Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2004
Publication Date: 5/1/2004
Citation: Watten, B., Boyd, C., Schwartz, M., Summerfelt, S., Brazil, B.L. 2004. Feasibility of measuring dissolved carbon dioxide based on head space partial pressure. Aquacultural Engineering. 30:83-101.
Interpretive Summary: Commonly used methods for measuring dissolved carbon dioxide require a submerged probe, wetted membrane or titration for determining the ambient concentration. In this study, two alternative methods for measuring dissolved carbon dioxide were tested for accuracy and precision. With the first method, a contact chamber is used to create and isolate headspace above a gas-liquid interface. Gas samples from the headspace were passed through an infrared detector, whose, measurement in combination with Henry's Law and water temperature were used to calculate the dissolved carbon dioxide concentration. In the second alternate method, the voltage developed by a pH electrode immersed in a sodium carbonate solution as air from the headspace device is sparged through the solution is measured. Using this method, a standard curve was established to correlate carbon dioxide concentration with voltage for subsequent determines under various test conditions. Dissolved carbon dioxide concentrations calculated from either method compared favorably the carbon dioxide concentrations determined titrimetrically. The average absolute and relative errors observed for the infrared detection and pH electrode methods were 0.9 mg/L and 7% and 0.6 mg/L and 9.6%, respectively. Lastly, the infrared detection unit was configured such that multiple sample locations could be monitored sequentially. A time-based controller was use to active a series of solenoid values that allowed gas samples to flow from different headspace units to a single infrared detected. Measurement error from the replicate headspace units averaged 3.1 mg/L when compared to the titration method, which was within the accepted error the titrimetric method. This multi-site sampling configuration demonstrated that carbon dioxide monitoring cost could be reduced 67% while maintaining accuracy.
Technical Abstract: We describe an instrument prototype that measures dissolved carbon dioxide (DC) without need for standard wetted probe membranes or titration. DC is calculated using Henry's Law, water temperature, and the steady-state partial pressure of carbon dioxide that develops within the instrument's vertical gas-liquid contacting chamber. Gas-phase partial pressures were determined with either an infrared detector (ID) or by measuring voltage developed by a pH electrode immersed in an isolated sodium carbonate solution (SC) sparged with recirculated headspace gas. Calculated (DC) concentrations were compared with those obtained by titration over a range of DC (2,4,8,12,16,20,24, and 28 mg/L), total alkalinity (35,120, and 250 mg/L as CaCO3), total dissolved gas pressure (-178 to 120 mm Hg), and dissolved oxygen concentrations (7, 14, and 18 mg/L). Statistically significant (P<0.001) correlations were established between headspace (ID) and titrimetrically determined DC concentrations (R2=0.987'0.999, N-96). Millivolt and titrimetric values from the SC solution tests were also correlated (P<0.001, R2=0.997, N=16). The absolute and relative error associated with the use of the ID and SC solution averaged 0.9 mg/L DC and 7.0% and 0.6 mg/L DC and 9.6%, respectively. The precision of DC estimates established in a second test series was good; coefficients of variation (100(SD/mean)) for headspace (ID) and titration analyses were 0.99% and 1.7%. Precision of the SC solution method was 1.3%. In a third test series, a single ID was coupled with 4 replicated headspace units so as to permit sequential monitoring (15 minute intervals) of a common water source. Here, appropriate gas samples were secured using a series of solenoid valves (1.6mm bore) activated by a time-based controller. This system configuration reduced the capital cost of per sample site from $2695 to $876. Absolute error averaged 2.9,3.1,3.7, and 2.7 mg/L for replicates 1-4 (N=36) during a 21 day test period (DC range, 36-40 mg/L). The ID meter was then modified so as to provide for DO as well as DC measurements across components of an intensive fish production system.