Determination of pCO₂ in Seawater

A fully automated equilibrator-gas chromatograph system was used during the expedition for the determination of partial pressure of CO₂ exerted by the seawater samples, and its design has been described by Chipman et al. (1993). Figure 6 gives a schematic diagram of this system.

The system consists of a pair of air circulation pumps (Spectrex Model AS-300-SS) plumbed to recirculated air through porous plastic gas dispersers which are immersed in two separate seawater samples. Electrically driven Valco 4-port valves were used to isolate each of the equilibrators during the initial equilibration prior to analysis of the equilibrated air. 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, to create the necessary headspace for equilibration. A drain line in each equilibrator insured that the ratio of water to air in each equilibrator was constant, allowing accurate corrections to be made for the effect of the perturbation of the sample seawater by the headspace air. Diaphragms (thin rubber balloons) were plumbed to each equilibrator to provide "soft walls" to the system, so that the pressure in the equilibrators was kept close to the ambient laboratory atmospheric pressure which was measured with a high precision electronic barometer. Since the partial pressure of CO₂ is strongly affected by temperature changes, the equilibration flasks were kept immersed in a constant temperature water bath, held at a temperature of 20.00^oC. An electrically driven Valco 6-port valve allowed the entire equilibration system to be isolated, simultaneously connecting a calibration gas selection valve (also electrically driven Valco, Model 4SD, with 4 input ports but with an eight-position driver, so that all gas flows were blocked at the four intermediate positions). A 2-way normally-closed Skinner solenoid valve on the output of the calibration selection valve allowed the gas flows to be controlled by the system controller, and provided a necessary second means of stopping the flow of the calibration gases to prevent the accidental loss of calibration gases in the event of control malfunction.

The analysis of the CO₂ in the equilibrated air or calibration gases was performed using a Shimadzu Mini- 2 gas chromatograph, which was equipped with a flame ionization detector. A one-ml sample loop and a pre-column and analytical column (both packed with Chromosorb 102, of 0.2 and 2.0 m lengths respectively) were attached to an electrically driven Valco 10-port valve within the column oven of the gas chromatograph. Ultra-high purity hydrogen gas (electrolytically generated by an Aadco hydrogen generator and purified by means of diffusion through a palladium foil using an Aadco hydrogen purifier) served as the carrier gas for the chromatographic separation of CO₂ from the other components of the air. The use of hydrogen for carrier gas also allowed the CO₂ to be converted to methane in an attached catalytic converter prior to quantification by the flame ionization detector. Unlike the method described by Weiss (1981), this system used a catalyst of ruthenium metal on Chromosorb W support and did not require a palladium pre-catalyst to remove oxygen from the carrier gas stream. Hydrocarbon-free air to support the combustion in the flame ionization detector was provided by means of a chromatographic air purifier (Aadco Model 737).

Integration of the output signal from the gas chromatograph and control of the entire equilibration and calibration procedure was provided by means of a Shimadzu Chromatopac (Model C-R6A) computing integrator.

The analytical procedure is as follows. Water samples for analysis were drawn from the 10-liter Niskin samplers of a rosette cast directly into 500-ml narrow-necked Pyrex flasks which serve both as sample containers and equilibration vessels. The samples were poisoned with 200 µl of 50% saturated mercuric chloride solution to prevent biological modification of the pCO₂, and were stored in the dark until measurement, which normally was performed within 24 hours of sampling. A headspace of 3 to 5 ml was left above the water in the flask to allow for thermal expansion during storage. The flasks were sealed air- tight using screw-caps with conical plastic liners.

The equilibrated air samples were saturated with water-vapor at the temperature of equilibration and had the same pCO, as the water sample. By injecting the air aliquot without removing the water vapor, the partial pressure of CO₂ was determined directly, without the need to know the water vapor pressure (Takahashi et al., 1982) using the relationship below:

PCO₂ (µ atm) = [Cmeas (ppm)] x [Total pressure of equilibration (atm)],

where Cmeas is the mole fraction concentration of CO₂ in equilibrated moist air. The total pressure of equilibrated air was measured by having the head space in the equilibrator flask always at atmospheric pressure which was, in turn, measured with the electronic barometer at the time each equilibrated air sample was injected into the gas chromatograph.

Corrections were made to account for the change in pCO₂ of the sample water due to the transfer of CO₂ to or from the water during equilibration with the recirculating air. The analytical steps yielding C_meas, which have been programmed in the on-line computer, are schematically shown in Figure 7-A; the pCO₂ correction routines in Figure 7-B; and a list of variables in Table 1. The overall precision of the pCO₂ measurement has been estimated to be about ± 0.10% based on the reproducibility of replicate equilibrations.

Table 1. List of variables used in Fig. 7-A and 7-B.