Soluble orthophosphate

While broad agreement existed on the forms and limits of detectability required for the adequate biological interpretation of nitrogen analyses there was less meeting of the biological minds on the required attributes of phosphorus form differentiation. It was pointed out that the present ability to make interpretive judgments from phosphorus data was limited so that differentiation beyond total P, soluble P, and soluble orthophosphate was probably not justifiable. The detectable limits recommended for the analysis of P forms ranged between 3 to 10 fig, P/hter. 

The amount of orthophosphate which could be recovered from a sample seemed to depend largely on how long and how vigorously the sample was shaken during the analysis. It appeared that orthophosphate which was either adsorbed on or associated with the sediment was being released into solution. Since it was also found that sahnity variations affected color development and that large turbidity corrections that were necessary when samples high in suspended sohds were analyzed, it was decided that only a soluble orthophosphate determination was possible. 

Total Soluble Phosphorus. Total soluble phosphorus includes soluble orthophosphate (soluble condensed phosphate if present) and soluble organic phosphorus. A membrane-filtered sample is treated to release phosphorus as orthophosphate from combination with organic matter and condensed phosphates by some form of oxidation (and hydrolysis). 

The results of the preservation study (Figure 2) show that for long periods (one month) the best preservation technique was storage at —10°C. with 40 mg. Hg Vliter. For storage periods of a few days, any of the methods, except storage at 4°C. with CHCI3, were satisfactory. The chloroform treatment produced a dramatic reduction in soluble orthophosphate and an accompanying rise in total insoluble phosphorus. 

Lead orthophosphate, Pb3(P04)2, is a white insoluble compound, which, like lead apatite, Pbio(P04)g(OH)2, can be reduced by hydrogen at comparatively low temperatures (4.7). Lead orthophosphate is one of the least soluble lead salts (Table 5.22) and the least soluble orthophosphates (Table 5.12). It exists as monoclinic a-Pb3(P04)2, rhombohedral 3-Pb3(P04)2, or as an apatite-type structure, Pb9(P04)g(0H)2 (Table 5.20). Also known are Pbio(P04)gO, Pb3(P04)2-Pb0 and Pb3(P04)2-5Pb0. 

Phosphides and other unreactive inorganic compounds are best detected and estimated as orthophosphates after suitable oxidation. Insoluble rocks and minerals are usually fused with sodium carbonate or sodium peroxide to secure conversion into soluble orthophosphates. Alternatively dissolution with HF and HCIO4 may be employed. 

If not easily hydrolysed to soluble orthophosphates, condensed phosphates are usually best identified and estimated by the techniques of paper chromatography or x-ray diffraction (XRD) (see below). Several purely chemical methods are, however, known. 

Orthophosphate Hquid mixtures are ineffective as micronuttient carriers because of the formation of metal ammonium phosphates such as ZnNH PO. However, micronutrients are much more soluble in ammonium phosphate solutions in which a substantial proportion of the phosphoms is polyphosphate. The greater solubiHty results from the sequestering action of the polyphosphate. The amounts of Zn, Mn, Cu, and Fe soluble in base solution with 70% of its P as polyphosphate are 10 to 60 times their solubiHties in ammonium orthophosphate solution. When a mixture of several micronutrients is added to the same solution, the solubiHty of the individual metals is lowered significantly. In such mixtures the total micronuttient content should not exceed 3% and the storage time before precipitates appear may be much shorter than when only one micronuttient is present. 

C2HgNg H4O2P2 (60). The pyrophosphate is reported to be only soluble to the extent of 0.09 g/100 mL water, whereas melamine orthophosphate is soluble to 0.35 g/mL. The pyrophosphate is the most thermally stable. Melamine orthophosphate is converted to the pyrophosphate with loss of water on heating. AH three are available as finely divided soflds. AH are used commercially in flame-retardant coatings (qv) and from patents also appear to have utihty in a wide variety of thermoplastics and thermosets. A detaHed study of the thermal decomposition of the these compounds has been pubHshed (61). 

The solubihty of a number of sodium orthophosphates is depicted in Figure 7. Some of the sodium phosphates dissolve or precipitate incongmendy, affording a different Na20/P20 ratio in the solution phase from that of the soHd phase. Sodium phosphates that precipitate are also a function of the temperature. As the temperature increases, the sodium phosphates that may precipitate from solution tend to be anhydrous or lower hydrates than those equiUbrium sodium phosphate phases at lower temperature. Whereas most of the phases in Figure 7 represent congmentiy soluble sodium phosphates, soHd phases appear or disappear upon changes in temperature. 

Uses. The principal use of monosodium phosphate is as a water-soluble soHd acid and pH buffer, primarily in acid-type cleaners. The double salt, NaH2P04 H PO, referred to as hemisodium orthophosphate or sodium hemiphosphate, is often generated in situ from monosodium phosphate and phosphoric acid in these types of formulations. Mixtures of mono- and disodium phosphates are used in textile processing, food manufacture, and other industries to control pH at 4—9. Monosodium phosphate is also used in boiler-water treatment, as a precipitant for polyvalent metal ions, and as an animal-feed supplement. 

Potassium Phosphates. The K2O—P20 —H2O system parallels the sodium system in many respects. In addition to the three simple phosphate salts obtained by successive replacement of the protons of phosphoric acid by potassium ions, the system contains a number of crystalline hydrates and double salts (Table 7). Monopotassium phosphate (MKP), known only as the anhydrous salt, is the least soluble of the potassium orthophosphates. Monopotassium phosphate has been studied extensively owing to its piezoelectric and ferroelectric properties (see Ferroelectrics). At ordinary temperatures, KH2PO4 is so far above its Curie point as to give piezoelectric effects in which the emf is proportional to the distorting force. There is virtually no hysteresis. 

Evolution of ammonia from a boiling dilute solution of diammonium phosphate gradually reduces the pH. This process is used commercially to control the precipitation of alkaH-soluble—acid-insoluble coUoidal dyes on wool. Other ammonium orthophosphate salts of interest are ammonium hemiphosphate [28537-48-6] NH4H2PO4 H3PO4, and its hydrate [28037-74-3], as well as the trihydrate [78436-07-4] of DAP. 

Alkali Meta.IPhospha.tes, A significant proportion of the phosphoric acid consumed in the manufacture of industrial, food, and pharmaceutical phosphates in the United States is used for the production of sodium salts. Alkali metal orthophosphates generally exhibit congment solubility and are therefore usually manufactured by either crystallisation from solution or drying of the entire reaction mass. Alkaline-earth and other phosphate salts of polyvalent cations typically exhibit incongment solubility and are prepared either by precipitation from solution having a metal oxide/P20 ratio considerably lower than that of the product, or by drying a solution or slurry with the proper metal oxide/P20 ratio. 

Silver Phosphates. Silver phosphate [7784-09-0], or silver orthophosphate, Ag3P04, is a bright yellow material formed by treating silver nitrate with a soluble phosphate salt or phosphoric acid. Silver pyrophosphate [13465-97-9], is a white salt prepared by the addition of a soluble... 

Both iron and aluminum are particulady troublesome because of their abiUty to act as coagulants. Also, their soluble and insoluble hydroxide forms can each cause precipitation of some water treatment chemicals, such as orthophosphate. Airborne contaminants usually consist of clay and dirt particles but can include gases such as hydrogen sulfide, which forms insoluble precipitates with many metal ions. Process leaks introduce a variety of contaminants that accelerate deposition and corrosion. 

Na[Sb(OH)g], respectively. The latter compound is one of the least soluble sodium salts known and is useful in sodium analysis. Numerous polyantimonate(V) derivatives are prepared by heat treatment of mixtures of antimony trioxide and other metal oxides or carbonates. Of these, K Sb O [12056-59-6] and K Sb O [52015-49-3] have been characterized by x-ray. These consist of three-dimensional networks of SbO in which corners and edges are shared with K" ions located in tunnels through the network (23). Simple species such as SbO and Sb20 2, analogous to orthophosphate and pyrophosphate, apparendy do not exist. 

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. 

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. 

Orthopedic devices, 3 721-735 joint replacement, 3 727-735 Orthopedic marrow needles, 3 743-744 Orthophosphate (PO4), in soil, 11 112 Orthophosphates, 18 830-841 20 637 magnesium, 18 839 manufacture of, 18 853-855 Orthophosphate salts, 18 836 Orthophosphoric acid, 18 815, 817-826 condensation of, 18 826 properties of, 18 817-819 solubility of boron halides in, 4 140t orf/zo-phthalic resins, 20 101, 113 formulation of, 20 102 Orthorhombic crystal system, 8 114t Orthorhombic phosphorus pentoxide, 19 49 Orthorhombic structure, of ferroelectric crystals, 11 95, 96 Orthorhombic symmetry, 8 114t Orthosilicate monomers, in silicate glasses, 22 453... 

Table I. Effects of 17o Orthophosphate, Tripolyphosphate, and Hexametaphosphate on Calcium Solubility from Ground Beef or Soy Protein Concentrate Subjected to In Vitro Gastric and Gastrointestinal Digestions...

Although all three phosphates tested caused substantial decreases in calcium solubility following the gastric digestion, this trend was reversed upon completion of the second phase of the digestion. Only orthophosphate caused a decrease in calcium solubility from the soy following the complete digestion, while all three phosphates enhanced calcium solubility from the meat. Thus, phosphates are unlikely to interfere with calcium absorption due to the formation of insoluble salts in the gut. 

Condensed (poly) phosphates may exert different effects on calcium utilization than the aforementioned effects of simple (ortho-) phosphates. Polyphosphates have a much greater affinity for calcium than do orthophosphates, and soluble calcium-polyphosphate complexes are readily formed in the gastric and intestinal environments. In addition, polyphosphates must be hydrolyzed by an intestinal alkaline phosphatase (27) prior to absorption. We have found polyphosphates to be incompletely (80.5%) hydrolyzed to orthophosphate during the digestive process in young adult males when calcium intake was low only 56% of a 1 g phosphorus supplement was absorbed from a polyphosphate sources as compared to 71% from an orthophosphate source (5). 

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