Free hydration

Each entry has the % of total metal present as the free hydrated ion, then the ligands forming complexes, in decreasing order of expected concentration. For instance, in inorganic freshwater at pH 9, Ag is present as the free aquo ion (65%), chloro-complexes (25%), and carbonato-complexes (9%). 

In addition to soil solution, speciation of trace elements in water of the Nahr-Ibrahim river valley of Lebanon was studied with the AQUACHEM model. The results indicate that a high percentage of Pb and Zn is present as carbonate species, but in low percentages in free hydrated ion species. Cadmium exhibits as a high percentage of a free hydrated Cd2+. 

Soil solution to soil ratios also strongly affect distribution of some trace elements such as Zn speciation in arid and semi-arid soils. Fotovat et al. (1997) reported that the proportion of free hydrated Zn2+ to total Zn ranged from 20-65% at field capacity soil water content and decreased with increases in solution to soil ratios, while the proportion of Zn complexed with organic ligands increased dramatically in soils. However, solution to soil ratios do not strongly affect the distribution of Cu speciation in soil solution since Cu primarily occurs as organic complexes in these soil solutions. 

The misconception rests on a confusion between the thermodynamic scale of activities (which measures the tendency of an element to react) and the concentration scale for free, hydrated, ions. Consider for example the not uncommon situation where the free... 

Electro-analytical techniques have been used extensively in studies of natural waters. For example, ion-selective electrodes allow measurement of the activity of free hydrated ions in solution (species highly relevant in toxicity studies) and voltammetric methods (polarography and ASV) exhibit a high degree of selectivity (for highly labile species). Electro-chemical techniques also facilitate identification of the valency state of elements such as Fe, Cr, Tl, Sn, Mn, Sb, As, Se, Y U and I. For several other elements only one state is electro-chemically active, and redox state speciation becomes a special case of labile/inert species discrimination. The toxicity of an ion can vary with valency, for example, Cr is more toxic than Crm, while for As, Sb and Tl, the lower valency form is more toxic. 

Na+ and K+ exist in seawater nearly exclusively (<98%) as free hydrated ions ... 

Measurements of the total concentration of microelements in environmental samples provide little information on their bioavailability. In water, most studies of the susceptibility of fish to heavy metal poisoning have shown that the free hydrated metals ions are the most toxic [118-120]. Ions that are strongly complexed or associated with colloidal particles are usually considered to be less toxic. 

Fig. 4. Transfer of positive holes from bulk of a photocatalyst through the interface to aqueous solution phase. Surface-adsorbed and free (hydrated) hydroxyl radicals and its derivatives can be a possible form of positive holes.

Turner et al. (1981) and Byrne et al. (1988). Turner et al. (1981) used a database of stability constants for more than 500 metal complexes to calculate the inorganic speciation for 58 trace elements in model seawater at pH 8.2, 25 °C, and 1 atm. Byrne et al. (1988) extended this work by considering the influence of temperature and pH on speciation. The free hydrated divalent cation dominates the dissolved inorganic speciation of Zn(II) and the first transition series metals Mn(II), Co(II), and Ni(II). Strongly hydrolyzed trace metals include Be(II), Al(III), Fe(III),... 

The major species of soluble copper found in freshwater, seawater, and a combination of the two over a range of pHs is Cu, CulHCOs), and Cu(OH)2. At the pH values and carbonate concentrations characteristic of natural waters, most dissolved Cu(II) exists as carbonate complexes rather than as free (hydrated) cupric ions. 

The pK of proteinous acids is pK > 4. Thus the fraction of active site retaining a free proton will be 10-4 or less. Consequently, the true presentation of a proton, even in a protogenic site, is not as a free hydrated proton but as a covalently linked hydrogen atom. 

Carbon dioxide is dissolved in the molecular form as a free hydrated CO2 and is usually denoted by the symbol C02(aq). Slightly less than 1% reacts with water to form non-dissociated molecules of H2CO3. Carbon dioxide dissolved in water is called free carbon dioxide and this term is used for the sum of the concentrations of free hydrated CO2 and H2CO3. 

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