Protocols for Measurement of pCO, Total CO, and
Alkalinity on the Equatorial Pacific JGOFS Survey Cruises
David Archer and Taro Takahashi
Our system consists of a gas-seawater equilibrator and a gas
chromatograph, which measures pCO in the equilibrated carrier gas. A
seawater sample (about 500 ml) is placed in a gas circulation system,
and its temperature is rigorously controlled at 20.00 C using a
constant temperature bath. The gas circulation system is first filled
with a CO-air gas mixture of a known CO concentration, and the
gas is recirculated through a plastic gas disperser immersed in the water
sample, and through a closed system loop (which includes a small gas
circulation pump and a gas sampling valve) for equilibration.
Ten minutes of recirculation have been found to be sufficient for
achieving equilibration between the water sample and the circulating gas.
A 1 ml gas sample is isolated from the loop using a gas sampling valve,
and is injected into the gas chromatograph for CO determination.
The gas chromatograph is similar to that described by Weiss (1981). The
CO molecules mixed with hydrogen gas are converted quantitatively to methane using a catalytic column of ruthenium, and the methane molecules produced are detected by a flame-ionization detector. The chromatograph itself yields a precision of about ±0.06 % for CO analyses, and is
calibrated using the WMO standard air-CO mixtures analysed by C.D. Keeling of SIO.
The gas sample isolated from the equilibrator and injected into the
chromatograph for CO analysis is saturated with water vapor at the equilibrium temperature of 20 C. Since the chromatograph measures
the number of CO molecules in a known volume of the sampling valve at a known temperature, the measurement yields pCO directly rather than the
mole fraction of CO in dry equilibrated carrier gas. All the shipboard
determinations of pCO have been performed at least in duplicate in order
to insure equilibrium. A reproducibility of ±0.25 % has been achieved
routinely under shipboard conditions.
Coulometric Measurements of the Total CO Concentration
The coulometer system is similar to that described by Johnson
et al. (1985), and has been modified from a commercial coulometer
(Model-5011) manufactured by UIC Inc. (Joliet, Ill). It consists of a
CO extraction vessel, a CO absorber cell, and a coulometer.
A known volume of a seawater sample (about 25 ml containing about
50 mmoles of CO) is forced into the extraction vessel by a CO-free
nitrogen gas, and is acidified using 1 ml of 10 % phosphoric acid.
The liberated CO gas is swept by a stream of nitrogen gas into the CO absorber cell, which is filled with an aqueous solution of
dimethylsulfoxide, monoethanolamine, and thymolphthalein. The CO is absorbed quantitatively by this solution, in which it reacts with the monoethanolamine to form hydroxyethylcarbamic acid and lower the pH, thus
causing a color change in the thymolphthalein indicator from blue to
colorless. The photocell in the coulometer detects the color change, and
instructs the unit to pass an electric current through the cell, so that
the water in the solution dissociates to generate OH- ions and hydrogen
gas. The OH- ions neutralize the acid until the original pH is restored
in the CO absorber solution. The product of current passed and
time represents the amount of CO released from the sea water sample.
We have calibrated the coulometer system using five independent methods:
1) gravimetrically prepared CaCO standards, 2) gravimetrically prepared
NaCO standards, 3) volumetrically prepared pure (99.999 %)
CO gas standards, 4) WMO air-CO gas mixture standards, and 5) a
calibrated electrical current meter. We have found that the results of
these calibrations agree within 0.1 %. Routine calibration is done using
CO gas standards. The accuracy of the measurements has been estimated to be about 0.1 % (or about 2 mM/kg).
We have replicated the setup and procedure of Brewer
(Bradshaw et al., 1981; Brewer et al., 1986) to measure
alkalinity. The procedure is automated using a titrator controlled by a
computer, which is also interfaced to the pH meter. The alkalinity is
calculated by modified Gran titration. Before the sample is measured, the
ratio of the volume of the sample cell to the acid strength is determined
by titration of a gravimetrically prepared tris buffer standard from NBS.
The standard is prepared in a solution of NaCl at the ionic strength of
seawater (0.7). The titrant is 0.1 M HCl, prepared from Dilu-it
analytical HCl in NaCl at the ionic strength of seawater. The blank in
the NaCl solution was determined by titration with HCl of the salt
solution with small known amounts of tris standard added (standard
addition). The effect of the HgCl preservative on the measured
alkalinity was also determined empirically. The reliability of the
alkalinity measurements will be verified by measuring the alkalinity of
standard solutions prepared by Andrew Dickson at SIO. We achieved a
reproducibility of 0.1 % on replicate samples with this method.