Ferric Orthophosphate

Ferric orthophosphate.—The dihydrate, FeP04.2H20, occurs in nature as strengite (see p. 27), and may be obtained in the laboratory in crystalline form by heating a concentrated solution of ferric chloride with phosphoric acid in a sealed tube at 180° to 190° C. for several hours, when it separates out as small rose-coloured monoclinic crystals, of density 2 -74 at 15 ° C. The crystals closely resemble those of strengite, but the latter are rhombic. They are insoluble in nitric acid, but dissolve in hydrochloric acid.5... 

Ferric dihydrogen orthophosphate, Fe(H2P04)3, results when ferric oxide is dissolved in phosphoric acid until a precipitate begins to appear. The solution is washed free from excess of acid with ether and dried,3 whereby a pink crystalline powder is obtained. When boiled with water, it yields ferric orthophosphate, FeP04.2H20. 

Dietary iron level does not seem to affect the efficiency with which dietary iron is converted into hemoglobin when ferrous sulfate (Table 1) or when ferric orthophosphate (Table 2) is the primary source of dietary iron. This is also true for white bread (Table 2) however, the source of the iron in the enriched flour used in the bread is unknown. That the efficiency of converting food iron into hemoglobin is not affected by dietary iron concentration is important to bioavailability testing because it is often difficult to formulate diets with precise amounts of iron, especially when foods are the sources of iron. 

Among experiments, the variability of the efficiency of converting iron from ferrous sulfate into hemoglobin (Table 1) was much greater than when ferric orthophosphate (Table 2) was the iron source. This variability is disturbing since ferrous sulfate is commonly used as a reference source of iron for bioavailability experiments, as well as an iron supplement clinically. Typically, this variability is dealt with by expressing the hematinic responses of the unknowns relative to ferrous sulfate (Shah et al., 1979 Coccodrilli et al., 1976 Amine et al., 1972). 

Figure 7. Percentage ionization of iron additives predicted by buffers and actually found in foods. El, elemental iron FS, ferrous sulfate FOP, ferric orthophosphate SFEDTA, sodium ferric EDTA trihydrate. (Reproduced, with permission, from Ref. 43. Copyright 1981, Institute of Food Technologists.)...

This does not mean that the bioavailability of iron from all compounds containing both phosphorus and iron is low. Wood, et al. (12) and Theuer and his associates (7, 8) have found that the bioavailability of iron from sodium iron pyrophosphate and ferric pyrophosphate was greatly improved when the foods containing these salts were processed with heat and pressure (Table II). Such processing did not, however, improve the bioavailability of iron from ferric orthophosphate or ferrous sulfate. The reason for this effect is not known but sugars in the foods may have formed chelates with the iron that facilitated absorption. 

Ferric orthophosphate Fewic (IIQ orthophosphate. See Feme phosphate Fdric oxido... 

Synonyms Ferric orthophosphate Ferric (III) orthophosphate Feme (III) phosphate Ferriphosphate Iron (III) orthophosphate Iron phosphate Empirical FeO,P xH20 Frnnula FePO, xHjO, x = 1 -4... 

Synonyms Ferric orthophosphate Ferric (III) orthophosphate Ferric (III) phosphate ... 

Ferric orthophosphate can be prepared from ferrous orthophosphate by heating it with iron powder at 800°C (5.80). This compound forms a colourless octahydrale (vivianite, see above), which will partially oxidise in the air to form a complex blue-coloured compound which is probably an oxide phosphate of some kind. 

Certain metaphosphates, for example, Cr and U will, on heating, decompose to phosphorus pent-oxide and pyrophosphate (5.107), while ferrous pyrophosphate can be prepared by reducing ferric orthophosphate (5.108), and mercury pyrophosphate by simply heating the orthophosphate (5.109). 

Another very likely way of producing POP linkages is based upon reduction of iron in a ferric orthophosphate. There are many reducing agents that could have worked, but H2S is a likely candidate. It will be seen in Chapter 3 that much phosphate chemistry is dictated by M2O-P2O5 ratios. A ferric ion, Fe, is equivalent to 3 Ms while a ferrous ion, Fe, is equivalent to 2 Ms. 

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