Ultraphosphates crystalline

The ultraphosphates are situated between P O q and the metaphosphates. These comparatively Htde-known, highly cross-linked polymers contain at least some of the phosphoms atoms as triply coimected branching points. This stmctural feature is quite unstable toward hydrolysis. Ultraphosphates undergo rapid decomposition upon dissolution. In amorphous ultraphosphates, the cross-linking is presumably scattered randomly throughout the stmctural matrix in contrast, crystalline ultraphosphates have a regular pattern. 

Condensed phosphates are derived by dehydration of acid orthophosphates. The resulting polymeric stmctures are based on a backbone of P—O—P linkages where PO tetrahedra are joined by shared oxygen atoms. The range of materials within this classification is extremely broad, extending from the simple diphosphate, also known as pyrophosphate, to indefinitely long-chain polyphosphates and ultraphosphates (see Table 1). Both weU-defined crystalline and amorphous materials occur among the condensed phosphates. 

Ultraphosphates are defined as phosphates in which the ratio Me1 is less than unity. The existence of such compounds, e.g. Ag2P60,o, PbP6Oio and Ca2PeOi7, was detected in the corresponding melts some time ago (165) and the compounds CaP4Ou and CasPeOn were described as crystalline solid phases (136). In such compounds P04 tetrahedra must be present which are linked by three of their O atoms with neighboring P atoms (349). 

Determine whether or not cross-1 inked potassium Kurrol s salt contains cross-links in the crystalline phase and is indeed a crystalline ultraphosphate or does the salt obey the phase diagram for the two component K2O-P2O5 system. 

Cross-linked potassium Kurrol s is prepared in exactly the same manner as the calcium polyphosphate fibers. The only difference is potassium is substituted for calcium in the preparation of potassium Kurrol s salt. There is strong reason to question whether or not the strange solution behavior of cross-linked potassium Kurrol s salt is a result of cross-linking in the crystalline phase or is a result of ultra long polyphosphate chains mixed with an independent ultraphosphate phase that contains the required cross-linking. All of the observations which led to the belief that the molecule-ions in the crystalline phase were cross-linked can be explained based upon the two phase model where the ultraphosphate phase is rather quickly degraded in aqueous solutions to form acidic groups. 

The evidence for cross-linking in crystalline inorganic polyphosphates to convert them to crystalline ultraphosphates with small degrees of cross-linking is circumstantial rather than direct. This is not to question the triply-linked phosphate groups in the... 

Three possibilities exist when a salt with a polyphosphate x-ray pattern crystallizes from a melt containing an excess of phosphorus pentoxide. 1. The phosphorus pentoxide is incorporated into the polyphosphate chains converting the chains to crystalline ultraphosphates. 2. The excess phosphorus pentoxide does not enter the polyphosphate crystal structure, but forms an amorphous phase between the crystals of polyphosphate. The amorphous phase is not detected by x-ray. 3. The excess phosphorus pentoxide does not enter the crystal structure of the polyphosphate, but forms as an ultraphosphate between the crystalline polyphosphate crystals as a eutectic phase. (This latter case is precisely what happens in the calcium sodium ultraphosphate system from which calcium phosphate fibers are grown (21) and the phase diagram of Hill et. al. is obeyed as it should be.)... 

In the potassium Kurrol s salt phase system the crystalline analogues to Ca2P60i7 or CaP O i were not found by a literature search. (22) The amorphous potassium ultraphosphate systems have been studied. (23) Amorphous condensed phosphates are seldom, if ever, single compounds, but are a random mixtures of compounds and can be large and very complex. If the ultraphosphates were embedded between crystals of pure potassium phosphate fibers they would be very difficult to detect. 

It is concluded that the crystalline phase of potassium Kurrol s salt is not cross-linked. The mixture of very long chain polyphosphates and amorphous ultraphosphates dissolve to form highly viscous aqueous solutions. It is also concluded that the seed crystals aiding the growth of ultra long polyphosphate molecule-ions is built up from small segments of polyphosphate formed in a phosphate melt. 

Depending on the cation, crystalline salts of the cyclic meta- and ultraphosphates... 

Comparatively few ultraphosphates have as yet been synthesised and isolated as crystalline individuals, although an almost endless number of structures are theoretically possible with triply-linked PO4 tetrahedra. In this respect they rival the great variety of the much more stable silicate mineral structures which are based on condensed Si04 tetrahedra sharing up to all four of their comers with neighbouring tetrahedra. 

Heating alkali metal or alkaline earth metal dihydrogen phosphates produces polymeric salts (cyclic metaphosphates and linear polyphosphates) and cross-linked polyphosphates (ultraphosphates), depending on temperature and the presence of other ingredients (11,12). This complex group of polymers includes materials with crystalline, glass-like, fibrous, or ceramic properties as well as some with thermoplastic and thermoset characteristics some are useful as binders for metals, ceramics, and dental restorations. Reviews are available on glasses (12,13), crystalline compounds (14), and polyphosphate fibers (15). 

There is no question whether or not cross-linking is a necessary part of a system with an ultraphosphate composition. It is a structural-composition requirement, but where does cross-linking reside in potassium Kurrol s salt crystallized in ultraphosphate melts Are cross-links contained in crystalline phases and cause the formation of crystalline potassium ultraphosphates, or does a salt obey its phase diagram for a two-component K2O-P2O5 system (See Figure 3.1.)... 

Let us approach question 1 first. Assume that when preparing cross-linked Kurrol s salt, all phosphoric oxide added to crystalline [KP03]n enters potassium Kurrol s salt as a cross-linking agent. Assume that all phosphate crystallizes to cross-linked Kurrofs salt. If these assumptions are true, cross-linked Kurrol s salt is an ultraphosphate and not a polyphosphate because the M2O-P2O5 must be less than unity. Moreover, there must be two different compounds present in a phase system exhibiting different X-ray patterns. 

Two different potassium Kurrofs salt compounds, one cross-linked and the other linear chains, do not appear in this phase diagram, nor could a crystalline compound other than potassium Kurrofs salt be found in ultraphosphate areas of this system. Other choices are (1) an amorphous phase is formed, or (2) some new undetected crystalline potassium ultraphosphate is formed. No other crystalline potassium ultraphosphates are known, and no new ones were discovered in this work. Both phase studies and X-ray analyses suggest that an amorphous phase is formed in all potassium Kurrofs salt systems crystallized in ultraphosphate phase systems. 

A question of interest arises in predicting ratios of amorphous phase to crystalline phase in a Kurrol s salt system prepared in ultraphosphate regions. Since... 

Kurrol s salt is very slowly soluble in distilled water and ultraphosphate glasses are infinitely and quickly soluble in water, it is possible to estimate the quantity of amorphous phase contained in a well-crystallized Kurrol s salt system. Sample mixtures of crystalline and amorphous phases are first milled to a fine powder and then any soluble phase dissolved from crystalline phases with distilled water. The aqueous filtrate solution is then analyzed both for ratios and concentrations of dissolved phosphates. The soluble part of a sample should be amorphous since potassium Kurrol s salt is almost insoluble under these conditions. 

No potassium ultraphosphates in the composition range 1 > K2O/P2O5 > 0.5 are crystalline. 

Although the above approach is easy and straightforward, care must be exercised when applying it to more complex systems. Growth of calcium polyphosphates in sodium ultraphosphate melts is a three-component system rather than two, even when all water is driven from the system. It is recommended that the simpler method of assuming all sodium and no calcium reported to an amorphous phase (as was done in Chapter 6, Section 6.5) be used to estimate ratios of amorphous to crystalline phases or portions of a melt that is expected to be water soluble when the melt is fully crystallized. These estimates will not be precise, but are useful in estimating potential yields. 

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