Anion hydration, phosphate

According to this mechanism, the reaction rate is proportional to the concentration of hydronium ion and is independent of the associated anion, ie, rate = / [CH3Hg][H3 0 ]. However, the acid anion may play a marked role in hydration rate, eg, phosphomolybdate and phosphotungstate anions exhibit hydration rates two or three times that of sulfate or phosphate (78). Association of the polyacid anion with the propyl carbonium ion is suggested. Protonation of propylene occurs more readily than that of ethylene as a result of the formation of a more stable secondary carbonium ion. Thus higher conversions are achieved in propylene hydration. 

When aluminium is immersed in water, the air-formed oxide film of amorphous 7-alumina initially thickens (at a faster rate than in air) and then an outer layer of crystalline hydrated alumina forms, which eventually tends to stifle the reaction In near-neutral air-saturated solutions, the corrosion of aluminium is generally inhibited by anions which are inhibitive for iron, e.g. chromate, benzoate, phosphate, acetate. Inhibition also occurs in solutions containing sulphate or nitrate ions, which are aggressive towards iron. Aggressive anions for aluminium include the halide ions F ,... 

Very interesting behavior of incorporating anions can be observed when a multicomponent electrolyte is used for oxide formation. Here, anion antagonism or synergism can be observed, depending on the types of anions used. The antagonism of hydroxyl ions and acid anions has been observed in a number of cases. Konno et a/.181 have observed, in experiments on anodic alumina deterioration and hydration, that small amounts of phosphates and chromates inhibit oxide hydration by forming monolayer or two-layer films of adsorbed anions at the oxide surface. Abd-Rabbo et al.162 have observed preferential incorporation of phosphate anions from a mixture of phosphates and chromates. 

In electrochemistry, the effect of acidic anions on the electrode interface is important. The interaction of the adsorbed water molecules is relatively weak with perchlorate ions (Cl04 ) and fluoride ions (F ), intermediate with chloride ions (Cl ) and sulfate ions (S04 "), and relatively strong with phosphate ions (P04 ") and bromide ions (Br ). In a series of halogen anions, fluoride ions (F ), which interact weakly with the metal surface, are adsorbed as hydrated ions (H3O F ) chloride and bromide ions (Cl and Br ), which interact strongly with the metal surface, are adsorbed as dehydrated ions (Cl M and Br M). 

For anionic monolayers, the reversal of the tt-A isotherms can be explained in terms of a competition between the anionic head groups and the alkali metal cations for molecules of water. If a modified Stern-type model of the plane interface is assumed, this interface will be composed of distinct adsorption sites, with counterions (cations) of finite size that can adsorb on these sites if the standard free energies of adsorption are favorable. If the anionic head group is more polarizable than water, as with carboxylic acids or phosphates, the hydration shell of the cation is incompletely filled, and the order of cation sizes near the interface is K+ > Na+ > Li+. When the polarizability of the anionic group is less than that of water, as with the sulfates, the lithium cation becomes the most hydrated one, and the order of cation sizes becomes Li+ > Na+ > K+. 

---