Andrew G. Dickson

A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media

The published experimental data of Hansson and of Mehrbach et al. have been critically compared after adjustment to a common pH scale based upon total hydrogen ion concentration. No significant systematic differences are found within the overall experimental error of the data. The results have been pooled to yield reliable equations that can be used to estimate pK1∗and pK2∗ for seawater media a salinities from 0 to 40 and at temperatures from 2 to 35°C.

The equilibrium speciation of dissolved components in freshwater and sea water at 25°C and 1 atm pressure

A data base summarising the stability constants of more than 500 complexes is used to calculate speciation pictures for 58 trace elements in model seawater (pH 8.2) and freshwaters (pH 6 and 9). Consideration of the results provides a general summary of the chemical periodicity of the speciation of trace components in natural waters. The polarising power of an element ((cation charge)2/(radius), z2/r) provides a useful index to the degree of hydrolysis in aqueous solution. The fully hydrolysed elements with a high polarising power form distinct groupings in the periodic table. The relative magnitudes of the acid dissociation constants are summarised by Paulings rules and the speciation of the fully hydrolysed elements in natural waters largely depends on pH and, to a lesser extent, on interactions with the major cations. The remaining cations of low and intermediate polarising power can be subdivided according to their tendency to form covalent bonds. An empirical parameter Δβ(= logβ0MF − log β0MCl) is used to define (a)-type (Δβ > 2), borderline (a)-type (2 >Δβ > 0), (b)-type (Δβ Δβ > −2) cations. Again these various categories form coherent groupings on the periodic table. By considering the interactions of cations from the various categories with the inorganic ligands commonly encountered in natural waters it is possible to assign the ligands themselves to ‘hard’ (e.g. F−, SO42−), ‘intermediate’ (e.g. OH−, CO2−3) and ‘soft’ categories (e.g. Cl−). These concepts can be summarised by constructing a Complexation Field Diagram in which the various cations are located on a plot of z2r vs δβ. The extension of the model to include redox equilibria and additional ligands is described.

Standard potential of the reaction: AgCl(s) + 12H2(g) = Ag(s) + HCl(aq), and and the standard acidity constant of the ion HSO4− in synthetic sea water from 273.15 to 318.15 K

Abstract The standard potential of the reaction: AgCl(s) + 1 2 H 2 (g) = Ag(s) + HCl(aq) , has been measured in synthetic sea water in a galvanic cell from 273.15 to 318.15 K, and at five ionic strengths corresponding to salinities from 5 to 45. Measurements were made at various hydrochloric-acid molalities, and the results extrapolated to a pure ionic medium. These results allow the cell to be used to measure accurate values for acidity constants in sea water. The results have also been interpreted to provide values for the acidity constant of the ion HSO4− in sea water.

The complexation of metals with humic materials in natural waters

Abstract Humic materials have been isolated from sea, river and lake waters by an adsorption technique, and the stability constants of their complexes with Ca, Mg, Mn, Co, Ni, Cu, Zn, Cd and Hg have been determined at pH 8.0 using a gel filtration chromatographic technique. In general, the order of increasing strength of binding of the metals followed the Irving-Williams series. The stability constant data have been used in conjunction with the HALTAFALL program of Ingri et al. (1967) to compute speciation models for freshwaters and seawater which take into account metal-humic interactions. In freshwater more than 90% of the copper and mercury was found to be complexed by humic materials. However, 99% of the humic material is complexed by calcium and magnesium because of their relatively high concentrations; metal chelation is only appreciable for copper (~10%). The pattern of trace element speciation with inorganic ligands in seawater differs somewhat from that computed by previous workers largely because of the use of more up-to-date stability constant data. Finally, an attempt has been made to compute the changes in metal speciation which would occur when river water becomes diluted with seawater as in an estuary.

Thermodynamics of the dissociation of boric acid in synthetic seawater from 273.15 to 318.15 K

E.m.f. measurements have been made using the cell: Pt | H2(g,101.325 kPa) | borax in synthetic seawater | AgCl; Ag over the temperature range 273.15–318.15 K, and at five salinities from 5 to 45. The results have been used to calculate the stoichiometric (ionic medium) dissociation constant for boric acid in seawater media on the “total” hydrogen ion scale.

Ocean pCO2 calculated from dissolved inorganic carbon, alkalinity, and equations for K1 and K2: validation based on laboratory measurements of CO2 in gas and seawater at equilibrium

Abstract The partial pressure of carbon dioxide in the oceans surface waters, precisely expressed as the fugacity ( f CO 2 ) is determined from dissolved inorganic carbon (DIC) and total alkalinity (TA), and the first and second dissociation constants of carbonic acid, K 1 and K 2 . The original measurements of K 1 and K 2 reported by Mehrbach et al. [Mehrbach, C., Culberson, C.H., Hawley, J.E., Pytkowicz, R.M., 1973. Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnol. Oceanogr. 18, 897–907] are reformulated to give equations for p K 1 and p K 2 (p K =−log 10 K ) as a function of seawater temperature and salinity, consistent with the “total hydrogen ion” concentration scale: p K 1 =3633.86/T−61.2172+9.67770 ln T−0.011555 S+0.0001152 S 2 p K 2 =471.78/T+25.9290−3.16967 ln T−0.01781 S+0.0001122 S 2 By equilibrating solutions of seawater with gas mixtures of known composition, we demonstrate that the above formulations of K 1 and K 2 give calculated f CO 2 values that agree with equilibrated values to 0.07±0.50% (95% confidence interval, f CO 2 up to 500 μatm). Formulations of K 1 and K 2 based on other studies resulted in calculated f CO 2 values approximately 10% lower than the measurements. Equilibrations at f CO 2 above 500 μatm yielded measured f CO 2 values higher than calculated values by on average 3.35±1.22% (95% confidence interval). The cause for the f CO 2 dependence of the results is not known. The uncertainties in p K 1 and p K 2 were combined with the analytical uncertainties typical of contemporary measurements of DIC and TA to reveal the expected reliability of seawater f CO 2 calculated from these parameters. For example, an uncertainty of 1.0 μmol kg −1 in DIC and 2 μmol kg −1 in TA (1 standard deviation (s.d.)) will result in uncertainty of the calculated f CO 2 of 1% or ±3.5 μatm at 350 μatm (1 s.d.).

An exact definition of total alkalinity and a procedure for the estimation of alkalinity and total inorganic carbon from titration data

Abstract The total (or titration) alkalinity of a natural water sample can be regarded as a measure of the proton deficit of the solution relative to an arbitrarily defined zero level of protons. The problem of unambiguously incorporating any particular acid-base system into the definition of alkalinity is thus the one of deciding which form to specify as the zero level of protons, and it is proposed that it be defined so that acids with a dissociation constant K > 10−4·5 (at 25°C and zero ionic strength) are considered as proton donors, whilst those bases formed from weak acids with K ⪕ 10−4·5 are cosidered proton acceptors. A non-linear least squares procedure is suggested in order to estimate the total alkalinity (AT) and total inorganic carbon (CT) of a seawater sample from potentiometric titration data. The approach offers a significant conceptual improvement over the currently used refined Gran functions. In addition, an estimate of the statistical uncertainty of the estimated values of AT and CT is available. As it unnecessary to titrate beyond the alkalinity equivalence point, it may also be possible to speed up the titrations.

Reference materials for oceanic CO2 analysis: a method for the certification of total alkalinity

Abstract This paper describes a method used to certify reference materials based on seawater for total alkalinity. The technique employs a two-stage, potentiometric, open-cell titration using coulometrically analyzed hydrochloric acid. The equivalence point is evaluated from titration points in the pH region 3.0–3.5 using a least-squares procedure that corrects for the reactions with sulfate and fluoride ions. The reproducibility (one standard deviation) of this technique is less than 1 μmol kg −1 ; the accuracy is within 2 μmol kg −1 .

pH buffers for sea water media based on the total hydrogen ion concentration scale

Published e.m.f. values measured using the cell where p° = 101.325 kPa, and BH+ and B are the conjugate acid-base pairs of 2-aminopyridine, 2-amino-2-hydroxymethyl-1,3-propanediol (tris), tetrahydro-1,4-isoxazine (morpholine), and 2-amino-2-methyl-1, 3-propanediol (bis), have been re-evaluated to assign pH values based on the “total” hydrogen ion concentration scale to equimolal (m =0.04 mol kg−1) buffer solutions based on these compounds. These pH values are consistent with the best available equilibrium constants for acid-base processes in sea water and such pH buffers can be used as pH calibration standards to measure accurate values for oceanic pH on the “total” hydrogen ion pH scale. In addition, the published e.m.f. results for these various amine bases have been used to calculate their respective acidity constants on this pH scale.

pH scales and proton-transfer reactions in saline media such as sea water

Abstract The three approaches to defining pH scales for use in sea water: the N.B.S. scale, the pH(SWS) or ‘total’ hydrogen ion concentration scale and the ‘free’ hydrogen ion concentration scale are described, and it is shown how these arise as a direct consequence of alternative experimental procedures for determining practical acidity constants. The advantages of conceptual simplicity and of experimental precision inherent in the use of concentration products to describe proton-transfer reactions in saline media are emphasised. In addition, the problems of theoretical interpretation and of reproducibility which result from the conventional nature of the N.B.S. pH scale are described, and the effect on the corresponding ‘apparent’ constants outlined. Insofar as it is concentration products rather than ‘apparent’ constants that are amenable to prediction using models for activity coefficients, the deliberate use of a ‘free’ hydrogen ion concentration scale should be applicable to many areas of aqueous geochemistry in addition to marine chemistry.