Metal ions hardness

Because OH is hard and is soft, OH is more likely to form insoluble salts with 34- transition metal ions (hard) and S is more likely to form insoluble salts with 24- transition metal ions (borderline or soft). 

Conversely, very non-poisonous or inert metals are needed when artificial prostheses have to be introduced during surgery and so metals which, if dissolved, would give soft ions are chosen, e.g. gold, silver, tantalum, and platinum or their alloys. Because they give soft ions there is negligible tendency for those metals to give up electrons to form soft metal ion—hard solvent bonds (water is hard). 

Class A Metal Ions ( hard ) - These are small, compact and not very polarizable this group includes alkali metal ions, alkaline earth metal ions, and lighter and more highly charged metal ions such as Ti4+, Fe3+, Co3+, Al3+. They show a preference for ligands (bases) also small and less polarizable. 

Hardness increases with electronegativity and with positive charge. Thus, for the halogens the order is E > CF > Br > F, and for second-row anions, E > HO > HjN- > H3C. Eor cations, hardness decreases with size and increases with positive charge, so that H+ > Li+ > Na+ > K+. The proton, lacking any electrons, is infinitely hard. In solution it does not exist as an independent entity but contributes to the hardness of some protonated species. Metal ion hardness increases with oxidation state as the electron cloud contracts with the removal of each successive electron. All these as... 

A widely applied concept in the evaluation of metal ion interaction is that of hardness and softness. In general terms, hard and soft suggest the resistance to deformation in response to electric forces. Thus, hard ions have greater resistance to deformation of the electron cloud and soft ions have lesser resistance to deformation. Quantitative scales for metal ion hardness or softness were developed in the 1960s, starting with the HSAB theory developed by Pearson (see Section 3.3.1). 

Such water, and also that containing salts of multipositive metals, (usually sulphates), is said to be hard since it does not readily produce a lather with soap. Experiments with alkali metal salts can be performed to verify that the hardness is due to the presence of the multipositive metal ions and not to any of the anions present. The hardness due to calcium and magnesium hydrogencarbonates is said to be temporary since it can be removed by boiling ... 

METHODS FOR REMOVING THE METAL IONS RESPONSIBLE FOR HARDNESS IN WATER... 

Description of the Method. The operational definition of water hardness is the total concentration of cations in a sample capable of forming insoluble complexes with soap. Although most divalent and trivalent metal ions contribute to hardness, the most important are Ca + and Mg +. Hardness is determined by titrating with EDTA at a buffered pH of 10. Eriochrome Black T or calmagite is used as a visual indicator. Hardness is reported in parts per million CaCOs. 

The chemistry of Th(IV) has expanded greatly since the mid-1980s (14,28,29). Being a hard metal ion, Th(IV) has the greatest affinity for hard donors such as N, O, and light haUdes such as F and CF. Coordination complexes that are common for the t7-block elements have been studied for thorium. These complexes exhibit coordination numbers ranging from 4 to 11. 

Hardness can also be calculated by summation of the individually deterrnined alkaline earths by means of atomic absorption analysis. Basic samples must be acidified, and lanthanum chloride must be added to minimise interferences from phosphate, sulfate, and aluminum. An ion-selective electrode that utilizes ahquid ion exchanger is also available for hardness measurement however, this electrode is susceptible to interferences from other dissolved metal ions. 

The presence of a sufficientiy strong chelating agent, ie, one where K in equation 26 is large, keeps the concentration of free metal ion suppressed so that pM is larger than the saturation pM given by the solubiUty product relation (eq. 29) and no soHd phase of MX can form even in the presence of relatively high anion concentrations. The metal is thus sequestered with respect to precipitation by the anion, such as in the prevention of the formation of insoluble soaps in hard water. 

Trisodium phosphate [7601-54-9] trisodium orthophosphate, Na PO, is an important constituent of hard-surface cleaners including those for ceramic, metal, or painted surfaces. It may be used with soaps, surfactants, or other alkaHes. It precipitates many heavy-metal ions but does not sequester to form soluble chelates. It is thus a precipitant builder and additionally an alkaH. 

Calcium ion enters the system not ordy in the form of water hardness but also in the form of calcium salts contained in the sod. Other heavy-metal ions such as aluminum and ferric iron may also be present in the sod, and must be removed by an appropriate budder to achieve good sod removal. Effective budders for cotton washing are those for which the calcium dissociation constant, expressed as or —logif -, is >4 and preferably >7 (33). 

In a study of the adsorption of soap and several synthetic surfactants on a variety of textile fibers, it was found that cotton and nylon adsorbed less surfactant than wool under comparable conditions (59). Among the various surfactants, the cationic types were adsorbed to the greatest extent, whereas nonionic types were adsorbed least. The adsorption of nonionic surfactants decreased with increasing length of the polyoxyethylene chain. When soaps were adsorbed, the fatty acid and the aLkaU behaved more or less independently just as they did when adsorbed on carbon. The adsorption of sodium oleate by cotton has been shown independently to result in the deposition of acid soap (a composition intermediate between the free fatty acid and the sodium salt), if no heavy-metal ions are present in the system (60). In hard water, the adsorbate has large proportions of lime soap. 

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