Sillen equilibrium model

Some limitations of the equilibrium model of sea water do exist. Sillen has pointed out that based on equilibrium calculations all the nitrogen in the ocean-atmosphere system should be present as N03 in sea water however, most of the nitrogen is present as Nj gas in the atmosphere. Also, the concentrations of the major alkaline earth elements. Mg, Ca, and Sr, in sea water may vary slightly with depth or geographic location. 

By comparing the actual composition of sea water (sediments + sea -f- air) with a model in which the pertinent components (minerals, volatiles) with which water has come into contact are allowed to reach true equilibrium, Sillen in 1959 epitomized the application of equilibrium models for portraying the prominent features of the chemical composition of this system. His analysis, for example, has indicated that contrary to the traditional view, the pH of the ocean is not buffered primarily by the carbonate system his results suggest that heterogeneous-equilibria of silicate minerals comprise the principal pH buffer systems in oceanic waters. This approach and its expansion have provided a more quantitative basis for Forchbammer s suggestion of 100 years ago that the quantity of the different elements in sea water is not proportional to the quantity of elements which river water pours into the sea but is inversely proportional to the facility with which the elements in sea water are made insoluble by general chemical actions in the sea. 

Some years ago, Sillen published (56) a most fundamental contribution to chemical oceanography. Assuming a simple equilibrium model he was able to obtain almost correct values for the concentrations of many major and of some minor components of sea water. In comparing Sillen s paper with our approach, the reader will find that a seven-year progress in equilibrium chemistry has generally confirmed and justified his model. There are, however, some indications that the ocean represents a steady-state system rather than a equilibrated solution. Whatever the accuracy of our calculations, the relationship between oversaturation and the rate of transport cannot be ignored. 

This last requirement has been the greatest stumbling block for accepting the Sillen and Mackenzie and Carrels equilibrium models. Most marine clays appear to be detrital and derived from the continents by river or atmospheric transport. Authigenic phases (formed in place) are found in marine sediments however, they are nowhere near abundant enough to satisfy the requirements of the river balance. For example, Kastner (1974) calculated that less than 1% of the Na and 2% of the K transported by rivers is taken up by authigenic feldspars. 

From the pH data alone, it can be concluded that solutions of hafnium salts for [Hf]tot — lO -lO M, pH 1, and at 25°C require a year or more to reach a steady state. Thus, the solutions are not at equilibrium. Nonetheless, it is possible to calculate much about the nature of the hafnium species from the data at any particular time, in accordance with Sillen s model (13). In order to make such calculations, it is necessary to calculate Z from pH data for each solution. Values of Z define the actual stoichiometry of the hafnium species, [Hf(OH)z] (4 z)iV+. For experimental solutions at 25°C after two days, the Z vs. pH plot (with [Hf]ToT as the parameter) is shown in Figure 3. Clearly, the hydrolysis products are polynuclear. 

The thermodynamic data compilations of Sillen and Martell catalyzed rapid advances in equilibrium models of seawater speciation. These works were followed by additional compilations that were critically important to modern sea-water speciation assessments. In view of these developments, and additional extensive experimental analyses appropriate to seawater. Principal Species assessments ten to fifteen years after the pioneering work of Sillen demonstrated a much improved awareness of the importance of hydrolysis in elemental speciation. 

Sillen constructed his models in a stepwise fashion starting with a simplified ocean model of five components [HCl, H2O, KOH, Al(OH)3, and Si02] and five phases (gas, liquid, quartz, kaolinite, and potassium mica) (Sillen, 1967). His complete (almost) seawater model was composed of nine components HCl, H2O, and CO2 are acids that correspond to the volatiles from the Earth KOH, CaO, Si02, NaOH, MgO, and Al(OH)3 correspond to the bases of the rocks. If there was an equilibrium assemblage of nine phases, the system would have only two independent variables. Sillen argued that a plausible set could include a gas phase and a solution phase and the following seven solid phases ... 

The earliest experimental data are from Hietanen and Sillen [1968HIE/SIL] in 3 M NaCl. They tested a number of different chemical models, all of which describe the experimental data with very nearly the same accuracy. Many of the species proposed occur in small amounts in the test solutions, cf. Appendix A, and this review suggests that most of them are artefacts, i.e., parameters that just improve the fit of the experimental data. The stoichiometry of these species does not have an obvious relationship to the known coordination chemistry of Th(IV), e.g., there are no known stmctures containing tri- and penta-nuclear complexes. However, the complex ThjlOH) is a predominant complex in a large concentration range and the equilibrium constant is not strongly dependent on the selection of other species accordingly, this review considers the complex (2,2) as well estabhshed. 

The paper [1958LEF] does not contain new experimental data and the proposed speciation scheme and equilibrium constants are not accepted by this review. The potentiometric data of [1954H1E] and [1954KRA/HOL] have been evalnated snbse-quently by Baes et al. [1965BAE/MEY] and Hietanen and Sillen [1968H1E/S1L] who found a consistent model and equilibrium constants for the hydrolysis of thorinm in 1 M NaC104 solntion. Their model, including the complexes (n,m) = (1,1), (2,1), (2,2), (8,4) and (15,6), is selected in the present review. 

The potentiometric data were interpreted with the formation of only two dinu-clear species, (2,2) = Th2(OH)j and (3,2) = Th2(OH)3. Other models were not tested however, the speciation model used is consistent with that of Hietanen and Sillen [1968HIE/SIL] in 3 M NaCl, which has been confirmed by test calculations in the review of Baes and Mesmer [1976BAE/MES] and selected in the present review. The complex (14,6) = Th (OH)[ which is also included in the selected model for chloride media is not relevant under the experimental conditions in [1981MIL2], [1981SUR/MIL], [1982SUR/MIL] (pH < 3.2), cf. speciation calculations in Appendix A review of [1968HIE/SIL]. The reported conditional equilibrium constants, with the statistical uncertainties given as 3ct are summarised in Table A-52. 

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