Phosphate Glass Chain Lengths

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. 

Chemical proof that the anions in Graham s salt arc linear chains of P04 tetrahedra follows from the observation that arsenato-phosphate glasses analogous to Graham s salt (see Section VI) are broken down on aqueous hydrolysis into ortho arsenate and mixture of low molecular polyphosphates with a mean chain length dependent only on the P As ratio in the glass (295, 308). 

Very recently, Brown and co-workers65 published a concise paper in which they presented short review of the possibilities and limitations of the refocused INADEQUATE and refocused INADEQUATE spin-echo (REINE) NMR spectroscopy. The authors discussed how these experiments can be used for obtaining various structural parameters, for example identification phosphate units, phosphate chain length, 2/p p couplings and correlation with chemical shift. It is worthy of note that the approach presented there can be applied not for phosphate glasses alone. 

The average chain lengths of polyphosphates produced by dissolving the glasses of calcium phosphate-silicates were determined by the pH titration method. As shown in Table III, the average chain length decreases with an increase of the silicon dioxide in the glass. 

Fig. 1. Average chain lengths of polyphosphates derived from glasses with various P/Si ratios (37). A Experimental values for the NaP0s-Na2Si03 system. B Experimental values for the NaP03-Si02 system. C Values calculated for the NaP03-Na2Si0s system assuming that there is no interaction between a sodium phosphate glass and silicon dioxide in a melted state.

The glasses of sodium meta-arsenate-phosphates are very rapidly dissolved in water, but their crystals, especially those with higher P/As ratios, are very slowly dissolved. As will be discussed in the latter part of this section, a greater part of As-O-P linkages of meta-arsenate-phosphates are cut by hydrolysis as soon as they are dissolved in water. As a result of the hydrolysis of As-O-P linkages, a mixture of polyphos-phates with relatively short chain lengths is produced. When the chemical compositions of the polyphosphates thus produced are determined, the secondary hydrolysis of the polyphosphates should be pre-... 

A (b) The Ag/P Ratios of the Silver Polyphosphates. Experimental values of Ag/P ratios of silver polyphosphates derived from the crystals and the glasses of sodium meta-arsenate-phosphates are shown in Table XIX. Also shown are calculated values of the Ag/P ratios, which are obtained on the assumption that the average chain length of the silver polyphosphate is equal to the P/As ratio of the initial sodium meta-arsenate-phosphate. The two sets of values for Ag/P ratios indicate a good agreement. Experimental values for the average chain length, w, of the silver polyphosphates are calculated from the experimental values for Ag/P ratios by the use of equation 15 (Table XIX). 

Example 1. Assume we choose to form an average chain length near 50 or above when the aluminum phosphate glass is cured to cause crystallization of AIPO4. From equation (3.1)... 

A number of spectacular applications in structural studies of glasses were reported employing the INADEQUATE experiment.174 175 The basic sequence and different modifications were discussed in Section 2.2.2. Smith and coworkers176 used 31P refocused INADEQUATE MAS NMR experiment for searching the length of phosphate chain and quantification of crystalline phases in ternary sodium calcium phosphate ceramic of composition (CaO)0.4(Na20)0.i(P205)o.5. 

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