Methods for Measurement and Computation

Three titration systems (Thurmond and Millero 1982) were used to determine the TALK. Each system consisted of a Metrohm 655 Dosimat titrator and an Orion 720A pH meter that was operated by a personal computer (PC) (Millero et al. 1993a). The titration was made by adding HCl to the seawater past the carbonic acid end point (pH ~ or = 3.5). The solutions were contained in water-jacketed cells (230 cm³) controlled to a constant temperature of 25°C with a Forma temperature controller. The computer program used to perform the titration was developed in the Rosenstiel School of Marine and Atmospheric Science (RSMAS) laboratory using RS232 interfaces. A BASIC program was used to run the titration and record the volume of the added acid and the emf of the electrode system. A typical titration recorded the emf after the readings became stable (0.09 mV) and added enough acid to change the voltage to a preassigned value (13 mV). A full titration (25 points) took about 20 minutes. Using two systems, a 24-bottle cast could be analyzed in 4-5 hours.

The electrode systems used to measure the emf of the sample during a titration consisted of a ROSS glass pH electrode and an Orion double-junction reference electrode. A number of electrodes were screened to select those to be used in the titration systems. Electrodes with non-Nerstian behavior (slopes 1.0 mV different from theoretical) in acidic solutions were discarded. The reliability of a glass-reference electrode pair was determined by titrating 0.7-M NaCl solutions with HCl, by using seawater buffers (Ramette et al. 1977), and by determining the TALK of TCO₂ CRMs provided by Dr. Andrew Dickson of SIO. The titrations of 0.7-M NaCl solutions were used to evaluate the electrode slope in acidic solutions (pH >2 and <4). Seawater buffers (Millero et al. 1993b) were used to evaluate the electrode slope near a pH of 8. The resulting experimental electrode slopes found for the cells used in the present study are given in Table 1. The slopes near a pH of 8 were lower than the theoretical value (59.16 mV), whereas the slopes near a pH of 3 were near the theoretical value. The electrodes were also evaluated by determining the TALK, TCO₂, and pH of TCO₂ CRMs. The results are given in Table 2. The results indicate that precise values of TALK (±1.8 µmol/kg), TCO₂ (±5 µmol/kg), and pH (±0.005) can be obtained on weighed samples of seawater. The precision of the pH measurements for a given electrode (0.003) is better than the average deviation from the mean (0.005).

Table 1. Summary of the calibration results for the cells at 25°C

Table 2. Titrations of certified reference materials (S = 33.82) in the laboratory

^aCalculated from the initial emf using TRIS buffer calibration.
^bFrom spectroscopic measurements.

The HCl acid solutions used throughout the cruise were standardized in the laboratory. The approximately 0.25-M HCl solutions used contained 0.45-M NaCl to yield an ionic strength equivalent to average seawater (0.7 M). Approximately 20 liters of acid were made up in the laboratory. The calibrated acid was stored in 500 cm³ bottles for use at sea. The acid was standardized by titrating weighed amounts of Na₂CO₃ and TRIS dissolved in 0.7-M NaCl solution. The blanks in the 0.7-M NaCl solutions were determined by using coulometery and by titrations of the NaCl solutions with and without added Na₂CO₃ and TRIS. The TCO₂ in the blanks and carbonate solutions was measured daily by a UIC coulometer. The coulometer was calibrated using CO₂ gas loops and CRMs. The blanks of the titrations of TRIS were obtained by extrapolation to zero-added salt. The alkalinity blanks in the NaCl were generally about 14 ± 1 µM. The concentrations of the standard acids obtained from Na₂CO₃ and TRIS were in good agreement (Millero et al. 1993a).

The volumes of the cells used at sea were determined in the laboratory by weighing the cells filled with water. The density of water at the temperature of the measurements (25°C) was calculated from the international equation of state of seawater (Millero and Poisson 1981). The nominal volumes of all the cells was about 230 cm³ and the values were determined to ±0.03 cm³. The reliability of the volumes was assessed by comparing the values of TALK obtained for standard solutions with open and closed cells.

A FORTRAN computer program has been developed to calculate the CO₂ parameters (pH_sws, emf, TALK, TCO₂, and pK₁*) in Na₂CO₃ and seawater solutions. These programs are patterned after those developed by Dickson (1984). This program requires an input of the concentration of the acid, the volume of the cell, the salinity, the temperature of analysis, volume of added HCl (V_HCl), and the corresponding measured values of the emf. To obtain a reliable TALK from a full titration, at least 25 points have to be collected. The precision of the fit is less than 0.4 µmol/kg when pK1* is allowed to vary, and 1.5 µmol/kg when pK1* is fixed. This titration program has been compared with the titration programs used by others (Dickson 1981; C. Goyet, WHOI, personal communication, 1992; Bradshaw and Brewer 1988), and the values of TALK agree to within ±1 µmol/kg. Copies of the titration and calculation programs used are available upon request from Frank J. Millero (fmillero@rsmas.miami.edu).