Free ionic concentration

Sanders J.R. The effect of pH on the total and free ionic concentrations of manganese, zinc and cobalt in soil solutions. J Soil Sci 1983 34 315-323. 

The model balance equation for each metal and ligand (e.g., Eqs. 2.49 and 2.52) is augmented to include formally the concentration of each possible solid phase. By choosing an appropriate linear combination of these equations, it is always possible to eliminate the concentrations of the solid phases from the set of equations to be solved numerically. Moreover, some of the free ionic concentrations of the metals and ligands also can be eliminated from the equations because of the constraints imposed by on their activities (combine Eqs. 3.2 and 3.3), which holds for each solid phase formed. The final set of nonlinear algebraic equations obtained from this elimination process will involve only independent free ionic concentrations, as well as conditional stability and solubility product constants. The numerical solution of these equations then proceeds much like the iteration scheme outlined in Section 2.4 for the case where only complexation reactions were considered, with the exception of an added requirement of self-consistency, that the calculated concentration of each solid formed be a positive number and that IAP not be greater than Kso (see Fig. 

In these expressions, the square brackets represent the molar concentrations of the species indicated, and y is the activity coefficient of a Z valent species. In order to calculate the free ionic concentrations in these expressions, it is necessary to take into account ion-pair and complex formation. The equilibria in pure calcium phosphate solutions are -... 

In their otherwise useful paper on the saturation of saliva with respect to Ca salts, Larsen and Pearce [54] make certain potentially misleading statements concerning CaF2, in our opinion. These authors start their abstract with It may be assumed that free ionic concentrations of Ca and phosphate in resting saliva tend to equilibrate with those in plaque fluid, and that salivary determinations can therefore be used to illustrate chemical conditions in both saliva and plaque. and conclude with Therefore, this salt (CaF2, sic), the outcome of... 

Lorenz et al. (1997) have stated that the free ionic concentration in soil solution did not predict the concentration of Cd and Zn in plants better than does their total concentration in soil solution, which suggests that analysis of Cd and Zn spe-ciation in soil solution is of little practical importance when their hioavailability is assessed. 

Free ionic silver readily forms soluble complexes or insoluble materials with dissolved and suspended material present in natural waters, such as sediments and sulfide ions (44). The hardness of water is sometimes used as an indicator of its complex-forming capacity. Because of the direct relationship between the availabiUty of free silver ions and adverse environmental effects, the 1980 ambient freshwater criterion for the protection of aquatic life is expressed as a function of the hardness of the water in question. The maximum recommended concentration of total recoverable silver, in fresh water is thus given by the following expression (45) in Fg/L. 

Fabiato, A. (1988). Computer programs for calculating total from specified free or free from specified total ionic concentrations in aqueous solutions containing multiple metals and ligands. Method. Enzymol. 157 378-417. 

In all cases a transicis selectivity of around 7/3 is obtained Numbers separated by dashes indicate results in successive reuses Bromine-free ionic liquid Catalyst concentration 25 mM... 

Aluminium toxicity is a major stress factor in many acidic soils. At soil pH levels below 5.0, intense solubilization of mononuclear A1 species strongly limits root growth by multiple cytotoxic effects mainly on root meristems (240,241). There is increasing evidence that A1 complexation with carboxylates released in apical root zones in response to elevated external Al concentration is a widespread mechanism for Al exclusion in many plant species (Fig. 10). Formation of stable Al complexes occurs with citrate, oxalate, tartarate, and—to a lesser extent— also with malate (86,242,243). The Al carboxylate complexes are less toxic than free ionic Al species (244) and are not taken up by plant roots (240). This explains the well-documented alleviatory effects on root growth in many plant species by carboxylate applications (citric, oxalic, and tartaric acids) to the culture media in presence of toxic Al concentrations (8,244,245) Citrate, malate and oxalate are the carboxylate anions reported so far to be released from Al-stressed plant roots (Fig. 10), and Al resistance of species and cultivars seems to be related to the amount of exuded carboxylates (246,247) but also to the ability to maintain the release of carboxylates over extended periods (248). In contrast to P deficiency-induced carboxylate exudation, which usually increases after several days or weeks of the stress treatment (72,113), exudation of carboxylates in response to Al toxicity is a fast reaction occurring within minutes to several hours... 

Eqs. (15), (17), and (21) can be used to define other observable quantities, such as relative surface excess concentrations of ions, which also comprise the contributions from the free ionic and ion-pair surface excesses, e.g., for the ideally polarized ITIES,... 

The total metal concentration in a solution can be easily determined using methods such as atomic absorption spectroscopy (AAS) however, the bioavailability of different metal species likely varies. In addition, much of the original concentration may have speciated into insoluble precipitates. Therefore, the concentration of some bioavailable species may be extremely low, perhaps even within or below the nanomolar range.99 Ion-selective electrodes are useful for measuring the bioavailable concentration of a metal because they measure only the free, ionic species, which is often most prevalent.102... 

Although buffer components are often present at the highest concentration in a medium, metals can also bind to inorganic ligands in solution. Ligands such as CL, OHand SO can form soluble complexes with many metals.127 These complexes remain in solution, but are considered to be less bioavailable than the free, ionic species under most conditions.97 Even though other soluble metal species are considered less bioavailable, they may play roles in metal toxicity. 

Free, ionic species of metals are at their highest concentrations at lower pH, so metals tend to be more bioavailable under these conditions.121128 At acidic pH, more protons are available to saturate metal-binding sites.99 For example, metals are less likely to form insoluble precipitates with phosphates when the pH of the system is lowered because much of the phosphate has been protonated. Under basic conditions, metal ions can replace protons to form other species, such as hydroxo-metal complexes. Some of the hydroxo-metal complexes are soluble, such as those formed with cadmium, nickel, and zinc, whereas those formed with chromium and iron are insoluble. 

In a study of the bioaccumulation of metals as colloid complexes and free ions by the marine brown shrimp, Penaeus aztecus [29] the colloids were isolated and concentrated from water obtained from Dickinson Bayou, an inlet of Galveston Bay, Texas, using various filtration and ultrafiltration systems equipped with a spiral-wound 1 kDa cutoff cartridge. The total colloidal organic carbon in the concentrate was found to be 78 lmgdm 3. The shrimps were exposed to metals (Mn, Fe, Co, Zn, Cd, Ag, Sn, Ba and Hg) as radiolabelled colloid complexes, and free-ionic radiotracers using ultrafiltered seawater without radiotracers as controls. The experiments were designed so that the animals were exposed to environmentally realistic metal and colloid concentrations. 

A plot of the spectroscopically computed maximum ionic concentrations [SD ions]m (sum of free ions and ion-pairs) at the end of the reactions (m denotes maximum values throughout this paper, the subscript 0 denotes initial values) against the final values of the specific conductivity Km. for the equivalent runs (Tables 1 and 2) gave a straight line through the origin, showing that virtually only free ions were present in our systems if there had been important quantities of ion-pairs, this plot would have been markedly curved. 

Once the composition of the aqueous solution phase has been determined, the activity of an electrolyte having the same chemical formula as the assumed precipitate can be calculated (11,12). This calculation may utilize either mean ionic activity coefficients and total concentrations of the ions in the electrolyte, or single-ion activity coefficients and free-species concentrations of the ions in the electrolyte (11). If the latter approach is used, the computed electrolyte activity is termed an ion-activity product (12). Regardless of which approach is adopted, the calculated electrolyte activity is compared to the solubility product constant of the assumed precipitate as a test for the existence of the solid phase. If the calculated ion-activity product is smaller than the candidate solubility product constant, the corresponding solid phase is concluded not to have formed in the time period of the solubility measurements. Ihis judgment must be tempered, of course, in light of the precision with which both electrolyte activities and solubility product constants can be determined (12). 

The effect of concentration of free (molecular) ammonia on the activity of the electrolyte was derived mainly from two 80 C data points of Miles and Wilson having 16 to 17 molal free ammonia concentration. Data points below 0.2 ionic strength were fitted by application of Kielland s estimation of ionic activity coefficients(6 2). Details are presented elsewhere(45), together with graphs giving partial pressures of ammonia and hydrogen sulfide for temperatures from 80 to 260 F over a range of liquid concentration. 

This calculation is for spherical micelles, but a similar calculation could be used to obtain estimates of salt concentrations for ionic wormlike micelles. Such salt concentrations for wormlike micelles are expected to be increased in comparison to spherical micelles. In fact, the addition of counterions or a sufficient increase in surfactant concentration often leads to a transition from spherical micelles to wormlike micelles. As the free counterion concentration in solution increases, so does the counterion binding. As a result, electrostatic repulsion between the charged head-groups is increasingly shielded and the mean cross-sectional (effective) headgroup... 

Calculated concentrations, using (4.9), for the various components, surfactant monomers, counter-ions and micelles, for the case of CTAB micellization (with a cmc of 0.9mM), is shown in Figure 4.5. Clearly, the micelle concentration increases rapidly at the cmc, which explains the sharp transition in surfactant solution properties referred to earlier. It is also interesting to note that the law of mass action (in the form of equation 4.9) predicts an increase in counterion (Br ions) concentration and a decrease in free monomer concentration above the cmc. It has been proposed that for ionic surfactants, a useful definition of the cmc would be... 

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