The Carbon Dioxide System in the Ross Sea during the JGOFS
Southern Ocean Process Study

1. Spectroscopic pH Measurements

The pH measurements of seawater were made using the spectrophotometric techniques of Clayton and Byrne (1993). The pH_T of samples using m-cresol purple (mCP) is determined from

pH_T = pK_ind + log [(R - 0.0069) / (2.222 - 0.133 R)] (1)

where pK_ind is the dissociation constant for the indicator, [H⁺]_T = [H⁺] + [HSO₄^-], and R (A₅₇₈/A₄₃₄) is the ratio of the absorbance of the acidic and basic forms of the indicator corrected for baseline absorbance at 730 nm. The pH_T of the samples is perturbed by the addition of an indicator. The magnitude of this perturbation is a function of the difference between the seawater acidity and indicator acidity; this correction was quantified for each batch of dye solution. To a sample of seawater( ~30 cm³), a normal volume of mCP (0.080 cm³, in this case) was added and the absorbance ratio was measured. From a second addition of mCP and absorbance ratio measurement, the change in absorbance ratio per cm³ of added indicator (R) was calculated. From a series of such measurements over a range of seawater pH, R was described as a linear function of the value of the absorbance ratio (R_m) measured subsequent to the initial addition of the indicator (i.e. R = -0.03540 - 0.1289 R_m). In the course of routine seawater pH_T analyses, this correction was applied to every measured absorbance ratio (R_m); i.e. the corrected absorbance ratio is calculated as

R = R_m - (-0.03450 - 0.1289 R_m) V_ind (2)

where V_ind (0.08 cm3) is the volume of mCP used. Clayton and Byrne (1993) calibrated the m-cresol purple indicator using TRIS buffers (Ramette et al., 1977) and the pH_T equations of Dickson (1993). They found that

pK_ind = 1245.69/T + 3.8275 + (2.11 x 10^-3) (35 - S)(3)

where T is temperature in Kelvin and is valid from 293.15 to 303.15 K and S = 30 to 37. The values of pH_T calculated from equations (1) and (3) are on the total scale in units of moles per kilogram. The total proton scale (Hansson and Jagner, 1973) defines pH_T in terms of the sum of the concentrations of free hydrogen ion, [H⁺], and bisulfate, [HSO4^-]

pH_T = -log[H⁺]_T = -log{[H⁺] + [HSO₄^-] } = -log{[H⁺] (1 + [SO₄^2-] / K_HSO4)}(4)

where the concentration of total sulfate, [SO₄^2-] = 0.0282 ( 35 / S, and K_HSO4 is the dissociation constant for the bisulfate in seawater (Dickson, 1990a).
Lee and Millero (1995) redetermined the value of pK_ind from 273.15 to 313.15 K using a 0.04 m TRIS buffer (Ramette et al., 1977). The pH_T of the TRIS buffer was determined from emf measurements made with the H₂,Pt|AgCl,Ag electrode system (Millero et al., 1993). At 25°C the buffer had a pH_T of 8.0760 and yielded spectrophotometric values of pH that were in excellent agreement (~0.0002) with those found using equations (1) and (3). Their results from 273.15 to 313.15 K (0 to 40°C) for S = 35 were fitted to the equation

pK_ind = 35.913 - 216.404 / T - 10.9913 log (T)(5)

with the standard error of 0.001 in pK_ind where the constants are on the total proton scale (moles per kilogram of H₂O). The use of equations (1) and (5) from 0 to 40°C makes the assumption that R is independent of the temperature. The values of pH_T calculated from equations (1) and (5) are on the total scale in units of mole per kilogram of H₂O. The conversion of the pH_T (mol (kg-H₂O)^-1) to the seawater pH_SWS (mol (kg-soln)^-1) can be made using (Dickson and Riley, 1979; Dickson and Millero, 1987)

pH_SWS = pH_T - log{(1 + [SO₄^2-] / K_HSO4 + [F^-] / K_HF ) /
(1 + [SO₄^-2] / K_HSO4])} - log (1 - 1.005 ( 10^-3 S)(6)

where the total concentration of fluoride, [F^-] = 0.000067 ( 35 / S, and K_HF is the dissociation constant for hydrogen fluoride (Dickson and Riley, 1979). The seawater pH_SWS scale was used in this report since the carbonate constants used are on this scale (Dickson and Millero, 1987; Millero et al., 1993; Millero, 1995). The absorbance measurements were made using a Diode Array 8452A spectrophotometer. The temperature was controlled to a constant temperature of 25°C with an Endocal RTE 8DD refrigerated circulating temperature bath that regulates the temperature to +/-0.01°C. The temperature was measured using a Guildline 9540 digital platinum resistance thermometer.

2. Discrete Coulometric TCO₂ Measurements (SOMMA)

The total inorganic carbon dioxide (TCO₂) in a volume of seawater was determined coulometrically after acidification with H₃PO₄. The system used in this cruise was developed by Johnson et al. (1987) and is called the SOMMA (Single-Operator Multiparameter Metabolic Analyzer). The system is composed of five components: a DICE (Dissolved Inorganic Carbon Extractor) which controls the movement and delivery of acid and sample to the stripper, a coulometer (UIC Inc., model 5011), a CO₂ free N₂ generator (Balston, model 74-5021), a personal computer and a printer. Nitrogen gas from the Balston generator is split into two streams, one for pneumatic control of the sample and acid movements, and the other for the carrier gas for the CO₂ stripped from the seawater sample. The sample is acidified (with 1 to 1.5 cm³ of 8.5 % phosphoric acid) and the carbon dioxide is extracted with N₂ and introduced into the coulometric cell where the CO₂ reacts quantitatively with ethanolamine producing hydroxyethylcarbamic acid. Hydroxyethylcarbamic acid is titrated by electrochemically generated hydroxide ion. The number of electrons utilized in generating the titrant is proportional to the amount of inorganic carbon in the original sample. The life time of the coulometer cell solution is about twelve hours, after which the cell solutions need to be changed. In addition to the cell solutions, the water trap was changed (Gelman, 0.2 um PTFE ACRODISC). Changing the cell solutions requires about 30 minutes. After which, about three hours are needed for the system to stabilize, determine new blank values, and confirm the calibration with analyses of CRM's. If the CRM values were not reproduced to within 2 umol kg^-1 TCO₂, a new cell was prepared. A single measurement takes about 25 minutes, and a 24 bottle station cast can be completed in eight hours. The electrical calibration of the coulometer is not perfectly accurate and the current efficiency of the electrode processes occurring in the coulometer cell has been shown to vary from 100 %. The consistency of the calibration was checked for each cell solution using the Certified Reference Material (Dr. Andrew Dickson, Marine Physical Laboratory, La Jolla, California).

3. Total Alkalinity Measurements, TA

The alkalinity titration systems are similar to the one used in our earlier studies (Millero et al., 1993). The titration systems consisted of a titrator (Metrohm, model 665 Dosimat) and a pH meter (Orion, model 720A) that is controlled by a personal computer. The temperature of both the acid titrant in a water jacketed burette and the seawater sample in a water jacketed cell were controlled to a constant temperature of 25 +/- 0.05°C with a constant temperature bath (Neslab, model RTE 221). The plexiglass water jacketed cells used during the cruise were similar to that used by Bradshaw et al. (1988) except a larger volume (about 200 cm³) was used to increase the precision. Each cell had a fill and drain valve which increased the reproducibility of the volume of sample contained in the cell. A Lab Windows-C program was used to run the titration, record the volume of the added acid and the emf of the electrodes using RS-232 communication interfaces. The seawater samples were titrated by adding HCl to exceed the carbonic acid end point. During a typical titration, the emf readings are recorded after the readings become stable (+/- 0.05 mV), and then a volume of acid is added to change the voltage to a pre-assigned increment (13 mV). In contrast to the delivery of a fixed volume increment of acid, this method gives an even distribution of data points in the range of rapid increase in the emf near the endpoint. A full titration (25 points) takes about 20 minutes. Using two systems, a 24 bottle station cast can be completed in 4 hours. The electrodes used to measure the emf of the sample during a titration consisted of a ROSS glass pH electrode (Orion, model 810100) and a double junction Ag, AgCl reference electrode (Orion, model 900200). A single large 55 gallon batch of ~0.25 m HCl acid was prepared by dilution of concentrated HCl, AR Select( Mallinckrodt. The acid was prepared in 0.45 m sodium chloride (NaCl) to yield a total ionic strength similar to seawater of salinity 35.0 (I ~ 0.7 M). The acid was standardized by a coulometric technique (Taylor and Smith, 1959; Marinenko and Taylor, 1968). Further checks on the acid molality were performed by alkalinity titrations on seawater with known alkalinity, and subsamples of the acid were sent to Andrew Dickson for an independent laboratory determination of the molality. The calibrated molality of the acid used during this cruise was 0.2554 +/- 0.0001 m HCl. The acid was bottled in 500 cm³ glass bottles for use in the field. The volumes of the cells used at sea were determined in the laboratory by several weight titrations using seawater of known total alkalinity. The volumes of the cells used were determined to 0.03 cm³. The volume of HCl delivered to the cell is traditionally assumed to have small uncertainties (Dickson, 1981) and equated to the digital output of the titrator. Calibrations of the burettes of the Dosimats with water at 25C indicate that the systems deliver 3.000 cm³ (the value for a titration of seawater) to a precision of 0.0004 cm³. This uncertainty results in an error of +/- 0.4 umol kg^-1 in TA and TCO₂. The consistency of the measurements was checked for each cast of samples using low nutrient surface seawater and a Certified Reference Material (Dr. Andrew Dickson, Marine Physical Laboratory, La Jolla, California). The total alkalinity of seawater was evaluated from the proton balance at the alkalinity equivalence point, pH_equiv = 4.5, according to the exact definition of total alkalinity (Dickson, 1981)

TA = [HCO₃^-] + 2[CO₃2^-] + [B(OH)₄^-] + [OH^-] + [HPO₄^2-] + 2[PO₄^3-] + [SiO(OH)₃^-]
+ [HS^-] + [NH₃] - [H⁺] - [HSO₄^-] - [HF] - [H₃PO₄](7)

An integrated computer program has been developed which controls the titration data collection and the calculation of the carbonate parameters (pH_sw, E*, TA, TCO₂ and pK₁*) (Millero et al., 1993). The program is patterned after those developed by Dickson (1981), Johansson and Wedborg (1982) and Dickson (DOE, 1994). It is written in C in the National Instruments LabWindows/CVI programming environment. The program uses a Levenberg-Marquardt nonlinear least-squares algorithm to calculate the TA, TCO₂, pK₁ and pH_SWS from the potentiometric titration data. The computer program is based on equation (1) and assumes that nutrients such as phosphate, silicate and ammonia are negligible. Although this assumption is valid only for surface waters, the concentration of nutrients in the seawater sample does not significantly affect the accuracy of TA.