Marine Chemistry, 44 (1993) 153-165 153 0304-4203/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

Titration alkalinity of seawater

Frank J. Millero, Jia-Zhong Zhang, Kitack Lee, Douglas M. Campbell Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA

(Received January 29, 1993; revision accepted June 7, 1993)

Abstract

The titration system is described that was used to measure the total alkalinity of seawater (TA) during the Joint Global Ocean Flux Study (JGOFS) sponsored by the National Oceanic and Atmospheric Administration (NOAA) in the equatorial Pacific. It consists of a piston titrator, a pH meter, and a glass thermostated cell. Since the new pH meters and titrators have RS232 interfaces the system can be easily connected to a personal computer. The computer programs used to carry out the titration and to determine TA, pHsw (pH on the seawater scale), and TCO2 from the full titration curve are described. A typical titration takes 20 min and consists of 25 points. Six separate titration cells were calibrated to be used on three systems at sea. The reliability of the electrodes was examined by titrations of 0.7 m NaCI with HC1 at a pH near 3 and using seawater buffers at a pH near 8. Although most electrodes did not have Nernstian behavior over the entire pH range, all gave precise values of TA for a given solution. The individual ceils were calibrated using standard Na2CO3 and seawater standards prepared in our laboratory and Certified Reference Material (CRM) provided by Dickson. The cells gave reliable values of TA, but the values of pHsw were low (0.02) and values of TCO2 were high (20 #mol kg -1) due to the non-Nernstian behavior of the electrodes at a pH near 8.0. If the slope determined from the buffers is used, the titrations yield reliable values of TA, TCO 2 and pHsw. Measurements on Dickson standards with the three ceils at sea indicate that the systems have a reproducibility of ±2-4 #mol kg-1 in TA. The titration values of TCO2 determined on the CRMs and the samples collected at sea were about 17 -4- 6/~mol kg -1 (fall) and 20 4- 6/zmol kg 1 (spring) too high. This offset in TCO2 is independent of depth and is due to the non-Nernstian behavior of the electrodes. The offset is not due to unknown protolytes.

1. Introduction

There is now a great interest in understanding

the carbon cycle on the earth because of the

greenhouse effect o f CO2 on the global climate.

At present it is estimated that about 40% of

anthropogenic CO2 being released to the atmos- phere is being taken up by the oceans (Post et al.,

1990). Studies are being made in an attempt to

detect the changes in the carbonate system in the

oceans as a result of the increases of CO2 in the

atmosphere due to the burning of fossil fuels.

The rate of increase o f total carbon dioxide in the ocean is about 1 # m o l k g -1 yr -1. To detect

these small changes in the CO2 system it is neces-

sary to make reliable and precise measurements

of the parameters of the carbonate system. The

four parameters suitable for shipboard measure-

ment are pH, total carbon dioxide (TCO2), the

partial pressure o f CO2 (pCO2), and the total

alkalinity (TA). The observable parameters are

interrelated by the thermodynamic constants

of the carbonate system in seawater. Earlier

workers usually measured the pH and total

alkalility and calculated the carbonate compo-

nents using the thermodynamic constants of

the carbonate system in seawater. The recent

development o f the coulometric technique

(Johnson et al., 1985, 1987) to determine T C O :

and the infra-red and gas-chromatographic tech-

niques to determine pCO2 has expanded the tools

that can be used to study the carbonate system in the oceans. The results of Transient Tracers in

the Oceans (TTO) Nor th Atlantic expedition

154 F.J. Millero et al./Marine Chemistry 44 (1993) 153-165

showed that the T C O 2 determined from titra- tions was about 20 #mol kg -1 higher than deter- mined by coulometry (Bradshaw and Brewer, 1989). This difference has been attributed to the presence of unknown protolytes in natural sea- water (Bradshaw and Brewer, 1988). There cer- tainly is a need to come up with a uniform protocol for the measurement and calculation of TA and TCO2 from potentiometric titrations before carrying out the world wide surveys of the carbonate system as part of the Joint Global Ocean Flux Study (JGOFS) studies of the car- bon system.

The titration alkalinity of seawater can be determined from a hydrochloric acid titration of seawater. Dyrssen and Sillen (1967) suggested the use of Gran plots to evaluate the titration endpoint. Edmond (1970) was one of the first to use this method and the technique was auto- mated for the measurements of TA and T C O 2

defined by:

TA = [HCO3] + 2[CO3 z-] + [a (on)4]

+ [ O H - ] - [H +] (1)

TCO2 = [H2CO3] + [CO2] + [HCO3] + ICOn-]

(2)

during the GEOSECS (Geochemical Ocean Section Study) program to a precision of +0.1% in TA and +0.2% in T C O 2 (Bradshaw et al., 1981; Bos and Williams, 1982). More recently, it has become common to use computerized high- precision potentiometric titrations of seawater to determine the alkalinity (Hansson and Jagner, 1973; Almgren et al., 1977; Dickson, 1981; Johansson and Wedborg, 1982; Barron et al., 1983; Bradshaw and Brewer, 1988). These studies indicate that it is possible to measure TA and TCO2 to a precision of 0.1%, but sug- gest that problems exist in representing all the side reactions and in accounting for unknown protolytes (Bradshaw and Brewer, 1988). By examining the difference between calculated and observed values for the entire pH range of a titration, Butler (1992) concluded that the

addition of unidentified acid-base pair with pKI~IA of 6.2 in seawater is impossible to be resolved using titration data. An alkalinity stan- dard must be established before accuracies of better than 0.1% are achieved (Barron et al., 1983). Dickson (pers. commun., 1992) has devel- oped reference materials for total dissolved inor- ganic carbon in seawater that may be used to achieve this accuracy in TA.

Although most of the computerized systems are based on a potentiometric titration, it is also possible to follow the titration and deter- mine the endpoint using spectrophotometric (Graneli and Anfalt, 1977; King and Kester, 1989) and thermometric techniques (Millero et al., 1974). Since the spectrophotometric and thermometric response times are equal to the mixing times, the titrations can be performed faster. Medium effects are also less important. Over the years, less complicated methods have been used to determine the titration alkalinity (Culberson et al., 1970; Keir et al., 1977; Perez and Fraga, 1987; Breland and Byme, 1992, 1993) generally with less precision. These methods are essentially a one or two point titration past the carbonic acid endpoint. These methods measure the pH after acidification and can be used with either the spectrophotometric or potentio- metric determination of the pH of seawater. It has a precision of about +4 #mol kg -1 and may be useful in making quick titration alkalinity measurements in situ (Breland and Byrne, 1993).

In the present paper, we describe the titration systems constructed to make high-precision measurements of total alkalinity and total CO2 from potentiometric titrations. We also discuss the calibration of the titrating systems and their use during the Joint Global Ocean Flux Study (JGOFS) cruises sponsored by National Oceanic and Atmospheric Administration (NOAA) in the equatorial Pacific Ocean (spring and fall, 1992). The results of using CRMs to monitor the performance of potentiometric titration systems during the cruises will also be presented in this paper.

F.J. Millero et al./Marine Chemistry 44 (1993) 153-165 155

2. Experimental

2.1 Titration system

The titration system is similar to the one used in our earlier studies (Thurmond and Millero, 1982) and that developed by Bradshaw and Brewer (1988). The titration systems used to determined TA consisted of a Metrohm 665 Dosimat titrator and an Orion 720ApH meter that is operated by a personal computer. 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°C with a Neslab constant temperature bath. The glass water jacketed cells (volume about 230cm 3) used during the spring cruise were patterned after our earlier design (Thurmond and Millero, 1982). The cell top was made of Plexiglas with inlets for electrodes, syringe and titrant injection tip. The cell top can be removed between runs to clean and fill the cell with sample. It is closed by pushing the top all the way down to the top edge of the cell. An O- ring on the cell top was used to secure a fixed volume. For the fall cruise we used a Plexiglas cell. The cell design was similar to that used by Bradshaw and Brewer (1988) except a larger volume (about 200cm 3) was used to increase the precision. This cell had a fill and drain valve and increased the reproducibility of the cell volume. A GW-BAsIC program was used to run the titration, record the volume of the added acid and the emf of the electrodes using RS232 interfaces. The titration is made by adding HC1 to the seawater past the carbonic acid end point. A typical titration records the emf reading after the readings become stable (4-0.05 mV) and adds enough acid to change the voltage to a pre- assigned increment (10mV). In contrast to the delivery of a fixed volume increment of acid, this method gives data points in the range of a rapid increase in the emf near the endpoint. A full titration (25 points) takes about 20min. Using three systems a 24 bottle station cast can be completed in 3.5 h.

2.2 Electrodes

The electrodes used to measure the emf of the sample during a titration consisted of a ROSS glass pH electrode and an Orion double junc- tion Ag, AgC1 reference electrode. A number of electrodes were screened to select those to be used in the titrators. Electrodes with grossly non-Nernstian behavior (slopes + l . 0mV differ- ent from theoretical) were discarded. A total of 20 electrode pairs were examined before the cruise and six electrodes were selected for use at sea.

The reliability of a glass-reference electrode pair was determined by HC1 titrations in 0.7 M NaC1 solutions, by using seawater buffers (Dickson, 1993) and by determining the TA of standard Na2CO3 solutions and Gulf Stream seawaters. The HC1 titrations in 0.7M NaC1 solutions were used to evaluate the experimental slope in acidic solutions (pH 2 to 4). Artificial seawater buffers (Millero et al., 1993-this issue) were used to evaluate the experimental slope near a pH of 8. The resulting experimental slopes found for the electrodes used in the pre- sent study are given in Table 1. The slopes near a pH of 8 were lower than the theoretical value (59.16mV) while the slopes near a pH of 3 were higher than the theoretical value. The elec- trodes were also evaluated by determining the TA, TCO2 and pHsw of Gulf Stream seawater to determine the reliability of the electrodes in seawater. The results (Table 2) indicate

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

Cell Volume [Buffer] [Acid] (cm 3 )

Slope Dev Slope Dev

1 212.59 58.4 -0 .8 59.0 -0 .2 2 238.18 58.6 -0 .6 59.5 +0.3 3 239.64 59.2 0.0 59.8 +0.6 4 235.04 58.8 -0 .4 59.8 +0.6 5 240.43 58.3 -0 .9 59.5 +0.3 6 218.50 57.9 -1 .3 59.6 +0.3

156

Table 2 Comparisons of open cell titration results obtained from weighed Gulf Stream seawater using various electrodes

Electrode TA TCO2 pHsw ~mol .kg- l ) (#mobkg-t)

R2 2378.1 2055.5 8.042 R5 2375.3 2055.4 8.041 R6 2381.2 2062.3 8.023 R7 2376.9 2054.4 8.045 R15 2382.2 2060.3 8.053 R 16 2375.0 2051.7 8.044 R18 2376.5 2055.1 8.042 R19 2379.5 2055.6 8.049 R20 2376.3 2058.3 8.042 R21 2380.6 2058.2 8.049 R22 2382.6 2059.6 8.043 C#2 2376.6 2053.7 8.036 C#3 2377.3 2061.1 8.034 C#4 2382.2 2060.0 8.053 C#8 2374.0 2051.7 8.044

Average 2378.3 4- 2.5 2056.9 4- 2.8 8.043 4- 0.005 Standard a 2036 8.063

a Standard value of TCO2 determined by coulometry and of pHsw by spectrometry.

that high-precision measurements of TA (+2.5 #mol kg-~), TCO 2 (+2.8/~mol kg -1) and pH (+0.005) can be obtained with weighed samples of seawater in an open cell. The vari- ations in the experimental slopes for the various electrodes had little or no effect on the values of TA, TCO 2 and pHsw. The precision of the pH measurements for a given electrode (0.003) is better than the average deviation from the mean. As will be discussed later, the accuracies of the values of TCO 2 and pH are affected by the reliability of the electrodes.

2.3 Standard acids

The HC1 acid solutions (~ 201) used through- out the cruise were made, standardized, and stored in 500 cm 3 glass bottles in the laboratory for use at sea. The 0.25 M HC1 solutions were made with 1 M Mallinckhodt standard solu- tions in 0.45 M NaCI to yield an ionic strength equivalent to that of average seawater (.~ 0.7 M).

F.J. Millero et al./Marine Chemistry 44 (1993) 153-165

Table 3 Calibration of the acids used during the JGOFS Cruise

Acid Na2CO 3 TRIS Coulometry

l 0.2424 4- 0.0001 0.2424 4- 0.0004 0.2422 4- 0.0003 2 0.2466 4- 0.0002 0.2462 4- 0.0006 0.2466 4- 0.0003

The acid was standardized by titrating weighed amounts of Na2CO3 and TRIS dissolved in 0.7 M NaC1 solutions. The blanks in the 0.7 M NaC1 solutions were determined by using coulometry and by titrations of the NaC1 solutions with and without added Na2CO3 and TRIS. The blanks of the titrations of TRIS were determined by extra- polation to zero added salt (Goyet and Hacker, 1992). The alkalinity blanks in the NaC1 were generally about ( 1 4 + l m M ) . The concen- tration of the standard acids obtained from Na2CO3 and TRIS were in good agreement (Table 3). Recently we have determined the concentration of HC1 by using a coulometric technique. The system we used is similar to the earlier design of Taylor and coworkers (Taylor and Smith, 1959; Marinenko and Taylor, 1968) and constructed by Dickson (pers. commun., 1992). The protons are titrated coulometrically and released as H2(g) on the Pt anode. The current is generated from a Sargent constant current supply and recorded as a function of time by measuring the voltage across a standard one ohm resistor with a Keithley 617 electrometer. An ORtON combination elec- trode was used to measure the pH of the solution during the titration. The endpoint is determined from the plot of the concen- tration of H ÷ and O H - in the solution as a function of the coulombs delivered (Fig. 1). The precision of the coulometric titration is very good (0.01%) and the results agree with titrations made in NaECO 3 and TRIS solutions (see Table 3).

2.4 Volume of the cells

The volumes of the cells used at sea were deter- mined in the laboratory by weighing the cells

F.J. Millero et al./Marine Chemistry 44 (1993) 153-165 157

Z

< >,

0 " r..r.1 0 n~

1300

1200 [oH ]

1100

1000

900 [H +]

8OO

700 t i 0 . 0 0 0 0 0 .0004 0 .0008

CONCENTRAT[0N

0 . 0 0 1 2

Fig. 1. The determination of the total equivalents of the HC1 using a coulometric titration.

filled with degassed Millipore 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 Pois- son, 1981). The nominal volumes of all the cells were about 230cm 3 and the values were determined to 4-0.03 cm 3. The reliability of the volumes was assessed by comparing the values of TA obtained for standard solutions with open (weighed sample) and closed cells. If the volume is correct the TA from the open and closed cells should be the same (provided the same acid, titrator, and electrodes are used). The open cell titration was found to affect the TCO2 derived from the titration, due to the loss of CO2, but not the TA. If an accurate weight of the cell cannot be made, the open and closed cell titration offers a precise way of determining the volume of the cell. The volumes of the plastic cells determined in this manner were more reproducible than the glass cells. The valves in the plastic cells close off a more reproducible volume than the O-ring seal used on the glass cells. For this reason we replaced the glass ceils with the plastic cells for the fall cruise. If the cells were modified during the cruise, adjustments were made to

the volumes using the daily titrations on low- nutrient surface seawater (collected before the first station and poisoned with HgC12) and the CRM.

2.5 Solutions

The NaC1, Na2CO 3 and NaHCO3 salts used to make up the solutions were Baker reagent grade. The details on the preparation and calibration of the seawater buffers (Dickson, 1993) are given elsewhere (Millero et al., 1993-this issue). Approximately 201 of standard carbonate solu- tions in 0.7 M NaC1 were prepared for the cali- brations of the acids. The solutions were equilibrated with air to provide an alkalinity and nearly constant TCO2 standard. The TCO2 in the blanks and carbonate solutions was measured daily by coulometry (UIC Inc.). The coulometer was calibrated using CO2 gas loops and monitored with the CRM. The Gulf Stream seawater used in the laboratory was collected off the coast of Miami. The seawater was filtered through a 0.45 mm Millipore filter and poisoned with HgCI2 (250 #1 of saturated HgC12 solution per liter of seawater). The salinity was deter-

158 F.J. Millero et al./Marine Chemistry 44 (1993) 153-165

mined with a Guildline salinometer on the prac- tical salinity scale. The densities of all the solu- tions were determined with a Mettler densimeter. The densimeter was calibrated with nitrogen, water and standard seawater (Millero and Pois- son, 1981).

2.6 Volume of titrant

The volume of HCI delivered to the cell is traditionally assumed to have small uncertain- ties (Dickson, 1981) and equated to the digital output of the titrator. Calibrations of the burettes of the Dosimats with water at 25°C indicate that the systems deliver 3.000 cm 3 (the value for a titration of seawater) to a precision of +0.0004cm 3. This uncertainty results in an error of +0 .4#molkg -1 in TA and TCO2. The accuracy of volume of acid delivered by the Dosimats, however, is ten times lower than that of precision. Since the titration systems are cali- brated using standard solutions, this error in the acuracy of volume delivery will be partially can- celled and included in the value assigned to the concentration of HC1 and the volume of the cell. When TA CRM becomes available the calibra- tion of the burettes must be incorporated into the calibration of the system.

3. Evaluat ion o f the carbonate parameters

The total alkalinity of seawater was evaluated from the proton balance at the alkalinity equiva- lence point, pHequiv = 4.5, according to the exact definition of total alkalinity (Dickson, 1981):

TA = [HCO~-] + 2[CO 2-] + [B(OH)~-]

+ [OH-] + [HPO4 2-] + 2[PO 3-]

+ [SiO(OH)3] + [HS-] + [NH3]

- [H +] - [HSO4] - [ H F ] - [H3PO4] (3)

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

equation:

(V0.TA - V . N ) / ( V o + 1I)

= [HCO3] + 2[CO 2-] + [B(OH)4]

+ [OH-] + [HPO 2-] + 2[PO]-]

+ [SiO(OH)3] + [HS-] + [NH3]

- [H +] - [ H S O 4 ] - [HF] -[H3PO4] (4)

where V0 is the initial volume of the cell or the sample to be titrated; N is the normality of 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.

FORTRAN computer programs have been developed to calculate the carbonate parameters (pHsw, E*, TA, TCO2 and pK]') in Na2CO3, TRIS, and seawater solutions. The program is patterned after those developed by Dickson (1981; Dickson and Goyet, 1991) and Johansson and Wedborg (1982). The fitting is performed using a routine called S'rEvrr (J.P. Chandler, Oklahoma State University, Still- water, OK 74074). The Sa'Evrr software package minimizes the sum of squares of residuals by adjusting the parameters E*, TA, TCO2 and pKf using a non-linear least-squares procedure. The computer program is based on Eq. (4) a n d assumes that the 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 TA, 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 dis- sociation constants used in the program were taken from Dickson and Millero (1987) for car- bonic acid, from Dickson (1990a) for boric acid, from Dickson and Riley (1979) for HF, from Dickson (1990b) for HSO4 and from Millero (unpubl. data) for water. The program requires an input of the concentration of the acid, the volume of the cell, the salinity, the temperature,

F.J. Millero et al./Marine Chemistry 44 (1993) 153-165 159

Table 4

Calibrations of the cells with Na2CO 3 solutions before the cruise (A = M e a s u r e d - Average)

Cell Na2CO3 NaHCO3

ATA ATCO2 ATA ATCO 2 (#mol.kg - l ) (#mol.kg -1) (#mol.kg - j ) (~mol.kg -I)

1 1 11 1 2 - 2 - 1 3 1 - 1 0 - 3 4 3 - 5 4 5 0 - 5 - 1 6 - 2 9

Average 2331 2251 1969 a 2 9 3 Standard a 2246

3 - 5 - 3

1940 4

1926

a Standard value of TCO 2 determined by coulometry.

the measured emf (E) and volume of HC1 (VHcl). To obtain a reliable TA from a full titration at least 25 data points were collected (9 data points between pH 3.0 to 4.5). The precision of the fit is less than 0.4#molkg -1 when pK~' is allowed to vary and 1.5 #mol kg -1 when pK~ is fixed. Our titration program has been compared to the titration programs used by others (Bradshaw et al., 1981; Johannsson and Wedborg, 1982; Bradshaw and Brewer, 1988; Dickson, 1991) and the values of TA agree to within +1 #mol kg -1 . Copies of the titration and calcu- lation programs used are available upon request.

Table 5

Calibrations of the cells with Gulf Stream seawater and Dickson N a 2 C O 3 Standards ( A = M e a s u r e d - Average)

Cell Batch #7 Gulf Stream SW (S = 36.52)

NaCI+ Na2CO 3 ATA ATCO 2 ATA ATCO 2 (#mol.kg -1) (/zmol.kg -1) (/zmol.kg -I) (#mol.kg -1)

4. Results and discussion

To determine the reliability of the titrators a number of measurements were made on N a 2 C O 3

and seawater solutions before and after the cruise. The measurements were made in closed cells and with weighed seawater samples in open cells. These measurements were used to make sure that the assigned volumes of the cell were correct. It also allowed us to check the precision and internal consistency of the various cells. The results of the titrations of NaHCO3, Na2CO3, and Gulf Stream seawater are given in Tables 4 and 5. The values of the TA for solutions are in good agreement ( + 2 - 3 # m o l k g -1) for all the cells independent of the solution being titrated. The titration values of the TCO2 showed more scatter with average deviations of -4-9- 16#molkg -1. The titration values of TCO2 were larger than the values assigned or measured by coulometry.

Before and after the cruise a number of titra- tions were made in the laboratory on TCO2 CRM (Batch #10 and 12). The laboratory titra- tion results for TA, TCO2, and pHsw are given in Table 6 along with the assigned value of TCO2 and the pH measured spectrophotometrically (Millero et al., 1993-this issue). The precisions in the values of TA (~:2-3#molkg-1), TCO2 ( + 5 - 6 # m o l kg -1) and pHsw (+0.02) were quite good and similar to our titrations of Gulf Stream seawater (Table 2). The titration values of TCO2 were 22-24#molkg -1 higher than the assigned values (determined manometrically) and

Table 6

Titrations of the CRM for ocean TCO 2 measurement in the laboratory a

1 -5 3 0 18 2 5 14 0 - 8 CRM 3 3 - 1 2 - I - 1 8 4 - 3 -11 2 - 5 5 - 2 - 7 - 4 - 9 Batch #10 6 1 10 2 20 Standard

Average 1980 1939 2370 2100 Batch #12 a 4 11 2 16 Standard Standard a 1925 2078

a Standard value of TCO2 determined by coulometry.

Salinity TA TCO 2 pHsw (#mobkg-i) (/zmol-kg-I)

34.57 2264.4 ~ 2.9 1983 ± 6 8.00 ~ 0.02 28 1961 8.034

33.82 2233.4± 1.8 2007± 5 7.91 ±0 .02 31 1984 7.949

a These titration were made on weighed samples with three different acids and four different electrodes.

160 F.J. Millero et al./Marine Chemistry 44 (1993) 153-165

DELTA

(/zM)

60

40

20

0

- 2 0

- 4 0

- 6 0

I I I

......... VARIABLE K 1

FIXED K l

I I I I I

5 7 . 6 5 8 . 4 5 9 . 2 60 .0 6 0 . 8

SLOPE (MY)

Fig. 2. Errors in TA and TCO 2 determined f rom the t i trat ion o f the C R M for ocean TCO2 measurements (Batch #12) in Cell 1 at 25°C when the electrode slope is varied.

measured by coulometry in our laboratory. The titration values of pH were 0.02 lower than the values measured by spectrophotometric methods (Millero et al., 1993-this issue). The differences in pH and TCO2 are caused by the non-Nernstian behavior of the electrodes near a pH of 8. Calibration of the electrodes using TRIS seawater buffers yield

4

DELTA

OzM) 3

6 i i i i i

5 .... FIXED K, / /

- - VARIABLE K I

2 \ . . , / "

k /

0 I I i I I 5 7 . 6 5 8 . 4 5 9 . 2 6 0 . 0 6 0 . 8

SLOPE (MV)

Fig. 3. Standard errors in TA determined from the t i trat ion o f the C R M for ocean TCO2 measurement (Batch #12) in Cell 1 at 25°C when the electrode slope is varied.

pHsw = 8.034 + 0.003 and 7.948 ± 0.002, respec- tively, for Batch #10 and 12. These results are in excellent agreement with the spectrophoto- metrically determined pHsw and show a lower standard error than the values determined from the titrations.

The program used to calculate pH, TA and TCO2 assumes that the electrodes respond to a change in pH with a Nernstian slope (59.16mV at 25°C). Since the evaluation of the electrodes using buffers and titrating with HC1 frequently gave non-Nernstian behavior, it is of interest to examine the effect that variations in the slope have on the evaluated parameters. We have thus examined the parameters produced by both programs when the electrode slope is varied. This was done by examining the effect of adjusting the slope of the electrodes for a number of titrations on CRMs that have known values of TCO2 and pH determined independently. The evaluation of the carbonate parameters was carried out with programs that evaluate three adjustable parameters (TA, TCO2 and E*) with a fixed pK~ at the literature value (Dickson and Millero, 1987) or four adjustable parameters including pKg. The titration results for the CRM for ocean TCO2 measurement batch #12 are shown in Figs. 2 and 3. The results in Fig. 2 show that the deviations due to changes in the electrode slope are much greater for TCO2 than for TA. Errors of 1.0 mV in the slope yield differences in TA and TCO2, respectively, of 2.1 and 22.8#molkg -1 when pK~ is a adjustable parameter evaluated from the least-square calcu- lation and differences in TA and TCO2, respec- tively, of 10 and 31.6#molkg -1 when pK~ is fixed at a literature value (Dickson and Millero, 1987). These results point out the advantage of using a program with pK~ as an adjustable para- meter (Almgren et al., 1977; Dickson, 1981; Barron et al., 1983). The values of TA deter- mined with these programs are not strongly affected by the behavior of the electrodes. The standard deviations in the TA from the fits (Fig. 3) also support this finding. The standard devia- tions are lowest near the theoretical Nernstian

F.J. Miller• et al./Marine Chemistry 44 (1993) 153-165 161

m:

E-~ .,d

- 4

- 8

SPRING NOAA/JGOFS CRUISE 92 CERTIFIED REFERENCE MATERIAL BATCH #I0

I I I I i I I I

[] o (a) • • v 0

• •

• v O 0 • ( ~ • • v O 0 0

I • • • • [] <0

I • • ~ • 3 V Iv • • D •

v ~7 ~7 •

• ~ v v •

v ~7 ~ ~7 ~7

v • , l , l I I , I , I , I I

10 20 30 40 50 60 70 80

T I M E ( d a y s )

CELL 2 0 CELL 3 • CELL 4 w CELL 5 • CELL 8 [3

b~

.< e- <I

B

0

4

0

- 4

-B

FALL NOAA/JGOFS CRUISE 92 CERTIFIED REFERENCE MATERIAL BATCH #12

I I I I I

O 0 • 0 o n

I

(b)

% • (~mmml • m~o • • • o • , .Oo_ o o • o m .o o o•

0 I ~ I ~ • I 0 ~ 0

• ~ O 0 0

0

I , I I , I , I

20 30 40 50 60

TIME(days)

0 lib 0

II •

0 0

] L

I0

CELL 2 0

CELL 3 •

CELL 4 •

Fig. 4. Results of total alkalinity titrations on the CRM for ocean TCO2 measurement [Batch # 10 (a) and #12 (b)] made during the spring (upper panels) and fall cruises (lower panels) of the NOAA/JGOFS Equatorial Pacific study.

slope (59.16 mV) when pKf is allowed to vary and near the slope determined with buffers (58.4 mV) when the pK~ is fixed at a literature value (5.852). The values of pHsw (7.949) and TCO2 (1984#molkg -I) determined using the buffer slope (58.4mV) are also closer to the values determined, respectively, by spectro- scopy and coulometry. These calculations indi- cate that the offset in the TCO2 derived from titrations is due to deviations in the slope of

electrode from Nernstian behavior not due to unknown protolytes (Bradshaw and Brewer, 1988). If the slope determined from the buffers is used, the titrations yield reliable values of pHsw, TA and TCO2. It is also possible to deter- mine the slope of a given electrode system by adjusting its value to give the lowest standard error in TA when pKf is fixed. When seawater standards for TA as well as for TCO2 become available, one should be able to determine the

162

appropriate electrode slope needed to give the most reliable parameters from a titration.

During the spring and fall NOAA/JGOFS cruises we titrated a number of samples of CRMs used for TCO2 measurement (batch #10 and 12) to monitor the precision of the potentiometric titration systems and the cells used at sea. The TA results from batch #10 and 12 using various cells are shown in Fig. 4 and summarized in Tables 7 and 8. The average values of TA = 2264 + 4 #mol kg- 1 and 2233 4- 3 #molkg -1, respectively, for batches #10 and #12 are in good agreement with the values determined in the laboratory. The aver- age deviations (2.7#molkg -1) with the plastic cells in the fall cruise are lower than found (4.4#molkg -1) in the spring using glass cells. As discussed earlier the plastic cells give a more reproducible volume than the glass cells and yield more precise values of TA. The offset of TCO2 at sea ( 2 0 + 5 # m o l k g -l) is similar to that found in the laboratory. Since this offset is similar to the differences obtained in the titration and coulometric values of TCO2 on seawater samples collected during the cruise and is inde- pendent of depth (see Fig. 5), we feel that the cause is due to the non-Nernstian behavior of the electrodes and not related to the presence of unknown protolytes in the seawater samples (Bradshaw and Brewer, 1988). Our results demonstrate that the CRM for ocean TCO2 measurement prepared by Dickson can also serve as a reliable TA standard for measure- ments at sea. Hopefully in the future Dickson will be able to provide certified values for the TA of these TCO2 standards that can be used to calibrate titrators and evaluate their performance at sea.

5. Conclusions

This study indicates that the offset in the TCO2 derived from potentiometric titration relative to direct measurement is due to deviations in the slope of electrode from Nernstian behavior, not due to unknown protolytes (Bradshaw and

F.J. Millero et al./Marine Chemistry 44 (1993) 153-165

Table 7 Titrations of the C R M for ocean TCO2 measurement at sea (Batch #10) using glass cells during the spring cruise

Cell Leg TA TCO2 N (/~mol.kg - l ) (#mol .kg -I)

3 1 2263 4- 3.7 1978 4- 5.2 23 2 2264 4- 5.8 1983 4- 3.4 9 3 2264 4- 5.0 1977 4- 3.0 13

4 1 2264 4- 3.7 1978 4- 4.3 23 2 2264 4- 5.8 1983 4- 6.4 9 3 2257 -4- 5.0 1977 4- 6.3 13

5 1 2264 4- 5.0 1978 4- 4.9 23 2 2265 4- 6.0 1986 4- 9.0 9 3 2269 4- 2.4 a 1983 4- 3.4 13

8 1 2 2264 4- 3.1 2043 4- 29 9 3 2264 1985 13

Average 2263.5 4- 4.4 1981 4- 4.8 b 113 Standard 2264.4 c 1961

a Electrodes from broken Cell 2 were put into Cell 5. b Excluding the results from Cell 8, new electrode placed in Cell 8 for leg 3. e Result obtained in the laboratory.

Brewer, 1988). Reliable values of TA, TCO2 and pHsw can be obtained from a potentio- metric titration using the experimental slope in the calculation program. Three different

Table 8 Titrations of the C R M for ocean TCO 2 measurement at sea (Batch #12) using plastic cells during the fall cruise

Cell Leg TA TCO 2 N (#mobkg- I ) (#mobkg- I )

2 4 2233 4- 5.4 2007 4- 3.8 5 2232 4- 2.2 2000 4- 2.6 6 2233 4- 5.0 2000 4- 2.6

3 4 5 2233 + 2.9 2002 -4- 3.4 17 6 2232 4- 2.5 2003 4- 2.7 5

4 4 2232 4- 3.3 2006 -4- 3.9 22 5 2233 4- 2.9 2000 + 3.1 20 6 22334-1.4 20014-3.3 15

2233 -4- 2.7 2003 + 3.5 114 2233.4 a 1984

Average Standard

13 19 19

a Results obtained in the laboratory.

F.J. Millero et al./Marine Chemistry 44 (1993) 153-165 163

DEPTH (M)

200

400

600

800

1000

0 lo¢ o o / Lo o qbd?,~ °o I0 oO@ o

o 0 @ ~ o

o o ( ~ o o o o o o

o oO cxD I~111~ ~m~o@ o

I I

10 20

~ @ ) 0 ( 2 0 0

1200 0 30

A TCOz (~zrnol kg -1)

Fig. 5. Offset of TCO2 from potentiometric titration relative to coulometric technique on the seawater samples collected during the cruises of the NOAA/JGOFS Equatorial Pacific study.

approaches can be used to determine the experi- mental slope of glass/reference electrode pair. First, one can measure the emf of the electrodes in TRIS, BIS, and 2-AMINOPYRIDINE seawater buffers (Millero et al., 1993-this issue) near a pH of 8 to calculate the experimental slope. Second, one can adjust the value of the slope in the calculation program to give the lowest stan- dard errors in TA (using only three adjustable parameters, TA, TCO2 and E*). Third, one can adjust the slope to give the correct independently determined values of pH (spectrophotometry) and TCO2 (coulometry). The three different approaches should give similar results if the calibrations of the acid and sample titrations use the same program. Although the TCO2 determined from potentiometric titrations is less accurate than direct measurements (using manomatery or coulometry), it offers an alternate inexpensive method provided the electrodes are properly calibrated.

The results of potentiometric titration on the CRM for ocean TCO2, prepared by Dickson, analysis (batch #10 and #12) during the spring and fall NOAA/JGOFS cruises indicate that

they can also serve as a reliable TA reference material to monitor the performance of titration system at sea. There is a need to provide certified values of TA for the TCO2 CRMs so that they can be used to calibrate titrators and evaluate their performance at sea.

Acknowledgements

The authors wish to acknowledge the support of the National Oceanic and Atmospheric Administration, the Oceanographic section of the National Science Foundation and the Office of Naval Research for supporting this study.

References

Almgren, T.D., Dyrssen, D. and Strandberg, M., 1977. Computerized high-precision titrations of some major constituents on board the R.V. Dmitry Mendeleev. Deep-Sea Res., 24: 345-364.

Barron, J.L., Dyrssen, D., Jones, E.P. and Wedborg, M., 1983. A comparison of computer methods for seawater alkalinity titrations. Deep-Sea Res., 30: 441-448.

Bos, D. and Willians, R.T., 1982. History and development

164 F.J. Millero et al./Marine Chemistry 44 (1993) 153-165

of the GEOSECS alkalinity titration system. In: H.G. Ostlund and D. Dyrssen (Editors), Workshop on Oceanic CO2 Standardization. US DOE Rep., CONF- 7911173, pp. 42 59.

Bradshaw, A.L. and Brewer, P.G., 1988. High precision measurements of alkalinity and total carbon dioxide in seawater by potentiometric titration--1. Presence of unknown protolyte(s)? Mar. Chem., 23: 69-86.

Bradshaw, A.L. and Brewer, P.G., 1989. High precision measurements of alkalinity and total carbon dioxide in seawater by potentiometric titration--2. Measurements on standard solutions. Mar. Chem., 24: 155-162.

Bradshaw, A.L., Brewer, P.G., Sharer, D.K. and Williams, R.T., 1981. Measurements of total carbon dioxide and alkalinity by potentiometric titration in the GEOSECS program. Earth Planet. Sci. Lett., 55:99-115.

Breland, J.A. and Byrne, R.H., 1992. Determination of the sea water alkalinity by direct equilibration with carbon dioxide, Anal. Chem., 64: 2306-2309.

Breland, J.A. and Byrne, R.H., 1993. Spectrophotometric determination of the total alkalinity of sea water using bromocresol green. Deep-Sea Res., 40: 629-641.

Butler, J.N., 1992. Alkalinity titration in seawater: how accurately can the data be fitted by an equilibrium model? Mar. Chem., 38: 251-282.

Culberson, C.H., Pytkowicz, R.M. and Hawley, J.E., 1970. Seawater alkalinity determination by the pH method. J. Mar. Res., 28:15 21.

Dickson, A.G., 1981. An exact definition of total alkalinity and a procedure for the estimation of alkalinity and total C02 from titration data. Deep-Sea Res., 28: 609- 623.

Dickson, A.G., 1984. pH scales and proton-transfer reactions in saline media such as seawater. Geochim. Cosmochim. Acta, 48:2299 2308.

Dickson, A.G., 1990a. Thermodynamics of the dissociation of boric acid in synthetic seawater from 273.15 to 318.15 K. Deep-Sea Res., 37: 755-766.

Dickson, A.G., 1990b. Standard potential of the (AgC1 + 1 /2 / /2 = Ag + HCl(aq)) cell and the dissocia- tion of bisulfate ion in synthetic sea water from 273.15 to 318.15 K. J. Chem. Thermodyn., 22: 113-127.

Dickson, A.G., 1993. pH buffers for seawater media based on the total hydrogen ion concentration scale. Deep-Sea Res., 40: 107-118.

Dickson, A.G. and Goyet, C. (Editors), 1991. DOE Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water, version 1.0, 101 pp.

Dickson, A.G. and Millero, F.J., 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Res., 34:1733 1743.

Dickson, A.G. and Riley, J.P., 1979. The estimation of acid dissociation constants in sea water media from potentio- metric titrations with strong base I. The ionic product of water --- Kw. Mar. Chem., 7: 89-99.

Dyrssen, D. and Sillen, L.G., 1967. Alkalinity and total carbonate in seawater; a plea for P-T-independent data. Tellus, 19: 113-121.

Edmond, J.M., 1970. High precision determination of titra- tion alkalinity and total carbon dioxide content of sea- water by potentiometric titration. Deep-Sea Res., 17: 737-750.

Goyet, C. and Hacker, S.D., 1992. Procedure for calibration of a coulometric system used for total inorganic carbon measurements in seawater. Mar. Chem., 38: 37-51.

Graneli, A. and Anfalt, T., 1977. A simple automatic photo- titrator for the determination of total carbonate and total alkalinity of sea water. Anal. Chim. Acta, 91: 175-180.

Hansson, I. and Jagner, D., 1973. Evaluation of the accu- racy of Gran plots by means of computer calculations: application to the potentiometric titration of the total alkalinity and carbonate content in sea water. Anal. Chim. Acta, 75: 363-373.

Johansson, O. and Wedborg, M., 1982. On the evaluation of potentiometric titrations of seawater with hydrochloric acid. Oceanol. Acta, 5: 209-218.

Johnson, K.M., King, A.E. and Sieburth, J.M., 1985. Cou- lometric TC02 analyses for marine studies; an intro- duction. Mar. Chem., 16: 61-82.

Johnson, K.M., Sieburth, J.M., Williams, P.J.L. and Brandstrom, L., 1987. Coulometric total carbon dioxide analyses for marine studies; automation and calibration. Mar. Chem., 21: 117-133.

Keir, R.S. Kounaves, S.P. and Zirino, A., 1977. Rapid determination of the "titration alkalinity" of sea water by equilibration with CO2. Anal. Chim. Acta, 9: 181- 187.

King, D.W. and Kester, D.R., 1989. Determination of sea- water pH from 1.5 to 8.5 using colorimetric indicators. Mar. Chem., 26:5 20.

Marinenko, G. and Taylor, J.K., 1968. Electrochemical equivalents of benzoic and oxalic acid. Anal. Chem., 40:1645 165l.

Millero, F.J. and Poisson, A., 1981. International one- atmosphere equation of state of seawater. Deep-Sea Res., 28: 625-629.

Millero, F.J., Schrager, S.R. and Hansen, L.D., 1974. Thermometric titration analysis of seawater for chlorinity, sulfate and alkalinity. Limnol. Oceanogr., 19:711 715.

Millero, F.J., Zhang, J.-Z, Fiol, S., Sotolongo, S., Roy, R., Lee, K. and Mane, S., 1993. The use of buffers to measure the pH of seawater. Mar. Chem., 44: 143-152, this issue.

Perez, F.F. and Fraga, F., 1987. A precise and rapid analytical procedure for alkalinity determination. Mar. Chem., 21:169 182.

Post, W.M., Peng T.-H., Emanuel, W.R. King, A.W. Dale, V.H. and DeAngelis, D.L., 1990. The Global Carbon Cycle. Am. Sci., 78:310 326.

Taylor, J.K. and Smith, S.W., 1959. Precise coulometric

F.J. Millero et al./Marine Chemistry 44 (1993) 153-165 165

titration of acids and bases, J. Res. Nat. Bur. Stand., 63A: 153-159.

Thurmond, V.L. and Millero, F.J., 1982. The ionization of carbonic acid in sodium chloride solutions at 25°C. J. Solution Chem., 11:447 456.

Weiss, R., 1981. Determinations of carbon dioxide and methane by dual catalyst flame ionization chroma- tography and nitrous oxide by electron capture chromatography. J. Chrom. Sci., 19: 611-616.