Methods for the Measurement of pCO2 and TCO2 in Seawater for JGOFS Southern Ocean Process Cruise; R/V Roger Revelle Cruise KIWI-9

Taro Takahashi, David Chipman, Stephany Rubin,
John Goddard, 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:

The coulometric analysis system used to measure the total CO2 (TCO2) concentration in seawater samples during this cruise is the very similar to that described in Chipman, D. W., Marra, J. and Takahashi, T. (Primary production at 47N and 20W 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). However, the way we introduced the sample is somewhat different as we will explain. 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. Thus, multiple water samples may be collected using syringes from a bottle of water sample, and the reproducibility of analyses may be tested by successively analyzing these syringes.

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. Two or more samples were collected simultaneously from a sample bottle and used for replicate analyses. 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 a 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 0.05C 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 Number 40 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) Process CRM TCO2 TCO2 at sea Difference Cruise Batch (umol/kg) (umol/kg) (umol/kg) Process2 40 1985.760.72 1984.481.11 +1.28 KIWI-9 (N=10) (N=202)

N is the number of analyses. The number of analyses (N=202) at sea includes the multiple syringe samples collected from each bottle of the Reference Solutions.

The attached figure shows the results graphically.

The mean values for Batch #40 agree well within one standard deviation for each set of measurements. Since we do not understand the source(s) of this discrepancy, we are reporting both observed and CRM-adjusted values in this data set.

Determination of pCO2 in Discrete Seawater Samples:

A fully automated equilbrator-infrared gas analyzer system was used during the expedition for the determination of partial pressure of CO2 exerted by the seawater samples. Its design is similar to that described by Chipman, D. W., Marra, J. and Takahashi, T. (Primary production at 47N and 20W in the North Atlantic Ocean: A comparison between the 14C incubation method and the mixed layer carbon budget. Deep-Sea. Res., 40 , 151-169, 1993) with the exception that the gas chromatograph was replaced with an infrared gas analyzer.

The system consisted of a circulation pump plumbed to recirculate air in a closed system through porous plastic gas dispersers immersed in a 450 ml seawater sample. The water was contained in a long-neck flask which served as an equilibration vessel. Two sets of flasks and circulation pumps were used so that two water samples could be processed concurrently. Electrically driven Valco 4-port valves were used to isolate each of the equilibrators during the initial equilibration. Manually operated 2-way and 3-way Whitey valves allowed part of the water in each equilibrator to be replaced with air of known initial CO2 concentration, creating the necessary headspace for equilibration. A drain line in each equilibrator insured that the volume of water was constant at 500 ml. This allowed us to make accurate corrections for the effect of the perturbation of the sample by the headspace air. The equilibrators were open to the laboratory air through isolation coils, keeping them at atmospheric pressure, and we measured lab atmospheric pressure with a high precision electronic barometer with an accuracy of better than 0.05%. Since the partial pressure of CO2 is strongly affected by temperature, the equilibration flasks were kept immersed in a water bath maintained at about 4C. An electrically driven Valco 6-port valve allowed the entire equilibration system to be isolated, while simultaneously connecting a calibration gas selection valve. A 2-way normally-closed Skinner solenoid valve on the output of the of the calibration selection valve allowed the gas flow to be controlled by the system controller. It also provided a necessary second means of stopping the flow of the calibration gases to prevent accidental loss of the calibration gases in case of a control malfunction.

The analysis of the CO2 in the gas equilibrated with the seawater sample was performed using an infrared gas analyzer (LICOR Model 6125) in a flow through mode. A 0.5 ml aliquot of equilibrated headspace gas, representing less than 1% of the circulating gas, was isolated using a gas pipette, and swept with CO2-free air flowing at a constant rate. For low pCO2 samples, a 1 ml sample volume was used. The gas was passed through a permeation drying tube, injected into the infrared gas analyzer cell (about 7 ml in volume) filled previously with CO2-free air, and discharged out of the cell into room. The small volume of the gas sample insured that all of the CO2 from the gas pipette were found in the cell at the same time, so that the peak height is proportional to the amount of CO2 present in the gas pipette. Drying of the sample gas avoids the effects of pressure-broadening and dilution caused by water vapor. The amount of CO2 in the sampling pipette is a function of the loop volume, temperature and pressure. The temperature was held constant and measured, and the pressure of sample gas was same as the barometric pressure, which was measured with an accuracy of better than 0.05%. The peak height, which represents the number of moles of CO2 in the sample gas, was calibrated every 1.5 hours using a quadratic equation fitted to three standard gas mixtures (368.6, 495.3 and 788.8 ppm in dry air).

The analytical procedure follows. Water samples were drawn from the 10-liter Niskin bottles of a rosette directly into 500 ml narrow-necked volumetric Pyrex flasks. These served as both sample containers and equilibration vessels. The samples were immediately inoculated with 200 ul of 50% saturated mercuric chloride solution, sealed airtight with screw caps with conical plastic liners to prevent biological modification of the pCO2, and stored in the dark until measurement. Measurements were normally performed within 24 hours of sampling. A headspace of 3 to 5 ml was left above the water to allow for thermal expansion during storage. Prior to analysis the sample flasks were brought to the water bath temperature of 4C in the constant temperature bath, and about 45 ml of water was displaced with air of known CO2 concentration. The air in the flasks and in the tubing connecting them to the gas pipette loop was recirculated continuously for about 20 minutes through a gas disperser immersed in the water. This provided a large surface area for gas exchange, and equilibrium for CO2 was attained in 15 minutes. The temperature of the bath water was assumed to be that for the sample water, and was measured at the time of equilibration with a precision of 0.01C using a thermometer calibrated against an NIST-certified thermometer. This temperature is reported in the data table as "TEMP PCO2." The sample temperature ranged from 3.93C to 4.11C, with a mean of 4.01C0.03C.

The equilibrated air samples were saturated with water vapor at the temperature of equilibration and had the same pCO2 as the water. By injecting the air aliquot into the infrared analyzer after the water vapor was removed, the concentration of CO2 was measured. Therefore, the effect of water vapor must be taken into consideration for computing pCO2 as follows:

pCO2 (uatm) = [Cmeas (ppm)] X [total pressure of equilibration (atm) - water vapor pressure (atm)]

where Cmeas is the mole fraction concentration of CO2 in dried equilibrated air. The total pressure of equilibrated air was measured by having the head space in the equilibrator flask always at atmospheric pressure. We measured the latter with an electronic barometer at the time each equilibrated air sample was injected into the infrared analyzer for CO2 determination. The water vapor pressure was computed at the equilibration temperature and salinity of the seawater. Cmeas was determined by using a quadratic equation fit to all 3 of the standards.

The concentrations for standard gases used are traceable to the WMO reference scale through analysis in the laboratories of C. D. Keeling of SIO, LaJolla CA, and of Pieter P. Tans of NOAA/CMDL, Boulder CO. The values of the standard gas mixtures used during this cruise are: 386.6 ppm CO2, 495.3 ppm CO2, and 788.8 ppm CO2.

Corrections were made to account for the change in pCO2 of the sample water due to the transfer of CO2 between the water and circulating air during equilibration. We know the pCO2 in equilibrated, perturbed water and the total CO2 (TCO2) by coulometry before the equilibration. We can also calculate the change in TCO2 in the water based on the change in pCO2 between the post-equilibrium value and the known concentration in the pre-equilibrium displacement gas. With the pre-equilibrium TCO2 plus the perturbation in TCO2 during equilibration, we obtain the post-equilibrium TCO2 value. Using the post-equilibrium TCO2 and measured pCO2 values, we calculate total alkalinity (TALK) at the end of the equilibration, knowing the temperature, salinity, phosphate, and silicate data. Since the perturbation does NOT change the TALK, we calculate the pre-equilibrium pCO2 from the pre-equilibrium TCO2, the calculated TALK, and the other parameters of temperature, salinity, etc. This is the value that we report as pCO2, the pre-equilibrium calculated value. The magnitude of this correction is generally less than 2 uatm. Details of the computational scheme are presented in a DOE technical report by Takahashi, et al. (1998) [Measurements of the Total CO2 Concentration and Partial Pressure of CO2 in Seawater during WOCE Expeditions P-16, P-17, and P-19 in the South Pacific Ocean, October, 1992 - April, 1993. Final Technical Report of Grant No. DE-FCO2-93ER61539 to U.S. Dept. of Energy, Lamont-Doherty Earth Obs., Palisades, NY 10964, p 124.].

The pCO2 values reported in this data set are in the unit of microatmospheres at the temperature of equilibration . These individual equilibration temperatures are listed in the data table.

We estimate the precision of the pCO2 measurement for a single hydrographic station to be about 0.15% based on the reproducibility of replicate equilibrations. We estimate the station-to-station reproducibility to be about 0.5%.