Many methods are available for the measurement of dissolved carbon dioxide in an aqueous environment.
Standard titration is the typical field method for measuring dissolved CO2 in aquaculture systems.
However, titrimetric determination of dissolved CO2 in marine water aquaculture systems is unsuitable
because of the high dissolved solids, silicates, and other dissolved minerals that interfere with the determination.
Other methods used to measure dissolved carbon dioxide in an aquaculture water included use
of a wetted CO2 probe analyzer, standard nomographic methods, and calculation by direct measurements
of the water’s pH, temperature, and alkalinity. The determination of dissolved CO2 in saltwater based on
partial pressure measurements and non-dispersive infra-red (NDIR) techniques with a CO2 gas analyzer
are widely employed for oceanic surveys of surface ocean CO2 flux and are similar to the techniques
employed with the head space unit (HSU) in this study. Dissolved carbon dioxide (DC) determination
with the HSU using a infra-red gas analyzer (IRGA) was compared with titrimetric, nomographic, calculated,
and probe measurements of CO2 in freshwater and in saltwater with a salinity ranging from
5.0 to 30 ppt, and a CO2 range from 8 to 50 mg/L. Differences in CO2 measurements between duplicate
HSUs (0.1–0.2 mg/L) were not statistically significant different. The coefficient of variation for the HSU
readings averaged 1.85% which was better than the CO2 probe (4.09%) and that for the titrimetric method
(5.84%). In all low, medium and high salinity level trials HSU precision was good, averaging 3.39%. Differences
existed between comparison testing of the CO2 probe and HSU measurements with the CO2 probe
readings, on average, providing DC estimates that were higher than HSU estimates. Differences between
HSU and titration based estimates of DC increased with salinity and reached a maximum at 32.2 ppt.
These differences were statistically significant (P < 0.05) at all salinity levels greater than 0.3 ppt. Results
indicated reliable replicated results from the head space unit with varying salinity and dissolved carbon
dioxide concentrations.