Margareta Wedborg

The sulphur-mercury(II) system in natural waters

Sulphur is an essential element for aquatic biosystems, the life processes of which lead to the formation of low molecular weight S compounds in the water. The results of our calculations indicate a pronounced tendency for Hg(II) to form HgS (or HgOHSH) and Hg(SR)2 complexes in the presence of H2S and thiols. Likewise, McHg will form CH3HgSH and CH3HgSR complexes, but in this case the chloride complex will dominate at low concentrations of H2S and thiols. In acidic low salinity water, CH3HgCl is the dominant McHg species at the lowest concentration of sulphide/thiols (0.1 nM), whereas a hundredfold increase of the sulphide/thiol concentration, or an increase of the pH to neutral or slightly alkaline conditions, will result in a total dominance for CH3HgSH and CH3HgSR.

The ammonia-ammonium equilibrium in seawater at temperatures between 5 and 25°C

The stability constantK1of the equilibrium NH3+H+⇌NH4+ was determined by potentiometric titration in synthetic seawaters of 5 to 40‰ (per mille) salinity and sodium chloride solutions between 0.2 and 0.7M ionic strength at temperatures of 5, 15 and 25°C. Moreover,K1was determined in sulfate-free seawater of 35‰ salinity and 25°C. The dependence of logK1on salinity (or ionic strength) and temperature can be reasonably well described by linear relations. TheK1obtained for sulfate-free seawater agrees very well with earlier experimental results and theoretical predictions. From the measuredK1and literature data on the equilibrium NH3(atm)⇌NH3(aq., NaCl), the atmospheric concentration of ammonia at equilibrium with seawater was estimated.

Acid-base properties of aquatic fulvic acid

Abstract The acid-base properties of aquatic fulvic acid, concentrated from the Gota River, were studied by means of potentiometri titrations. The forward and reverse titrations exhibited hysteresis, which decreased as the fulvic acid underwent repeated titration. A simple model based on six titratable groups, within the pH range 2.5–10.4, could explain the acid-base properties. Electrostatic polymeric effects and heterogeneity of the fulvic acid (i.e., a distribution of constants rather than a set of distinct constants) were not included in the model. It was found that the three most acidic groups were carboxylic.

Stability constants of phosphoric acid in seawater of 5–40‰ salinity and temperatures of 5–25°C

Abstract The stability constants K1 and K12 of phosphoric acid were determined for artificial seawater of six different salinities (5, 10, 20, 30, 35, 40‰) and in 0.2, 0.4 and 0.7 M NaCl at three temperatures (5, 15, 25°C). The results are compared with those of Kester and Pytkowicz (1967), Atlas et al. (1976) and Dickson and Riley (1979). The ionic product of water, Kw, was determined in sodium chloride media at 5, 15 and 25°C. Complex formation among Ca2+, Mg2+, HPO42− and PO43− is discussed.

Complexation of aluminum with DNA.

The extent of complexation of aluminum(III) with DNA (Calf thymus, Sigma type I) was estimated by means of two experimental techniques: potentiometric titration with a fluoride selective indicator electrode and dialysis followed by aluminum determination by graphite furnace AAS. Both types of experiments indicate that aluminum(III) is bound to DNA. The data are treated by assuming an ion exchange reaction with the phosphate diester groups. Using Rt to denote the concentration of these groups the values of log [AlMn-3R]/(Rt-3[AlMn-3R])[Al3+] decrease from approx. 7.6 to 5.6 when the concentration of sodium chloride is increased from 1 to 100 mM. In the pH range 4.5-5.5 the ion exchange constant increases approximately 0.5 log units. Dialysis gives lower values for the complex formation constant than potentiometry.

Titration of sulphides and thiols in natural waters

Abstract The concentrations of the species present during the procedure of Boulegue for determination of sulphide, thiols, thiosulphate and sulphite in mineral waters by titration with mercury(II) chloride have been calculated and functions suitable for the evaluation of the equivalence points have been derived. It is shown that the halide ions in seawater interfere only in the titration of sulphite.

Stability of ion pairs from gypsum solubility degree of ion pair formation between the major constituents of seawater

The stability constants of the ion pairs NaSO4−, KSO4−, MgSO4−, CaSO4, MgCl+ and CaCl+ were determined at 25°C and 0.7 M formal ionic strength, by measuring the solubility of gypsum (CaSO4 · 2H2O) in different media. The media used contained one or two of the following electrolytes: NaCl, KCl, MgCl2, NaClO4, Mg(ClO4)2, Na2SO4. Values for the stability constants are 1.22, 1.84, 12.3, 30.6, 0.48 and 1.20 M−1, respectively, and the solubility product for gypsum is 2.87 · 10−4 M2. The distribution of the main constituents of seawater was calculated using these results and the values of the carbonate and bicarbonate constants given by Dyrssen and Hansson (1972–1973). The solubility of gypsum in seawater as calculated and determined experimentally was 21.43 mM and 21.10 mM, respectively.

Stability constants of NaSO4−, MgSO4, MgF+, MgCl+ ion pairs at the ionic strength of seawater by potentiometry

Abstract The stability of the ion pair NaSO 4 was determined by measuring the change in sodium activity with medium composition at constant ionic strength, using a sodium-sensitive glass electrode. The stability constants of MgSO 4 and MgCl + were determined indirectly from measurements of the stability of MgF + in different media. All measurements were performed at 1 atm pressure, 25 ± 0.1 °C and 0.7 M formal ionic strength. The stability constants for NaSO 4 − , MgSO 4 , MgF + and MgCl + are 1.8 ± 0.1, 6.3 ± 0.1, 22.9 ± 0.1 and 0.34 ± 0.02 M −1 , respectively.

Sulphate complexation in seawater: A relation between the stability constants of sodium sulphate and magnesium sulphate in seawater from a determination of the stability constant of hydrogen sulphate and a calculation of the single ion activity coefficient of sulphate

Abstract The conditional stability constant of HSO 4 − has been determined at 25°C, 1 atm and a formal ionic strength of 0.7 M in solutions containing sodium, magnesium, chloride and sulphate. This was done spectrophotometrically (UV), using diphenylamine as indicator. The value obtained was 17.0 ± 0.1 (molar scale). Single ion activity coefficients for Na 2 SO 4 , K 2 SO 4 and MgSO 4 have been calculated according to the Bates et al. (1970) model, assuming that the sulphate ion is not hydrated. It was found that the single ion activity coefficient of sulphate changes very little between Na 2 SO 4 , K 2 SO 4 and MgSO 4 when the formal ionic strength is kept constant. These results have been used to obtain relations between the stability constants of NaSO 4 − and MgSO 4 valid for seawater.

The influence of the partial pressure of carbon dioxide on the total carbonate of seawater

Abstract The dependence of the total carbonate concentration of ocean water on temperature and atmospheric partial pressure of carbon dioxide is calculated. The results show that the increase in total carbonate caused by the increase of carbon dioxide in the atmosphere is ca. 25–50 times larger than the precision in the experimental determination of C t .