Orthophosphoric acid solutions

Here we are concerned with the cement-forming reaction between orthophosphoric acid solutions and basic oxides and silicates where the reaction is much simpler. Polymeric phosphates are not involved, there are no P-O-P bonds, and the structural unit is the simple [POJ tetrahedron. 

Concentrated solutions of orthophosphoric acid, often containing metal salts, are used to form cements with metal oxides and aluminosilicate glasses. Orthophosphoric acid, often referred to simply as phosphoric acid, is a white crystalline solid (m.p. 42-35 °C) and there is a crystalline hemihydrate, 2H3PO4.H2O, which melts at 29-35 °C. The acid is tribasic and in aqueous solution has three ionization constants (pA J 2-15,7-1 and 12-4. 

Results from infrared spectroscopy indicate that the only species present in 50 % phosphoric acid are H3PO4, HjPOj and their oligomers (Wilson Mesley, 1968). There is evidence that HgPjOg, the phosphoric acid dimer, and HsPjOg, the triple ion H2PO4.. HjPOj, are also present (Elmore, 

Mason Christensen, 1946 Selvaratnam Spiro, 1965). Akitt, Greenwood Lester (1971), on the basis of PNMR studies, suggest further that there are oligomers of the type (HgP40) . 

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Although this account of gelation is made with reference to organic polyelectrolytes, it is of wider application and may be applied to phosphoric acid cements. Orthophosphoric acid solutions used in these cements contain aluminium, and soluble aluminophosphate complexes are formed. Some appear to be multinuclear and there is evidence for polymers based on the bridging Al-O-P unit. These could be termed polyelectrolytes (Akitt, Greenwood Lester, 1971 Wilson et al., 1972 O Neill et al., 1982). 

As we have already shown, the presence of cations in orthophosphoric acid solution can have a decisive effect on cement formation. As noted above, Kingery (1950b) found it necessary to modify orthophosphoric add, by the addition of calcium, to obtain cement formation with calcium oxide. Also, Finch and Sharp (1989) had to modify orthophosphoric add, with either ammonium or aluminium, to achieve cement formation with magnesium oxide. 

Later, better cements appeared based on c. 50% solutions of orthophosphoric acid. But even these were far from satisfactory. As always with dental cements, the problems revolved around the control of the setting reaction the reaction between zinc oxide and orthophosphoric acid was found to be far too fierce. By the time of Fleck s 1902 paper these problems had been solved. The importance of densifying and deactivating the zinc oxide powder to moderate the cement reaction had been recognized. Of equal importance was the realization that satisfactory cements could be produced only if aluminium was incorporated into the orthophosphoric acid solution. The basic science underlying this empirical finding was elucidated only in the 1970s. 

Early workers, and some later ones, ignored the fact that aluminium is always found in the orthophosphoric acid liquid of the practical cement its presence profoundly affects the course of the cement-forming reaction. It affects crystallinity and phase composition, and renders deductions based on phase diagrams inappropriate. Nevertheless we first describe the simple reaction between zinc oxide and pure orthophosphoric acid solution, which was the system studied by the earliest workers. 

Those based on the reaction between magnesium oxide powder and orthophosphoric acid solution. 

The early history of the cement is obscure. Dreschfeld (1907) and Sanderson (1908) attributed its invention to Fletcher. Fletcher (1878,1879) certainly described cements formed from concentrated orthophosphoric acid solutions and sintered mixtures of oxides which included SiOj, AljOj, CaO and ZnO. One was reported by Fletcher (1879) as being slightly translucent. These cements were not successful in clinical use. 

Reaction of Sodium Bromide and Potassium Iodide with Concentrated Orthophosphoric Acid. Put a small amount of sodium bromide in one test tube and of potassium iodide into another one. Add a 60 % orthophosphoric acid solution to both tubes. What gases evolve from the test tubes Write the equations of the reactions and explain their course. 

Put 1-2 ml of a 95% orthophosphoric acid solution into a porcelain bowl. Heat it on a sand bath until it acquires a syrupy consistence, after which roast it at 350 °C. How can you prove that me-taphosphoric acid has been obtained Write the equation of the reaction. 

An orthophosphoric acid solution, 1 volume of reagent-grade 85% phosphoric acid to 5 volumes of distilled water, is saturated with reagent-grade calcium phosphate. After allowing the excess calcium phosphate to settle, the clear, saturated solution is decanted into a beaker. This solution is heated to its boiling point and well-crystallized calcium... 

The procedure for the preparation of monetite is the same as that given for brushite in part A, except that the reaction medium is kept at 100° throughout the course of the reaction. The wash solutions are slightly different. The product is first washed with three 330-ml. portions of ap orthophosphoric acid solution with a pH of approximately 3, and then with 250 ml. of absolute ethanol. The yield is 65 g. (95%). Loss of weight upon ignition at 900°. Calcd. 6.62. Found 7.13. (This represents an excess of water, over that required by the equation, of 0.0416 mol per mol of calcium hydrogen orthophosphate.)... 

Liquid ammonium phosphate fertilizers Ammonium polyphosphates can be manufactured from phosphoric acids containing either high or low concentrations of poly-phosphoric acid, or from orthophosphoric acid solutions. 

Cyclohexanedione by Bromate in Perchloric or Orthophosphoric Acid Solution. 

Mix 200 uL of sample, standards, and controls with 25 uL of BHT solution (54 mmol/L in methanol) and 200 uL orthophosphoric acid solution (200 mmol/L). Mix well. 

In a 500 ml. three-necked flask, equipped with a thermometer, a sealed Hershberg stirrer and a reflux condenser, place 32-5 g. of phosphoric oxide and add 115-5 g. (67-5 ml.) of 85 per cent, orthophosphoric acid (1). When the stirred mixture has cooled to room temperature, introduce 166 g. of potassium iodide and 22-5 g. of redistilled 1 4-butanediol (b.p. 228-230° or 133-135°/18 mm.). Heat the mixture with stirring at 100-120° for 4 hours. Cool the stirred mixture to room temperature and add 75 ml. of water and 125 ml. of ether. Separate the ethereal layer, decolourise it by shaking with 25 ml. of 10 per cent, sodium thiosulphate solution, wash with 100 ml. of cold, saturated sodium chloride solution, and dry with anhydrous magnesium sulphate. Remove the ether by flash distillation (Section 11,13 compare Fig. II, 13, 4) on a steam bath and distil the residue from a Claisen flask with fractionating side arm under diminished pressure. Collect the 1 4-diiodobutane at 110°/6 mm. the yield is 65 g. 

All three fluorophosphoric acids are commercially available. The mono- and difluoro acids can be made as anhydrous or hydrated Hquids. Commercial hexafluorophosphoric acid is an aqueous solution. Anhydrous hexafluorophosphoric acid maybe prepared at reduced temperatures and pressures but it dissociates rapidly into PF and HF at 25°C (56). When diluted with water all the fluorophosphoric acids hydrolyze producing orthophosphoric acid. The hexafluoro acid is the most stable of the three fluorophosphoric acids. 

At equihbrium, the specific composition of a concentrated phosphoric acid is a function of its P2 s content. Phosphoric acid solutions up to a concentration equivalent of about 94% H PO (68% P2O5) contain H PO as the only phosphoric acid species present. At higher concentrations, the orthophosphoric acid undergoes condensation (polymerization by dehydration) to yield a mixture of phosphoric acid species (Table 5), often referred to genericaHy as polyphosphoric or superphosphoric acid, H20/P20 = - 3, or ultraphosphoric acid, H20/P20 = - 1. At the theoretical P2O5 concentration for orthophosphoric acid of 72.4%, the solution is actually a mixture containing 13% pyrophosphoric acid and about 1% free water. Because the pyrophosphoric acid present is the result of an equihbrium state dependent on the P2 5 content of the solution, pure orthophosphoric acid can be obtained because of a shift in equihbrium back to H PO upon crystallization. 

Orthophosphate salts are generally prepared by the partial or total neutralization of orthophosphoric acid. Phase equiUbrium diagrams are particularly usehil in identifying conditions for the preparation of particular phosphate salts. The solution properties of orthophosphate salts of monovalent cations are distincdy different from those of the polyvalent cations, the latter exhibiting incongment solubiUty in most cases. The commercial phosphates include alkah metal, alkaline-earth, heavy metal, mixed metal, and ammonium salts of phosphoric acid. Sodium phosphates are the most important, followed by calcium, ammonium, and potassium salts. 

Spray solution For sugars [3] dissolve 3 g 2-aminodiphenyl in 100 ml glacial acetic acid and add 1.5 ml 85% orthophosphoric acid. 

Figure 12.18 Neutralization curve for aqueous orthophosphoric acid. For technical reasons the curve shown refers to 10 cm of 0.1 M NaH2P04 titrated (to the left) with 0.1 m aqueous HCl and (to the right) with 0.1m NaOH solutions. Extrapolations to points corresponding to 0.1m H3PO4 (pH 1.5) and 0.1m Na3P04 (pH 12.0) are also shown.

Aluminium alloys Dip for 5-10 min in an aqueous solution containing 2wt.% chromic acid (CrOj) plus 5 vol.% orthophosphoric acid (H3PO4, 85%) maintained at 80°C. Ultrasonic agitaion will facilitate this procedure. 

Phosphoric acid esters are strong acids similar to orthophosphoric acid. Potentiometric titration of a 0.1 N aqueous solution of an acid phosphoric acid ester clearly shows two potential jumps which lie at pH values of 6.5 and 11.5. The pH value of diluted aqueous solutions of acid esters lies in the range of 1-3. Phosphoric acid esters are stable against hydrolysis, but adducts of free phosphoric acid esters with ethylene oxide are generally less stable. 

The phosphate bonded cements described in this chapter are the products of the simple acid-base reaction between an aqueous solution of orthophosphoric acid and a basic oxide or silicate. Such reactions take place at room temperature. Excluded from this chapter are the cementitious substances that are formed by the heat treatment of aqueous solutions of acid metal phosphates. 

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