Polyvalent Anions

To accomplish any separation of two cations (or two anions) of the same net charge, the stationary phase must show a preference for one more than the other. No variation in the eluant concentration will improve the separation. However, if the exchange involves ions of different net charges, the separation factor does depend on the eluant concentration. The more dilute the counterion concentration in the eluant, the more selective the exchange becomes for polyvalent ions. 

Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature (106), can be greatly accelerated by the addition of certain reducing agents or small amounts of polyvalent metal salts, or both (87). By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with a thermally initiated system (75—90°C). 

Ion Removal and Metal Oxide Electrodes. The ethylenediamine ( )-functional silane, shown in Table 3 (No. 5), has been studied extensively as a sdylating agent on siUca gel to preconcentrate polyvalent anions and cations from dilute aqueous solutions (26,27). Numerous other chelate-functional silanes have been immobilized on siUca gel, controUed-pore glass, and fiber glass for removal of metal ions from solution (28,29). 

Ghelants and Precipitation Inhibitors vs Dispersants. Dispersants can inhibit crystal growth, but chelants, such as ethylenediaminetetraacetic acid [60-00-4] (EDTA), and pure precipitation inhibitors such as nitrilotris(methylene)tris-phosphonic acid [6419-19-8], commonly known as amino trismethylene phosphonic acid (ATMP), can be more effective under certain circumstances. Chelants can prevent scale by forming stoichiometric ring stmctures with polyvalent cations (such as calcium) to prevent interaction with anions (such as carbonate). Chelants interact... 

In solutions, the counterions of poly electrolytes are HjO (for the polyacids) and OH (for the polybases), cations such as K+ and Na, or anions such as Cl (for the polysalts). The addition of polyvalent counterions (such as Ca, Mg, Cu, AT ) produces ionic cross-linking interfering with solubility The polyelectrolyte precipitates and may be redissolved upon addition of a strong acid (such as HCl). This can be regarded as a special case of ion exchange. 

For chemical and electrostatic reasons, some ions are bound more strongly than others. Polyvalent cations are much more strongly bound than univalent cations the same holds for anions. Thus, when a solution containing Ca ions (hard water) is passed through a cation exchanger which is present in its Na or H+ form, the Ca ions are held back in it and the Na or H+ ions leave instead. Similarly, an anion exchanger in the OH form will retain Cl or SO ions from the solution. By a cation and anion exchanger in series, the former in the H+ and the latter in the OH... 

Special polymerization techniques are described [298] in which the polymerization is performed in an aqueous solution together with a polyvalent anionic salt in the presence of a water-soluble cationic polymer acting as a dispersant polymer. Furthermore, a seed polymer that is water soluble and a cationic polymer that is insoluble in the aqueous solution of the polyvalent anionic salt are present. 

Although it is possible, by the loss of several electrons, for certain metal atoms to form polyvalent cations upto a maximum valency of four (e.g., tin forms the tetravalent stannic ion, Sn4+), the formation of polyvalent anions is extremely difficult since for the acquisition of each additional electron the attractive force exerted by the nucleus on each individual electron becomes progressively smaller. It is for this reason that the maximum valency for a simple anion is found to be two. 

Chitosan-clay bio-nanocomposites are very stable materials without significant desorption of the biopolymer when they are treated with aqueous salt solutions for long periods of time. In this way, they act as active phases of electrochemical sensors for detection of ions (Figure 1.8). The particular nanostructuration of the biopolymer in the interlayer region drives the selective uptake of monovalent versus polyvalent anions, which has been applied in electrode arrays of electronic tongues [132]. 

Because of its capacity to separate effectively cations and anions ion chromatography has also found application in chromatographic practice. A nonlinear model has been developed for the prediction of the retention of polyvalent weak acid anions in anion chromatography. In the case of strong acid anions containing only one acidic group eluted by a mobile phase with monoanionic additive, the retention can be described by... 

There are two general types of liquid-membrane ISEs, namely one which involves liquid-phase ion exchange, with the response being selective to the anion or cation under scrutiny (generally polyvalent ions), while the other type involves... 

Sometimes the activities are replaced by braces, e.g., Uml = ML -, here we prefer the former formality. The estimation of y, values by equations such as Eqs. (2.38) and (2.43) is unreliable for ionic strengths above about 0.5 M for univalent cations and anions and at even lower ionic strengths for polyvalent species. Consequently, values of (3° are rarely calculated, except for very exact purposes (see Chapter 6). Instead, measurements of equilibrium concentrations of the species involved in the reaction are used in a medium of fixed ionic strength where the ionic strength, /, is defined as I=V2l. Ci Zi M [see Eq. (2.38)]. A solution of fully ionized CaCl2 of 0.5 M concentration has an ionic strength / = H (0.5 X 2 -I- 2 X 0.5 X (-1) ) or 1.5 M. Stability constants are reported, then, as measured in solutions of 0.1 M, 0.5 M, 2.0 M, etc., ionic strength. In practice. 

The ready formation of polyanions from poly(acrylic acid) grafts or of polycations from a polyaminated surface leads to a polyvalent anionic or cationic interface. Such interfaces can be modified ionically (as in ionic... 

In summary, the NS-300 membrane system actually comprises a family of membranes, with reverse osmosis properties determined by the isophthallc trimesic ratio. Exceptionally high fluxes are possible at high retentivity levels for dissolved salts containing polyvalent anions. This membrane type may find applications in the desalination of brackish sulfate ground waters or industrial... 

A comparative study of the products of dehydration of the dihydrogen monophosphates of polyvalent cations showed that the stable end-products for cations with ionic radii between 0.57 and 1.03 A. (Cu++, Mg++, Ni++, Co++, Fe++, Mn++, Zn"1-1", Cd++, A1+++) are tetrametaphosphates. When the cations are either larger or smaller the end-products of dehydration are crystalline high-molecular polyphosphates (Li+, Be++, K+, Rb+, Cs+, Ag+, Zn++, Cd++, Hg++, Ca++, Sr++, Ba++ Pb++, Cr+++, Fe+++, Bi+++). In the case of the alkali salts only sodium trimetaphosphate occurs as a condensed phosphate with a cyclic anion (304, 305). Up to the present, an alkali tctrametaphosphate has not been observed as the dehydration product of a dihydrogen monophosphate. Consequently, alkali tetrametaphosphates arc obtainable only indirectly. Reference is made later (Section IV,C,4) to the fact that the tetraphosphates of barium, lead, and bismuth are formed as crystalline phases from melts of the corresponding composition. There are also reports of various forms of several condensed phosphates of tervalent iron and aluminum (31, 242, 369). 

Not only hydrogen ions are relatively strongly bound to the pyrophosphate anion. Thus, for example, the salt BasPaCh, which is not easily soluble in water, is brought into solution by an excess of pyrophosphate. Furthermore, electrometric and conductimetric studies have established (16,I4O) that most polyvalent cations and also the T1+ ion (265) are bound in complexes of various sorts. Quantitative studies show that the formation constants for the calcium complexes of the P2O74- ion have values which are about a power of ten smaller than those for the corresponding complexes of the triphosphate anion (I48). (For data, see Section IV.B.S.)... 

Alkali oligophosphates are soluble in water either individually or when in the form of glass-like mixtures, and their solutions form precipitates with polyvalent cations which are soluble in excess of the phosphate. Their physical properties are described in Section IV,E,/,e since they are related to those of glasses of the type of Graham s salt with anions of large chain length. 

Ion bridging is a specific type of Coulombic interaction involving the simultaneous binding of polyvalent cations (e.g., Ca, Fe, Cu ) to two different anionic functional groups on biopolymer molecules. This type of ionic interaction is commonly involved in associative self-assembly of biopolymers. As a consequence it is also an important contributory factor in the flocculation (via bridging or depletion) of colloidal particles or emulsion droplets in aqueous media containing adsorbed or non-adsorbed biopolymers (Dickinson and McClements, 1995). 

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