The estimation of acid dissociation constants in seawater media from potentionmetric titrations with strong base. I. The ionic product of water — Kw

The various assumptions implicit in the calculation of acid dissociation constants (based on ionic medium standard states) from potentiometric titrations using a cell with liquid junction (i.e. a pH measuring cell) have been examined. It was concluded that results can be obtained having an accuracy commensurate with the experimental precision. It has been shown that although the precise composition of the medium is a function of the hydrogen ion concentration (because of the protolytic nature of some of the ions in the media, e.g., sulphate and fluoride), the effect of such variations in the medium composition can be compensated for when defining the activity of hydrogen ion on an ionic medium standard state by defining the concentration of hydrogen ion as:

where β_HSO4 and β_HF are the relevant association constants and S_T and F_T are the total concentrations of sulphate and fluoride, respectively.This approach was used to obtain values for the ionic product of water (K_W) in artificial seawater media at various temperatures and ionic strengths. These were fitted to give the equation (molal concentration units):

$\text{pK}{}_{\mathrm{w}}\text{=}\text{3441.0}\text{T}\text{+2.256-0.7091}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(}\text{rms deviation 0.01}\text{)}$

where I is the formal ionic strength of the artificial seawater medium and T is the absolute temperature. The values obtained are in reasonable agreement with those found by previous workers.     

The significance of oxygen as oxides and hydroxides in controlling the abundance and residence times of elements in seawater

A model is presented which signifies the role of oxygen (as oxides and hydroxides) in controlling the composition of seawater. Using the regression equations

$\text{log}\text{K}{}_{\mathrm{SW}}\text{=-0.77+0.03ΔO}{}^{\mathrm{2-}}\text{M}\text{and}\text{[M]}{}_{\mathrm{SW}}\text{=K}{}_{\mathrm{SW}}\text{[M]}{}_{\mathrm{crust}}$

$\text{log}\text{t}\text{=4.73+0.04ΔO}{}^{\mathrm{2-}}\text{M}$

respective concentration and residence times for the unknown elements can be estimated. Geometric and statistical indices of Legget and Williams (1981) are used to evaluate the accuracy of the model. This reveals from the known values of ΔO^2?M that the present model estimates log $\text{t}{}_{\mathrm{y}}$ values within a factor of 1.77. The predicted oceanic residence times for Am, Ir, Ra and Rh are 3.6 × 10², 3.7 × 10², 2.2 × 10⁵ and 6.4 × 10² years, respectively.     

Equilibrium and structural studies of silicon(iv) and aluminium(iii) in aqueous solution. V. Acidity constants of silicic acid and the ionic product of water in the medium range 0.05–2.0 M Na(CI) at 25°C

The apparent ionization constants for silicic acid, k₁ and k₂, and the ionic product of water, k_w, have been determined in 0.05, 0.1, 0.2, 0.4 and 2.0 M Na(CI) media at 25°C. The medium dependence of these constants was found to fit equations of the form

$\text{log}\text{k}{}_{\mathrm{i}}\text{=}\text{log}\text{K}{}_{\mathrm{i}}\text{+a}{}_{\mathrm{i}}\text{I}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(1+I}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)+b}{}_{\mathrm{i}}\text{I}$

where K₁ is the ionization constant in pure water, α_i and b_i are parameters of which b_i has been adjusted to present data. The following results were obtained (α_i, b_i): pK₁ = 9.84, (1.022, ?0.11); pK₂ = 13.43, (2.044, ?0.20); and pK_w = 14.01 (1.022, ?0.22). k_i values are collected in Tables I and II. Attempts have been made to explain the medium dependence of k₁ and k₂ with weak sodium silicate complexing according to the equilibria

$\text{Na}{}^{\mathrm{+}}\text{+}\text{SiO(OH)}{}^{\mathrm{?}}{}_3\text{?}\text{NaSiO(OH)}{}_3\text{;k}{}_{11}$

$\text{Na}{}^{\mathrm{+}}\text{+}\text{SiO}{}_2\text{(OH)}{}^2{}_2\text{?}\text{NaSiO}{}_2\text{(HO)}{}^{\mathrm{?}}{}_2\text{; k}{}_{21}$

giving k₁₁ = 0.37M^?1 and k₂₁= 3.0M^?1. However, these weak interactions cannot be interpreted unambiguously from potentiometric data at different 1-levels. Probably the medium dependence could equally well be expressed by variations in the activity coefficients.The measurements were performed as potentiometric titrations using a hydrogen electrode. The average number of OH^- reacted per Si(OH)₄, Z, has been varied within the limits 0 ? Z ? 1.1 and B₁, the total concentration of Si(OH)₄, between 0.001 M and 0.008 M. k₁ was evaluated from experimental data with B ? 0.003 M, and k₂ with B ? 0.008 M and Z ? 0.95.     

Copper(II) interaction with carbonate species based on malachite solubility in perchlorate medium at the ionic strength of seawater

Equilibrium constants for copper(II)-carbonate and -bicarbonate species have been determined at 25°C from consideration of malachite, Cu₂(OH)₂CO_3(s), solubility in UV-photo-oxidized perchlorate solutions of 0.72 m ionic strength. The ratios of total dissolved copper, T(Cu), to free copper(II) ion, [Cu ²⁺], in 30 malachite saturated experimental solutions of 1–10 × 10^?3eq kg^?1 H₂O initial total alkalinity (TA_i in the pH range 5.0–9.3 were fitted to a copper(II)-ion speciation model. The experimental data indicate the existence of CuCO₃⁺, CuHCO₃⁺ and Cu(OH)CO₃^? in addition to the hydrolys and Cu(OH)CO₃^? in addition to the hydrolysis products in the range of conditions defined by this study. The stoichiometric equilibrium constants, applicable to seawater at 0.72 m ionic strength, 25°C and 1 atm are

A speciation model employing the equilibrium constants determined in this study and copper(II) hydrolysis constants from previous work suggests that the inorganic speciation in seawater (pH = 8.2, TA = 2.3 meq kg ^?1, 25°C) is dominated by the CuCO₃⁰ complex (82%) and that only 2.9% of the total inorganic copper exists as the free copper(II) ion. Hydrolysis products, CuOH⁺ and Cu(OH)₂⁰, account for 6.5% while CuHCO₃⁺ and Cu(OH)CO₃^? species comprise 1.0 and 6.3% of the total inorganic copper, respectively.     

Inhibition of sulphate reduction in anoxic marine sediment by Group VI anions

The addition of various concentrations (1, 10 and 20 mM) of Group VI anions to sediment slurry resulted in inhibition of the rate of sulphate reduction at the two higher concentrations, the degree of inhibition being in the order of molyb-date $\text{(MoO}{}_4{}^{\mathrm{=}}\text{)>selenate(SeO}{}_4{}^{\mathrm{=}}\text{)>tungstate(WO}{}_4{}^{\mathrm{=}}\text{)}$. The addition of 20 mM concentrations of these inhibitors almost entirely eliminated sulphate reduction. Doubling the sulphate concentration while using the highest concentration of inhibitors (20 mM) led to the re-establishment of some sulphate reduction in the $\text{SeO}{}_4{}^{\mathrm{=}}$ and $\text{WO}{}_4{}^{\mathrm{=}}$ treated slurries whereas no such reversal was noticed with $\text{MoO}{}_4{}^{\mathrm{=}}$. These observations suggested that $\text{SeO}{}_4{}^{\mathrm{=}}$ and $\text{WO}{}_4{}^{\mathrm{=}}$ are competitive inhibitors of sulphate reduction, while $\text{MoO}{}_4{}^{\mathrm{=}}$ is a non-competitive inhibitor.     

Trace metals in the Pagassitikos Gulf,Greece

The concentrations of Ni, Mn, Cr, Cu, Co, Zn, Fe, Pb and organic carbon have been determined in sediments of the Pagassitikos Gulf. They vary with the ‘sand equivalent’ content, s′, characteristic of the grain size, and are related to it by the equation,

$\text{c = c}{}_1\text{·k·s′}{}^{\mathrm{d}}$

in which the constants c₁, k and d depend on the element. Consequently, since the metal concentrations in a sample are much less representative of the area from which it has been taken than its ‘sand equivalent’ content, a contour map of the latter constitutes a reliable guide to the general pattern of the elements. Superimposed on it, we detect isolated patches of moderate pollution close to the harbour of Volos and a few, slight, natural anomalies, such as increased Mn and Ni, the former in the east and the latter in the south of the Gulf.     

Apparent dissociation constants of hydrogen sulfide in chloride solutions

Spectrophotometric measurements are reported for the first apparent dissociation constant of hydrogen sulfide in seawater over the temperature range 7.5–25°C and 2–35.8‰ salinity. These data are described by the expression $\text{p}\text{K}{}_1\text{′ = 2.527 ? 0.169 Cl}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{+ 1359.96/T}$. The second apparent dissociation constant in potassium chloride solution was estimated potentiometrically using a sulfide specific ion electrode. A value of ～13.6 was found for pK₂′ at a KCl concentration of 0.67 M. It is suggested that explicit reference to the sulfide ion, S^2?, in describing equilibria in marine waters be dropped in favor of a formulation involving the bisulfide ion, HS^?.     

Nitrogen uptake,ammonium oxidation and nitrous oxide (N2O) levels in the coastal waters of western Cook Strait,New Zealand

Vertical measurements of $\text{NH}{}_4{}^{\mathrm{+}}$, $\text{NO}{}_3{}^{\mathrm{?}}$ and N₂O concentrations, $\text{NO}{}_3{}^{\mathrm{?}}$ and $\text{NH}{}_4{}^{\mathrm{+}}$ uptake, and $\text{NH}{}_4{}^{\mathrm{+}}$ oxidation rates were measured at 5 sites in western Cook Strait, New Zealand, between 31 March and 3 April 1983. Nitrate increased with depth at all stations reaching a maximum of 10.5 μg-atom $\text{NO}{}_3{}^{\mathrm{?}}\text{N l}{}^{\mathrm{?1}}$ at the most strongly stratified station whereas $\text{NH}{}_4{}^{\mathrm{+}}$ was relatively constant with depth at all stations (～0.1 μg-atom $\text{NH}{}_4{}^{\mathrm{+}}\text{N l}{}^{\mathrm{?1}}$). The highest rates of $\text{NH}{}_4{}^{\mathrm{+}}$ oxidation generally occurred in the near surface waters and decreased with depth. N₂O levels were near saturation with respect to the air above the sea surface and showed no obvious changes during 24 h incubation. $\text{NH}{}_4{}^{\mathrm{+}}$ oxidation by nitrifying bacteria may account for about 30% of the total $\text{NH}{}_4{}^{\mathrm{+}}$ utilization (i.e. bacterial+agal) and, assuming oxidation through to $\text{NO}{}_3{}^{\mathrm{?}}$, may supply about 40% of the algal requirements of $\text{NO}{}_3{}^{\mathrm{?}}$ in the study area. These results suggest that bacterial nitrification is of potential importance to the nitrogen dynamics of the western Cook Strait, particularly with respect to the nitrogen demands of the phytoplankton.