constant over a period of 14 days, but the resistance increased by 41 Ω, presumably due to a reaction between the alkali and glass container.
The error due to interfering substances in the orthophosphoric acid is very small, being equivalent to about +0 ·05 mg./l. carbon dioxide, but the same amount of undiluted lactic (used as a 1 N solution in the usual van Slyke procedure) can cause errors up to +3 ·5 mg./l., presumably due to the presence of a volatile acid.
The total carbon dioxide content of this distilled water was 0-8 mg./l. and this amount is included in the total value for each standard solution.
