Ferrous orthophosphate

Ferrous orthophosphate occurs in nature in the octahydrated form as vivianite (see p. 26), Fe3(P04)2.8H20, in monoclinic crystals.8 When perfectly pure the crystals are colourless,9 but most specimens are tinged writh a greenish blue in consequence of slight oxidation.10 They melt at 1114° C.11... 

Ferrous pyrophosphate, Fe2P207, is obtained 5 as an unstable white powder by double decomposition of ferrous sulphate and sodium pyrophosphate by heating ferrous orthophosphate and by reduction of ferric phosphate with hydrogen. Upon exposure to air it turns green and ultimately brown. 

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. 

The salt may be obtained by heating diferrous orthophosphate with water to 250° C. 12 by heating in a sealed tube mixed solutions of sodium phosphate and ferrous sulphate 13 in an atmosphere of carbon dioxide or by electrolysis of sodium phosphate solution, using an iron anode.14... 

Ferrous hydrogen orthophosphate, FeHP04.H20, has been obtained by dissolving iron in boiling phosphoric acid solution. The salt crystallises out in colourless needles which become bluish in air it is insoluble in water, but readily dissolves in dilute acids and in ammonia. When heated with water to 250° C., it is converted into vivianite.2... 

Ferrous thio-orthophosphate, Fe3(PS4)2, results 6 on heating together ferrous sulphide and phosphorus pentasulphide, the latter being present in excess of that required by the equation... 

Ferrous thio-orthophosphate is a black crystalline substance which... 

In vinyl compound polymerization of vinyl acetate, alcohol, bromide, chloride, or carbonate, ascorbic acid can be a component of the polymerization mixture (733-749). Activators for the polymerization have been acriflavine (734), other photosensitive dye compounds (737,738), hydrogen peroxides (740,741,742), potassium peroxydisulfate (743), ferrous sulfate, and acyl sulfonyl peroxides (747). Nagabhooshanam and Santappa (748) reported on dye sensitized photopolymerization of vinyl monomers in the presence of ascorbic acid-sodium hydrogen orthophosphate complex. Another combination is vinyl chloride with cyclo-hexanesulfonyl acetyl peroxide with ascorbic acid, iron sulfate, and an alcohol (749). Use of low temperature conditions in emulsion polymerization, with ascorbic acid, is mentioned (750,751). Clarity of color is important and impact-resistant, clear, moldable polyvinyl chloride can be prepared with ascorbic acid as an acid catalyst (752) in the formulation. 

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. 

Iron (III) orthophosphate. See Ferric phosphate Iron oxide. See Iron oxide red Jron oxide black Iron (II) oxide. See Ferrous oxide Iron (III) oxide. See Iron oxide black Ferric oxide... 

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). 

If ammonium molybdate is added to an acidified orthophosphate solution, followed by a reducing agent such as stannous chloride or ferrous sulphate, an intense blue colour ( molybdenum blue ) will develop. A more satisfactory and sensitive version of this test is to use benzidine as the reducing agent in which case an intense blue colour arises from both the molybdenum blue and a reduced product from the benzidine which is also blue. 

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. 

When treated with oxidants such as ferricyanide or hydrogen peroxide, uteroferrin and other two-iron add phosphatases, freed of orthophosphate or other strongly interacting anionic inhibitors, are driven to their purple forms characterized by a broad intense absorption maximum between 550-570 nm and a prominent near-UV shoulder between 315-320 nm The pink form of these proteins, generated by mild reductants such as 2-mercaptoethanol, ascorbate, or ferrous ion, show absorption maxima shifted to 505-510 nm and their near-UV shoulders, now less conspicuous, shifted to 310 As expected, both redox forms of these proteins have a sharp, protein-... 

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