Total Alkalinity Measurements

Section P16S_2005

Dr. Andrew Dickson's group (SIO) was responsible for the TALK measurements during Section P16S_2005. Samples for TALK were collected in glass bottles made from Schott Duran® glass. They were preserved by the addition of 0.02% by volume of a saturated mercury (II) chloride solution (HgCl₂) (DOE 1994 - SOP 01), and analyzed-typically within 24 h-on board ship.

TALK measurements were made using an open-cell, two-stage, potentiometric titration procedure similar to that used to certify reference materials for TALK (see Dickson et al. 2003), except that samples were not weighed into the titration vessel but instead were dispensed using a 120-mL glass syringe. A metal frame attached to the syringe barrel and plunger controlled the maximum extent the plunger could be withdrawn in the barrel. This ensured that a reproducible amount of seawater was dispensed. The analytical procedure was as follows (equipment is listed in Table 3):

1. An aliquot of seawater was dispensed into the titration vessel (a jacketed glass beaker with its temperature controlled to ±0.02 °C at about 20.0 °C), a stirrer bar was added, and the temperature probe and burette tip were inserted in the solution
2. The solution was then acidified to a pH of about 3.6 with a single aliquot of the titration acid and stirred vigorously while CO₂-free air was bubbled through for about 6 min to remove CO₂
3. The main titration was then started and the solution was titrated using 0.05 mL increments to a pH of about 3.0. Data from the pH range 3.5 - 3.0 were used in a non-linear least squares process that corrects for the reactions with sulfate and fluoride ions to estimate the TALK of the sample - see Dickson et al. (2003) for more details

The hydrochloric acid used for the titration was made up in bulk and then stored in 1 L Pyrex bottles with greased ground-glass stoppers. The acid strength was approximately 0.100 mol/kg. The acid was made up in a 0.6 mol/kg sodium chloride background so as to approximate the ionic strength of seawater. Selected bottles of the acid were then analyzed coulometrically (Dickson et al. 2003) to assign a concentration to the batch.

The at-sea repeatability of the method was estimated by analyzing duplicate samples, collected on each cast. These results were used to estimate a standard deviation using the standard expression (DOE 1994, SOP 23). The repeatability was 1.06 µmol/kg based on 89 pairs of analyses.

In addition, analyses were made of the alkalinity of CO₂ reference material. These analyses were carried out regularly throughout the cruise, typically a pair of analyses every 12 h. The results are shown in Figure 2.

The measured average value for the CO₂ reference material was: 2258.02 ± 1.09 (200) µmol/kg, slightly lower than the certified value.

An examination of Figure 2 suggests that there was no significant unambiguous change in the system calibration throughout the course. Therefore, the alkalinity data was adjusted by multiplying by a correction factor of 1.00011, derived by dividing the certified value by the average calculated CRM value: 2258.02 µmol/kg.

Finally, the adjusted alkalinity data results were multiplied by a factor of 1.0002 to correct for the dilution inherent in adding mercury (II) chloride to the sample to preserve it for analysis.

Once the at-sea alkalinity measurements had been adjusted in this fashion, they were normalized to a salinity of 35 and the resulting values plotted in Ocean Data View (ODV) to help identify any questionable data. As a result of this analysis, 15 points were identified as either questionable or bad, and flagged accordingly. (Outliers found in replicate data were thus identified at this stage).

Section P16N_2006

Dr. Frank Millero's group (RSMAS/UM) was responsible for the TALK measurements during Section P16N_2006 Legs 1 and 2.The titration systems used consisted of a Metrohm 665 Dosimat titrator and a computer-controlled Orion 720A pH meter (Millero et al. 1993b). 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 ± 0.1°C with a Neslab (RTE-17) constant temperature bath. The Plexiglas water jacketed cell used is shown in Figure 3. The cells had fill-and-drain valves, which increased the reproducibility of the cell volume.

The TALK system consists of a manifold that allows the automated measurement of six samples in sequence. A set of pumps, valves, and relays are used to rinse, fill, and drain the TALK cell. The titration is controlled programmatically using National Instrument's Labwindows/CVI environment. The titration is made by adding HCl to seawater past the carbonic acid end point. A typical titration records the electromagnetic field (emf) reading after the readings become stable (± 0.05 mV) and adds enough acid to change the voltage to a pre-assigned increment (13 mV). A full titration (25 points) takes about 15 min. Using two automated systems, a 36-bottle station cast can be completed in 6 h.

The electrodes used to measure the emf of the sample during a titration consisted of a ROSS 8101 glass pH electrode and an Orion 90-02 double junction Ag/AgCl reference electrode. The filling solution used in the reference electrode was a 0.7 M NaCl solution to maintain a consistent junction potential between the solution and electrode.

The HCl used throughout the cruise was made, standardized, and stored in 500 mL glass bottles in the laboratory for use at sea. The 0.243402 ± 0.000022 M HCl solutions were made from 1 M Mallinckrodt standard solutions in 0.45 M NaCl to yield an ionic strength equivalent to that of average seawater (~ 0.7 M). The acid was originally tested on seawater of a known TALK to determine the acid's reliability and later sent for final standardization using a coulometric technique (Taylor and Smith, 1959; Marinenko and Taylor, 1968) by Dickson's group.

The volumes of the cells used at sea were determined in the laboratory by making numerous measurements of seawater with a known TALK. Once the TALK values agree to ± 1.0 µmol/kg, the volume of the cell is determined to ± 0.01 mL from the value required to reproduce the TALK. Measurements on CRM samples were made to confirm the volume and reproduce the known TALK to ± 0.5 µmol/kg.

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. Calibration of the burette of the Dosimat with Milli-Q water at 25°C indicates that the system delivers 3.000 mL (the value for a titration of seawater) to a precision of ± 0.0004 mL. This uncertainty results in an error of ± 0.4 µmol/kg in TALK and TCO₂ . Since the titration systems are calibrated using standard solutions, the error in the accuracy of volume delivery will be partially canceled and included in the value of cell volumes assigned. The laboratory precision of the system was ± 1.0 µmol/kg.

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):

TALK = [HCO₃^-] + 2[CO₃^2-] + [B(OH)₄^-] + [OH^-] + [HPO₄^2-] + 2[PO₄^3-] + [SiO(OH)₃^-] - [H⁺] - [HSO₄^-] - [HF] - [H₃PO₄] (7)

At any point of the titration, the total alkalinity of seawater can be calculated from the equation

(V_o TA - VM)/(V_o + V) = [HCO₃^-] + 2[CO₃^2-] + [B(OH)₄^-] + [OH^-] + [HPO₄^2-] + 2[PO₄^3-] + [SiO(OH)₃^-] - [H⁺] - [HSO₄^-] - [HF] - [H₃PO₄] (8)

where V_o is the volume of the cell, M is the molarity of the acid titrant, and V is the volume of acid added. In the calculation, all the volumes are converted to mass using the known densities of the solutions (Millero et al. 1993b).

A computer program has been developed in Labwindows/CVI to calculate the carbonate parameters (pH_sw, E^*, TALK, TCO₂, and pK₁) in seawater solutions. The program is patterned after those developed by Dickson (1981), Johansson and Wedborg (1982), and DOE 1994. The fitting is performed using the STEPIT routine. The STEPIT software package minimizes the sum of squares of residuals by adjusting the parameters E^*, TALK, TCO₂, and pK₁. The computer program is based on equation (8) and assumes that nutrients such as phosphate, silicate, and ammonia are negligible. This assumption is valid only for surface waters. Neglecting the concentration of nutrients in the seawater sample does not affect the accuracy of TALK, but does affect the carbonate alkalinity.

The pH and pK of the acids used in the program are on the seawater scale, [H⁺]_sw = [H⁺] + [HSO₄^-] + [HF] (Dickson 1984). The Mehrbach et al (1973) dissociation constants used in the program were taken from Dickson and Millero (1987) for carbonic acid, from Dickson (1990a) for boric acid, from Dickson and Riley (1979) for HF, from Dickson (1990b) for HSO₄^-, and from Millero (1995) for water. The program requires as input the concentration of acid, volume of the cell, salinity, temperature, measured emf (E) and volume of HCl (VHCl). To obtain a reliable TALK from a full titration, at least 25 data points should be collected (9 data points between pH 3.0 to 4.5). The precision of the fit is better than 0.4 µmol/kg when pK₁ is allowed to vary and 1.5 µmol/kg when pK₁ is fixed. The titration program has been compared to the titration programs used by others (Johansson and Wedborg 1982, Bradshaw and Brewer 1988) and the values of TALK agree to within ± 1 µmol/kg.

The spectrophotometric pH and potentiometric TALK of CRM used during the cruise have been measured in the laboratory before the cruise to characterize the pH of the standard and make sure the titration systems were performing to the desired precision. During the cruise, titrations on CRM were made to ensure that the two titration systems were giving consistent values. The values of pH and TALK for CRM #73 are summarized in Table 4. The precision of the potentiometric measurements of batch #73 were ± 3.0 µmol/kg for TALK and ± 0.006 for pH. For spectrophotometric measurements the average values agreed to ± 0.002 for CRM batch #73 and ± 0.003 for TRIS buffer solution. The deviations are within 2σ for most of the measurements. Small correction factors were made to the TALK to account for the offset with the CRM. To correct the TALK values, a ratio of the CRM value to the measured value, for each system, was taken and multiplied to each of the sample measurements. For pH, the average value was subtracted from the CRM value for each system, and this value was added to each of the sample measurements. These correction factors were made at the end of each station. The TALK values for System A appeared to drift over the course of the cruise; however, the correction factors made accounted for this drift as is evident in the duplicate results.

The precision of the instruments was tested by making duplicate or replicate measurements of samples throughout the cruise. These samples were taken from the same Niskin bottle, equilibrated for an equal amount of time, and then measured on each system for duplicates and the same system for replicates. A total of 62 duplicate samples were made on the titration systems yielding a precision of 0.3 ± 2.3 µmol/kg for TALK and -0.001 ± 0.008 for pH. These results validate the correction factors applied to each system as the deviations between the two systems are within the experimental error of the titrators (± 3.0 µmol/kg). A total of 59 and 74 replicate samples were run on Systems A and B, respectively, and 1051 replicate samples were made on the spectrometer. Results showed that the average replicate difference for TALK were 0.1 ± 1.2 µmol/kg for System A, 0.1 ± 1.0 µmol/kg for System B and, 0.0004 ± 0.0025 for spectrophotometric pH. The Summary of duplicate measurements is presented in Table 5.