Arsenic phosphate

Fig. 3. Sorption of arsenate (ASO4) onto ferrihydrite or Al(OH)x in the presence of phosphate (PO4) or phosphate and malate (Mai) at 50% surface coverage of arsenate and at initial ASO4/PO4 molar ratio of 1.0 or 0.5. Arsenate and phosphate were added as a mixture (As04+P04 As04+2P04) or phosphate was added 24 hrs before arsenate (P04 before As04) or arsenate was added 24 hrs before phosphate (As04 before P04). Arsenate, phosphate and malate were added as a mixture (As04+ P04/Mal molar ratio of 1). The numbers in parenthesis indicate the effectiveness of phosphate in preventing arsenate sorption. From Del Gaudio (2005).

Benson, A.A. 1984. Phytoplankton solved the arsenate-phosphate problem. Pages 55-59 in O. Holm-Hansen, L. Bolis and R. Gilles (eds.). Lecture Notes on Coastal and Estuarine Ecology. 8. Marine Phytoplankton and Productivity. Springer-Verlag, Berlin. 

Devoto 115)has described an indirect procedure for the determination of 0.1 ppm arsenic in urine. The arsenomolybdic acid complex is formed and extracted from 1 ml of urine at pH 2 into 10 ml of cyclohexanone. The molybdenum in the complex is then measured. Before extracting the arsenic, phosphate in the urine is separated by extracting the phosphomolybdic acid complex at pH 1 into isobutyl acetate. The direct determination of arsenic in biological material and blood and urine is best done using a nitrous oxide-acetylene flame 116>. The background absorption by this flame is low at 1937 A, and interferences are minimized due to the high temperature of the flame. 

Bertrand Pelletier, 1761-1797. French chemist and pharmacist who investigated the arsenates, phosphates, and phosphides of many metals, studied the action of phosphorus on platinum, and devised new methods for making soap and refining metal for clocks. He served as inspector of the hospitals m Belgium. His son, Joseph Pelletier (1788-1842), and Joseph Caventou discovered quinine, cmehomne, strychnine, and brucine. See also ref. (89). 

See Graham, Researches on the Arsenates, Phosphates and Modifications of Phosphoric Acid, Phil. Trans., 1833, X23, 253. 

Large amounts of sulphates, thiosulphates, nitrites, arsenates, phosphates, and oxalates interfere with the test. 

It may be pointed out that if arsenite is also present it may be readily detected in the filtrate obtained by treating the original mixture of arsenate, phosphate, and arsenite with the magnesium nitrate reagent upon acidifying with 2m hydrochloric acid and passing hydrogen sulphide, an immediate yellow precipitate of arsenic(III) sulphide is produced. 

Other examples of redox-sensitive elements include heavy elements such as uranium, plutonium, and neptunium, all of which can exist in multiple oxidation states in natural waters. Redox conditions in natural waters are also indirectly important for solute species associated with redox-sensitive elements. For example, dissolution of iron (hydr)oxides under reducing conditions may lead to the solubilization and hence mobilization of associated solid phase species, e.g. arsenate, phosphate (see Sections 3.3.2.1, 3.3.3.2, and 3.3.4.1). 

Figure 5.8. (a) Effect of contact time on the sorption of phosphate (PO4) and arsenate (ASO4) on an Andisol at pH 5.0. The oxyanions were added alone (filled symbols) or as a mixture at an initial arsenate/phosphate molar ratio of 1 (open symbols). (6) Effect of contact time on rf (rf = sorbed AsO4/sorbed PO4 molar ratio). (Modified from Violante and Pigna, 2002.)... 

The condensed compounds of this type so far investigated are mainly phosphate-silicate, sulfate-phosphate, vanadate-phosphate, and arsenate-phosphate. All the component 0x0 acids or the anions of these condensed compounds can form isopoly acids or their anions. As can be seen in silicate minerals, there are several structural types of isopoly silicates which are polymers of interconnected Si04 tetrahedra. For example, pyroxene has a chain structure 1 (13). In the structure of... 

From the experimental results, which will be described later in this chapter, it appears that the condensed compounds of phosphate-silicate, vanadate-phosphate, and arsenate-phosphate are hydrolyzed, when dissolved in water, by the scission of P-O-X linkages and, in some cases, X-O-X linkages. Monomer and polymer units of PO4 tetrahedra inserted between two atoms of X are released into the solution when the system is dissolved in water. Consequently, orthophosphate and/or chain phosphates are produced. As for condensed compounds of sulfate-phosphate, it has been found that copolymers of sulfate-phosphate persist in aqueous solutions. 

Ohashi and Matsumura (51) have investigated the NaVOs-NaPOs system and presumed the existence of copolymers of vanadate and phosphate, which have V-O-P linkages. Both glasses and crystals of alkali meta-arsenate-phosphates, when dissolved in water, decompose by the scission of As-O-P linkages to give arsenate and mixtures of condensed phosphates (see Section V). As is the case with condensed arsenate-phosphates, it is assumed that condensed vanadate-phosphates, if present, decompose by the scission of V-O-P linkages, when dissolved in water, to give vanadate and mixtures of condensed phosphates. 

All the work on condensed arsenate-phosphates discussed here has been carried out by Thilo and co-workers (24,57-62). Some reviews on these compounds in connection with the condensed phosphates have also been published by Thilo (25,63-65). 

Crystals of sodium meta-arsenate-phosphates with P/As ratios of 1, 2, 3, and 4 are prepared by the following procedure (57,60). A mixture of calculated amounts of sodium dihydrogen orthophosphate and sodium dihydrogen orthoarsenate is melted at 620°C. and tempered at 470°C. for 24 hr. The resulting product is hard, needle-shaped crystals. When a mixture of the phosphate and the arsenate with a P/As ratio of 5 is treated by a similar procedure, needle-shaped crystals with considerable amounts of sodium trimetaphosphate are produced. 

If the melt is tempered for a longer time, the crystals are decomposed into trimetaphosphate and meta-arsenate-phosphate with a smaller P/As ratio. If the period of tempering is too short, crystals buried in a glass are obtained. 

When starting materials with P/As ratios more than 6 are treated by the same procedure, pure crystals are not obtained, but, instead, mixtures of glasses, trimetaphosphate, and crystals of meta-arsenate-phosphate that do not have any constant compositions are produced. 

Densities of the crystals and the glasses of sodium meta-arsenate-phos-phates vary linearly with composition, between those of pure sodium metaarsenate and pure sodium metaphosphate, as shown in Figure 3. Therefore, the density of crystalline or glassy sodium meta-arsenate-phosphate, dAsP is represented by equation 13 ... 

Fig. 3. Dependence of densities of sodium meta-arsenate-phosphates on their compositions (60). A Crystals. B Glasses.

X-ray diffraction patterns of the crystals of sodium meta-arsenate-phosphates vary little by little with the increase of the P/As ratio, approaching that of Maddrelhs salt. A lattice constant, 6, along the fiber axis decreases linearly, to the first approximation, as the fraction of phosphorus atoms increases, as illustrated in Figure 4. 

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-... 

Crystals of sodium meta-arsenate-phosphates, being similar to crystals of sodium metaphosphate, are dissolved in solutions of salts of alkali metals other than sodium. These dissolution reactions are considered to be based on ion-exchange reactions (67). About 10 g. of a fine powder of sodium meta-arsenate-phosphate can be dissolved in about 1 liter of a solution of 0.5M potassium nitrate and 0.05M potassium hydroxide. The potassium hydroxide is added to neutralize hydrogen ions produced by the hydrolysis of the arsenate-phosphate. After about 1 hr. almost all the crystals are dissolved. 

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