Phosphate dissolution/precipitation

When calcium phosphate is precipitated from aqueous solutions of high supersaturation and pH values above 7, the solid phase appearing initially is an ACP with the formula Ca9(P04)61U u2 - If this amorphous precipitate is allowed to remain in contact with the solution, it transforms to crystalline HA through a process of dissolution, nucleation and crystal growth77, unless stabilized in some manner. 

When an alkaline oxide such as MgO is stirred in phosphoric acid, the pH of the solution rises slowly due to the neutralization of this acid. Initially, the phosphoric acid has pH 0, but initial dissolution of the oxide and reaction with phosphate anions precipitate phosphate salts. This neutralization of the acid raises the pH of the solution to >2. Even in this pH range, the acid dissolves sufficiently, and protons and H2PO4 anions are readily available to react with the ions produced by the dissolution of metal oxides. Subsequently, consolidation of the precipitate in the neutral solution leads to the formation of ceramics. 

PHEMA into an SBF solution. At the higher released citric acid concentrations, the formation of calcium citrate crystals can bring about a depletion of the calcium ion concentration in the SBF solution, so less calcium phosphate will precipitate onto the polymer. At lower citric acid concentrations, the inhibition effect of citrate anions on formation of calcium phosphates has been attributed to the complex equilibria between calcium, citrate and phosphate ions (27-29). Precipitated calcium phosphates may undergo dissolution via surface exchange between phosphate and citrate ions (calcium citrate is much more soluble than calcium phosphate). 

London (1994). This amount of P2O5 is equivalent to the dissolution of 0.3 to over 3.0 modal per cent apatite. If the melt does not leave the rock, the phosphates will precipitate as the melt crystallizes. Apatite solubility is also enhanced in low ASI melts (peralkaline), because of the presence of alkali phosphate complexes (Ellison and Hess 1988). 

The reduction and dissolution of iron and its reprecipitation to form ferrous minerals are thought to be the dominant processes controlling phosphorus solubility in anaerobic systems. Soil pore water profiles shown in Figure 9.56 show that the concentrations of soluble phosphorus and dissolved iron are low in the water column and increase with depth. Similar patterns in these profiles suggest that the solubilities of phosphorus and iron are strongly coupled. In the water and in the aerobic soil layer, iron is in the immobile ferric form, which reacts with phosphate and precipitates. Thus, under aerobic conditions, oxidation and precipitation of iron control phosphorus solubility and limit... 

LC. Chow, Next generation calcium phosphate-based biomaterials, Dent. Mat. J., 28, 1-10 (2009) K. Ishikawa, Bone substitute fabrication based on dissolution-precipitation reactions, Materials, 3, 1138-1155 (2010). 

Dichloromethane (9 mL), carbontetrachloride (23.5 mL), triethylamine (0.94 g), and methanol (0.72 mL) were placed in a three-necked flask equipped with a magnetic stirrer, thermometer, reflnx condenser, and a dropping fnnnel. A solution of poly(aIkylene H-phosphonate) (2.3 g, 0.0036 mol, 646 molecnlar weight of the repeating unit) in dichloromethane (13.5 mL) was added dropwise at ambient temperature under continuous stirring. The reaction mixture was allowed to stand at room temperature for 24 h. After filtration of the precipitated triethylamine hydrochloride, the filtrate was concentrated and the poly(alkylene phosphate) was precipitated by addition of diethyl ether. Poly(alkylene phosphate) was purified by dissolution in A,A-dimethylformamide and precipitation in diethyl ether. The isolated product was dried at 30-40 C under reduced pressure (1 mmHg). Yield was 2.4 g, 100%. 

Titrimetric determination The mannitol or glycerol acid-base titration procedure is applied to the dissolved sodium carbonate melt for the determination of milligram amounts of B2O3. A blank prepared from a glass that does not contain boric oxide should be subjected to the same procedure. There are certain interferences the effect of fluoride is eliminated by adding calcium carbonate to the fusion mixture lead and zinc can be filtered off after dissolution of the melt phosphate is precipitated by addition of iron(ni) chloride. The influence of some interference from such elements as aluminum or titanium can be eliminated by adding ethylenediaminetetraacetic acid (EDTA). 

Electrochemical dissolution of sacrificial anodes, for example, iron, aluminum, or magnesium, has been proposed for phosphate removal from urine. Ikematsu et al. [15] used an electrochemical reactor craisisting of two DSA and one iron electrode for combined nitrogen oxidation and phosphate precipitatirai. First, urea was oxidized at the DSA, then the current direction was changed, and phosphate was precipitated by dissolving the iron electrode. Zheng et al. [29, 30] used synthetic and real fresh urine for their experiments with iron and aluminum electrodes. With both types of electrodes, complete phosphate removal was achieved. At 40 mA cm and a gap width of 5 mm, 1.3 mol Fe. mol had to be dosed to remove 98 % of the phosphate (calculated by assuming a current efficiency... 

Other Salts. Indium nitrate trihydrate [13770-61 -1], In(N02)3 3H20, is a soluble salt prepared by dissolution of the metal or oxide in nitric acid. Indium phosphate [14693-82-4], InPO, is precipitated by adding phosphate ions to a solution of an indium salt. It is soluble in water. 

The Zinc Phosphating Process. The zinc phosphating reaction involves acid attack on the substrate metal at microanodes and deposition of phosphate crystals at microcathodes (8). Liberation of hydrogen and the formation of phosphate sludge also occur. The equation for the dissolution of iron together with precipitation of dissolved iron as sludge in a nitrite accelerated system is as foUows ... 

Pure tin is completely resistant to distilled water, hot or cold. Local corrosion occurs in salt solutions which do not form insoluble compounds with stannous ions (e.g. chloride, bromide, sulphate, nitrate) but is unlikely in solutions giving stable precipitates (e.g. borate, mono-hydrogen phosphate, bicarbonate, iodide) . In all solutions, oxide film growth occurs and the potential of the metal rises. Any local dissolution may not begin for several days but, once it has begun, it will continue, its presence being manifested... 

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

Cerium Ce(IV) co-precipitation with ferric hydroxide, dissolution in hydrochloric acid, then passed through a column of bis (2 ethyl hexyl) phosphate on poly(vinylchloride), eluted with 0.3 M perchloric acid Spectrofluorimetry at 350 nm (excitation 255 nm) [626]... 

It is interesting to note that many crystal poisons not only interfer with nucleation and the growth of crystals but may also retard their dissolution. As we have seen (Chapter 6), precipitation and dissolution of solids proceed by the attachment or detachment of ions most favorably at kink sites of the crystalline surface. Solutes such as organic substances, or phosphates may upon adsorption immobilize kinks and thus retard dissolution. 

Fe(III)(hydr)oxides introduced into the lake and formed within the lake - Strong affinity (surface complex formation) for heavy metals, phosphates, silicates and oxyanions of As, Se Fe(III) oxides even if present in small proportions can exert significant removal of trace elements. - At the oxic-anoxic boundary of a lake (see Chapter 9.6) Fe(III) oxides may represent a large part of settling particles. Internal cycling of Fe by reductive dissolution and by oxidation-precipitation is coupled to the cycling of metal ions as discussed in Chapter 9. 

The oxidized form of As, arsenate, As(V), which is present as HAs04 at neutral pH (p f values in Table 7.8), is sorbed on soil surfaces in a similar way to orthophosphate. The reduced form arsenite, As(lll), which is present in solution largely as H3As03(p fi = 9.29), is only weakly sorbed, hence mobility tends to increase under reducing conditions. Mobility will also increase without reduction of As(V) because, as for phosphate, reductive dissolution of iron oxides results in desorption of HAs04 into the soil solution. Under prolonged submergence As(lll) may be co-precipitated with sulfides. 

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