Speciation models TITRATOR

Inorganic speciation in solution can also affect the mobility of metal ions (Doner, 1978). The formation of an ion-pair with Cl can more than double the mobility of Cd in the presence of 200molm 3NaCl. At the same chloride concentration, however, the mobilities of Cu2+ and Ni2+ are only increased slightly (5-10%), presumably because of very weak complexation with Cl. This mechanism could increase the leaching of Cd from saline soils but it may not be effective in non-saline soils because the ratio of the total concentrations of Cd Cl must be >1 106 before >50% of total Cd is complexed by Cl (estimated using the computer model TITRATOR (Cabaniss, 1987), which considered the chloro and hydroxy complexes of Cd at pH 5.0 and a total Cd concentration of 0.1 mmolm-3 equilibrium constants were taken from Lindsay (1979)). 

Many of the titration-based chemical models employed to describe Al-citrate speciation were validated usmg ancillary spectrometry data (Lakatos et al., 2001 Harris et al., 2003). Results from spectroscopic studies of Fe(III)-citrate systems do not support the titration-based models, which are defined by mononuclear monocitrate species. Instead, mononuclear dicitrate, dinuclear, and trinuclear Fe(III)-citrate species are observed. It may be concluded that the aqueous speciation of Fe(III)-citrate in soil solutions cannot be predicted. The current speciation models are not applicable to the pH range of soil solutions, or as in the case of the Essington et al. (2005) model, have not been validated. There is also evidence... 

The prediction by our speciation model that in multiple oxides (as in quartz) Si sites are not susceptible to adsorption by H+ is confirmed by the results of surface titrations of albite, labradorite, and anorthite (Fig. 6). One can see that the net adsorption of H + at the feldspar solution interface increases markedly with the relative number of Al sites to Si sites, which is reflected by the stoichiometry of Al and Si in the different feldspars. These results help clarify the typical features of the dependence on pH of silicate dissolution (Table 2) ... 

Ruzic [278 ] considered the theoretical aspects of the direct titration of copper in seawaters and the information this technique provides regarding copper speciation. The method is based on a graph of the ratio between the free and bound metal concentration versus the free metal concentration. The application of this method, which is based on a 1 1 complex formation model, is discussed with respect to trace metal speciation in natural waters. Procedures for interpretation of experimental results are proposed for those cases in which two types of complexes with different conditional stability constants are formed, or om which the metal is adsorbed on colloidal particles. The advantages of the method in comparison with earlier methods are presented theoretically and illustrated with some experiments on copper (II) in seawater. The limitations of the method are also discussed. 

The chemical speciation of copper in river water and model solutions was investigated by a titration technique in which cupric ion activities were measured at constant pH as the total copper concentration ([Cujoj]) was varied by incremental additions of CUSO4. pCu(-log cupric ion activity) was measured with a cupric ion-selective electrode (Orion 94-29) and pH with a glass electrode (Beckman 39301) both coupled to a single junction Ag/AgCl reference electrode (Orion 90-01) in a temperature controlled (25 + 0.5°C) water bath. Total copper concentrations in the titrated solutions were determined directly by atomic absorption spectrophotometry (Perkin Elmer 603) using a graphite furnace (Perkin Elmer 2200). Measurement of total copper concentrations is necessary because of adsorptive loss of copper from solution onto container and/or electrode surfaces. 

Titrations were performed on untreated, filtered, and UV-treated filtered river water samples at in situ and adjusted pH values. The effect of pH on copper speciation was investigated by titration of filtered Newport River water at pH 7.0 and filtered Newport and Neuse waters at pH 8.0. Newport River water was adjusted to pH 7.0 by decreasing the partial pressure of CO2 from the initial ambient value of about 10 times the atmospheric level. To adjust the pH to 8.0, sodium bicarbonate was added to bring the river water samples to a concentration of 0.5 mM with subsequent adjustment of Pc02 Titrations were also conducted at pH 7,0 in model solutions consisting of 0.01 KNO3 and 0.1 mM NaHC03 with and without the addition of 0.75 histidine to test electrode behavior in solutions of known chemistry. 

Studies of metal speciation in oceanic water show that experimental titration data fit models which consider one class of ligands (33-36, 57, 108) or two classes of ligands (32, 37, 69, 109). Attempts to fit experimental data to models which consider more than two classes of ligands did not improve goodness of fitting. 

The protonation of organic acid anions during alkalinity titration can cause serious errors in alkalinity determination and the use of the data for carbonate speciation and equilibrium chemical modeling. Errors in alkalinity depend on the same factors as for anion-cation balance and may vary from negligible values to as much as 1270% error as documented by Beck et (1974). Accurate values for alkalinity and carbonate speciation can be determined by direct measurement of CO2 evolved from acidified water samples. 

Harris Model Harris et al. (2003) examined Al-citrate speciation using potentiometric titrations and electrospray ionization mass spectrometry. The titrations were performed in the pH range 2 to 9 at 25°C in 0.10 mol... 

Numerous potentiometric titration-based chemical models have been proposed, a small number of which has been verified by H, and Al NMR and electrospray mass spectroscopic techniques. The models range in character relative to the number and nature of species considered, from simple (a small number of mononuclear monocitrate species) to inordinately complex (several mono- to trinuclear and mono- to tricitrate species). The models are incongruous, in that no two models are composed of the same species, nor do they predict similar Fe(III)-or Al-citrate aqueous speciation. Similarly, the models differ relative to the predicted impact of citrate on the solubility of Fe(III)- or Al-bearing accessory minerals. 

Chemical speciation of two acidic stream waters from Scotland was calculated by Lumsdon and Evans (2002), using the MINTEQA2 model. An estimate of the DOM concentration was based on the titratable acidity of 7 mol kg, which is typical of the humic substances found in Scottish surface waters (Anderson et al.. 

However, the major recent advance in the modeling of mineral dissolution has been the presentation by Stumm and coworkers (Furrer and Stumm, 1986 Zinder et al., 1986 Stumm arid Furrer, 1987) of a surface coordination approach to explain the proton- and ligand-promoted dissolution of simple oxides (A1203, BeO, a-FeOOH). This approach, which relies on surface titrations and double layer concepts for characterizing the chemical speciation on mineral surfaces, offers several advantages ... 

Most applications in the regulatory environment have used speciation-solubility models. A few used surface complexation models. Applications of surface complexa-tion models mostly used the model and data from Dzombak and Morel (1990). Reaction path calculations are mostly limited to the titration and mixing calculations of two fluids. 

Therefore, we take two steps in modeling. First, the acidic water is neutralized by titrating of calcite. The amounts of Fe and A1 hydroxides precipitated from this titration are then used in the subsequent speciation and complexation modeling. 

---