Polyphosphate-Cation Complexes

The Biochemistry of Inorganic Polyphosphates I. S. Kulaev, V. M. Vagabov and T. V. Kulakovskaya 

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R. P. Huang, and R. N. Reusch (1995). Genetic competence in Escherichia coli requires poly-beta-hydroxybutirate calcium polyphosphate membrane complex and certain divalent cations. J. 

Complex Ion Formation. Phosphates form water-soluble complex ions with metallic cations, a phenomenon commonly called sequestration. In contrast to many complexing agents, polyphosphates are nonspecific and form soluble, charged complexes with virtually all metallic cations. Alkali metals are weakly complexed, but alkaline-earth and transition metals form more strongly associated complexes (eg, eq. 16). Quaternary ammonium ions are complexed Htde if at all because of their low charge density. The amount of metal ion that can be sequestered by polyphosphates generally increases... 

Sequestration forms the basis for detergent and water-treatment appHcations of polyphosphates. Sequestration of hardness ions by sodium tripolyphosphate used in detergent formulations prevents the precipitation of surfactants by the hardness ions. Sodium polyphosphate glass (SHMP) may be added to water system to prevent the formation of calcium or magnesium scales by reducing the activity of the hardness ions. However, if the ratio of cation to polyphosphate is too high at a given pH, insoluble precipitates such as may result instead of the soluble polyphosphate complexes. The... 

Various polyphosphates are effective sequestering agents under appropriate conditions. The best known of these is sodium hexametaphosphate (10.14), the cyclic hexamer of sodium orthophosphate. Further examples are the cyclic trimer sodium trimetaphosphate (10.15), as well as the dimeric pyrophosphate (10.16), the trimeric tripolyphosphate (10.17) and other linear polyphosphates (10.18). All of these polyanions function by withdrawing the troublesome metal cation into an innocuous and water-soluble complex anion by a process of ion exchange as shown in Scheme 10.7 for sodium hexametaphosphate. Hence these compounds are sometimes referred to as ion-exchange agents. 

This Group IIA (or Group 2) element (atomic symbol, Ca atomic number, 20 atomic weight, 40.078 electronic configuration = ls 2s 2p 3s 3p 4s ) loses both As electrons to form a divalent cation of 0.99A ionic radius. Ionic calcium combines readily with oxygen ligands (chiefly water, phosphates, polyphosphates, and carbox-ylates) to form stable metal ion complexes. Ca under-... 

As in the hydrolysis of polyphosphates (Sections IV,B, and IV,E,1), hydrolysis of the trimetaphosphate anion is eatalytically accelerated by added cations. Their effect is attributed, as in the ease of the polyphosphates, to the formation of complex ions of the type CafPsOs)- or Ba(P309)- and Na PsOs)3 (130, I44, 146). 

Partial replacement of cations when using excess of alkali phosphate was at first interpreted by supposing that the cations were in part bound as a complex with the polyphosphates. This led to incorrect conclusions regarding the molecular size of polyphosphates, which persist even now in the designation of Graham s salt as hexametaphosphate. The correct interpretation of these very complicated relationships stems from the discovery that in dilute solutions of polyphosphates even salts of the alkali metal ions are only partially dissociated—up to a maximum of 30-40% 260, 280, 282, 288, 333). The degree of dissociation a for alkali polyphosphates depends on many factors. Thus, for example, the effective degree of... 

The effect of cations on the rate of hydrolysis of cycio-tri- and cvc/o-tetra-phosphates has also been studied.260 Alkali metal cations retarded the hydrolysis of both cyclo-phosphates in acidic solution but accelerated the reaction in basic solution. The catalytic activity followed the sequence Li+ > Na+> K+. Mg11, Ca11 and Ni11 retarded the hydrolysis of the cyclo-phosphates in the pH range 1.0-2.0 or 1.0-2.7, but accelerated the reaction at pH>3. Catalysis by Cu11 is observed above pH 2, while Alni has a marked catalytic effect at pH 1 and 2. A variety of other papers have appeared dealing with metal ion-promoted hydrolysis of polyphosphates.261-264 However, the mechanistic details of these reactions remain unclear and the catalytically active complexes remain undefined. 

The linear polyphosphates and cyclophosphates are hydrolysed extremely slowly at neutral pH and room temperature in comparison with other polyacids such as polyarsenates and polyvanadates, and are unique in this respect. The half-hydrolysis time for the P-O-P bonds in linear polyphosphates at pH 7 and 25 °C is several years (Van Wazer, 1958). The rate of hydrolysis of these bonds is increased by raising the temperature, reducing the pH, and by the presence in the solution of colloidal gels and complex cations. The hydrolysis of these bonds is dependent on the ionic strength of the solutions (Van Wazer, 1958). 

Phytate. Phytic acid is an organic polyphosphate found widely in plants, particularly cereals, nuts and legumes. It has been shown to complex with various divalent cations in the gastrointestinal tract and thus reduce mineral bioavailability (33,44,52). Davis et al. (53) reported that feeding a diet based on isolated soybean... 

Polyphosphates are able to bond different cations into complexes keeping them in this way in the soluble form even in the presence of some anions with which they would otherwise form insoluble compounds. Therefore, they find wide application possibilities in the textile industry, laundries. 

The artificial model of a cell membrane was prepared using biomimetic PHB and the calcium polyphosphate complex (poly-P) incorporated into lipid bilayers of 1,2-dierucoylphosphatidylcholine. It was found that PHB/poly-P channels show high conductance for Ca and Na cations . 

Almost all metal cations will react with alkali metal polyphosphate solutions to form insoluble precipitates. Most of these are soluble in excess of polyphosphate as a result of complex ion formation. Polarographic and conductivity studies indicate the existence, in dilute solutions, of several complex pyrophosphate and triphosphate anions. These include (MP207) , where M = Cu, Co, Cd, Pb, Mg, Ni, Zn, and (MP2O7), where M = Al, Fe. Some crystalline double salts of type MM P207 which can be isolated are probably more correctly formulated as M+(M P207). ... 

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