Taro Takahashi, David Chipman, Stephany Rubin, Colm Sweeney, and S.C. Sutherland Lamont-Doherty Earth Observatory (CO2 Group) 61 U.S. Route 9W P.O. Box 1000 Palisades, NY 10964-8000 U.S.A.TOTAL CO2 in Seawater:
Except for the way we introduced the sample, the coulometric analysis system used to measure the total CO2 (TCO2) concentration in seawater samples during these cruises is the same as described in Chipman, D. W., Marra, J. and Takahashi, T. (Primary production at 47°N and 20°W in the North Atlantic Ocean: A comparison between the C14 incubation method and the mixed layer carbon budget. Deep-Sea. Res., 40 , 151-169, 1993). The system and procedures used are summarized below.
This system consists of a Model 5011 coulometer, manufactured by UIC Inc., Jolliet, IL, and a sample introduction/CO2 extraction system of a Lamont-Doherty Earth Obs. (CO2 Group) design. It differs from the Single Operator Multiparameter Metabolic Analyzer (SOMMA) system used by 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. The syringe is a hand-ground Pyrex glass medical syringe with two firm reference stops which allow the quantitative sampling of seawater. Our experience with syringe and pipette methods is that accumulation of coatings formed on the glass surfaces from the repeated filling and emptying with seawater will not significantly affect the volume contained of either, but that it will affect the volume delivered by the passively drained pipette in the SOMMA system. The positive displacement of the plunger of a syringe will keep the delivered volume constant, even in the presence of surface coatings. Additionally, since the water sample in the syringe has no air space, changes in TCO2 due to gas exchange with the air in head space are eliminated.
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 and pressed in using two strong rubber bands. Immediately after sample collection, 200 ul of 50% saturated mercuric chloride solution was added to prevent biological alteration of the TCO2. A small head space (~5 ml) was left in the bottle to prevent thermal expansion of the water from causing a leak or breaking the bottle. Samples were normally analyzed within 24 hours of collection. For analysis, a water sample was sucked into a syringe, and a calibrated volume (19-20 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 using the measured salinity, the temperature at the time of injection, and 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 distilled, deionized water dispensed. The measurements were corrected for the buoyancy of air displaced by the water, amounting to about 0.1% of the weight of the water. The volume was then computed using the density of pure water at the temperature of the measurement. Repeated measurements gave a precision of ±0.03% or better.
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 continuously monitored the transmissivity of the solution. The electronic circuitry of the coulometer, in detecting the change in the color of the pH indicator, caused electrical current to flow through the cell, generating hydroxyl (OH-) ions from a small amount of water in the solution. The OH- generated then titrated the acid, returning the solution to its original pH and color, at which point the current flow was stopped. The product of current passed through the cell and time was related by the Faraday constant to the number of moles of OH- generated, and hence to the number of moles of CO2 absorbed to form the acid. A thermostated, double walled titration cell was used during titration, to eliminate the shifting of the endpoint of the titration due to a change in the 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, and the ambient atmospheric pressure in the laboratory was measured using a high precision electronic barometer with an accuracy of better than 0.05%. The loop temperatures were measured to ±0.05°C with a thermometer calibrated against one traceable to the NIST. The non-ideality of CO2 was incorporated into the computation of the loop contents. Prior to the expeditions, the volumes of the loops were determined by the difference in weight between the loop injection valve assembly when empty and filled with water. Repeated measurements gave a precision of ±0.02%. During the expedition the coulometer was calibrated several times a day using this gas sampling system.
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 on the condition of the solution in the titration cell, this factor varies around the ideal ratio of unity by a few tenths of a percent. It commonly starts from less than unity when the cell solution is new and increases to greater than unity as increasing amounts of carbon are titrated. This change can be represented by a quadratic equation relating values of calibration factor with the total amount of carbon titrated in a given cell. The CO2 concentration in each seawater sample was corrected using a factor estimated from the equation fit to the calibration data for each cell. Generally a cell had to be cleaned and filled with fresh solution after about 40 samples. After this number the cell began to behave erratically with unreliable analytical results.
Analyses of Certified Reference Solutions:
For the purpose of quality control of total CO2 determinations, SIO Reference Solutions Batch Numbers 31 and 32 were run through our analytical system at sea as unknowns. Our shipboard analyses compare with the SIO manometric values as follows:
SIO Manometric LDEO Coulometric (SIO-LDEO) Mooring CRM TCO2 TCO2 at sea Difference Cruise Batch (umol/kg) (umol/kg) (umol/kg) NBP 96/5 31 1876.57±1.27 1878.88±2.32 -2.31 (N=10) (N=22) NBP 96/5 32 1997.57±1.35 1999.30±2.13 -1.73 (N=10) (N=33)N is the number of analyses.
The two attached figures show the results graphically.
Figure A
Figure B
The mean values for both batches agree within one standard deviation for each set of measurements. However, since we do not understand the source(s) of this discrepancy, we are reporting both observed and CRM-adjusted values in this data set. The value used for this adjustment is the weighted average of the two differences above, -1.96.
