3a. Measurements of the Total CO2 Concentration in Seawater

The coulometric analysis system which was used to measure the total CO2 concentration in seawater samples (TCO2) during the expedition is described by Chipman et al. (1993) and is summarized below. This system consists of a coulometer (Model 5011) manufactured by UIC Inc. (Jolliet, IL) and a sample introduction/CO2 extraction system of the LDEO design, which differs from the Single Operator Multiparameter Metabolic Analyzer (SOMMA) system used by most of the other participants of the DOE/CO2 program. In the LDEO system, a precisely known volume of seawater sample is introduced manually into a CO2 extraction vessel using a calibrated syringe instead of the automated pipette used by the SOMMA system.

Samples for TCO2 analysis were drawn from the Niskin bottles of the rosette casts directly into 250 ml glass reagent bottles with ground standard-taper stoppers, sealed with silicone vacuum grease. Immediately after sample collection, 200 µl of 50% saturated mercuric chloride solution was added to prevent biological alteration of the TCO2, and samples were analyzed within 24 hours of collection. For analysis, a calibrated volume (ranging between 19 and 20 ml) of water sample was introduced into a CO2 extraction chamber through a rubber septum. The mass of the seawater sample delivered was determined from the density of seawater calculated at the temperature of injection using the International Equation of State of Seawater (Millero et al., 1980). Prior to the expedition, the volume of each sampling syringe between two reference stops was determined by repeatedly weighing aliquots of double distilled water dispensed. The measurements were corrected for the buoyancy of air displaced by the water, which amounted to approximately 0.1% of the weight of the water, and the volume was computed using the density of pure water at the temperature of the measurement. Repeated measurements yielded a precision of ±0.03%.

The seawater sample in the extraction vessel was acidified with ~1 ml of 8.5% phosphoric acid introduced through a sidearm of the extraction chamber. The evolved CO2 was stripped from the sample and transferred into the electrochemical cell of the CO2 coulometer by a stream of CO2-free air. In the coulometer cell, the CO2 was quantitatively absorbed by a solution of ethanolamine in dimethylsulfoxide (DMSO). Reaction between the CO2 and the ethanolamine formed the weak hydroxyethylcarbamic acid. The pH change of the solution associated with the formation of this acid resulted in a color change of the thymolphthalein pH indicator in the solution. The color change, from deep blue to colorless, was detected by a photodiode, which continually monitored the transmissivity of the solution. The electronic circuitry of the coulometer, on detecting the change in the color of the pH indicator, caused a current to be passed through the cell, electro-generating hydroxyl (OH-) ions from a small amount of water in the solution. The OH- generated titrated the acid, returning the solution to its original pH (and hence color), at which point the circuitry interrupted the current flow. The product of current passed through the cell and time was related by the Faraday constant to the number of moles of OH- generated to titrate the acid and hence to the number of moles of CO2 absorbed to form the acid. A thermostated double walled titration cell was used to extract the heat generated in the cell during titration, to eliminate the shifting of the endpoint of the titration due to change in temperature of the cell solutions.

The coulometer was calibrated using research grade CO2 gas (99.998% pure) introduced into the carrier gas line upstream of the extraction chamber, alternately using two fixed-volume sample loops on a gas sampling valve. The loops were vented to the atmosphere, of which the pressure was measured using a high precision electronic barometer used with the pCO2 system; the loop temperatures were measured to ±0.05 °C with a thermometer calibrated against one traceable to the NIST, and the non-ideality of CO2 was incorporated in the computation of the loop contents. Prior to the expedition, the volume of the loops was determined by the weight difference between the loop/injection valve assembly empty and filled with water. Repeated measurements indicated that the volumes of the loops were precise to ±0.02%. During the expedition, the coulometer was calibrated several times daily using the gas sampling system described above.

The calibration factor, which represents the ratio between the number of moles of CO2 in the loop and the reading of the coulometer, changes during the use of a titration cell. Depending upon the condition of the coulometric solution in the titration cell, the calibration factor varies around the ideal ratio of unity by a few tenths of percent. Fig. 2 shows the typical variation of the calibration factor as a function of the cumulative amount of CO2 titrated by a cell and indicates that it may be represented by a quadratic form. If changes in the calibration factor were not taken into consideration, a systematic error of as much as 4 to 5 µmol/kg would be introduced between the samples analyzed in the early and late stages of a single coulometric solution. Accordingly, the CO2 concentration in each seawater sample was determined using a calibration factor estimated from an equation fit to the calibration data obtained for each titration cell. Generally, a titration cell had to be cleaned and filled with new solution after about 40 samples were analyzed. Beyond this number of analyses, the cell began to behave erratically yielding unreliable analytical results. The working equation used for computing the coulometer/cell calibration factor (CF) is as follows.

CF = (12.011*106)*PA*[LPV*(1+3*a)*(TK(calib)-TK(lp))] / {MV(CO2)*[RD-(TM*BL)]}

where

12.011*106  =  Atomic weight (in grams) of carbon,
PA  =  Pressure (in atm) of CO2 gas in loop at time of calibration,
LPV  =  Volume of calibration valve loop (in ml) at TK(lp),
α  =  Linear thermal expansion coefficient of stainless steel,1.73*10-5 °K-1,
TK(lp)  =  Temperature (in °K) at which loop volume was determined,
TK(calib)  =  Temperature (in °K) of CO2 in loop at time of calibration,
MV(CO2)  =  Molar volume of CO2 (in ml) at temperature at which loop volume was determined,
RD  =  Coulometer reading (in µgrams-carbon),
TM  =  Length of calibration run (in minutes), and
BL  =  Instrumental blank (or background) rate (in grams of carbon/min).

The following relationships were used for the computation of the total CO2 concentration in seawater samples using the coulometer;

TCO2 (µmol/kg) = CF*DF*[RD-AB-(TM*BL)]/(12.011*VL*RHO)

where

CF  =  Calibration factor of coulometer/cell combination interpolated to the time when the measurement was made,
DF  =  Dilution factor to account for dilution of seawater sample by CO2-free mercuric chloride poisoning solution, DF = [(sample volume) + (poison volume)]/(sample volume) = 1.0008 for 200 µl of mercuric chloride solution in 250 ml sample,
RD  =  Coulometer reading (in µgrams of carbon ),
AB  =  Acid blank (in grams) of carbon to account for a small amount of CO2 in phosphoric acid solution added to sample; determined by measuring CO2 stripped from larger volume of acid, typically less than 0.03% of amount of CO2 in seawater sample.
TM  =  Length of analytical run (in minutes),
BL  =  Instrumental blank rate (in µgrams of carbon/min.), typical blank rate being 0.01 to 0.02 grams of carbon/min.; the maximum acceptable blank rate of 0.05 grams of carbon/min results in a correction of about 0.1% over the normal length of an analytical run,
VL  =  Volume of seawater sample (in liters) injected into stripping chamber, determined by use of pre-calibrated fixed-volume syringes, typical sample volume being 0.019 to 0.020 liters,
RHO  =  Density of seawater sample at the temperature of injection into stripping chamber, calculated using the UNESCO equation of state for seawater (Millero et al., 1980), the salinity, and the temperature measured on the water remaining in the syringe immediately after injecting sample,
12.011  =  Atomic weight (in grams) of carbon.

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