System Lime

Since the phosphorus compounds which are used in the arts, as well as phosphatic fertilisers, are derived from the decomposition of phosphates of lime, and the interactions of these are also of importance in biochemistry, a short account of the best investigated of these compounds will be given here.

The phosphates of lime which occur in nature or are produced during the course of manufacture of phosphorus compounds are salts of orthophosphoric acid. The hydrogen is replaced by calcium in stages giving successively mono-, di- and tri-calcium phosphate. Of these the mono-calcium salt alone is freely soluble. The solids deposited on evaporation, or obtained by double decomposition, are generally mixtures of the di-, tri- or more basic compounds, but pure crystalline forms have been prepared, especially of the more acid phosphates. The more basic phosphates are very sparingly soluble, and the solubilities are not definite. The solids are not in equilibrium with solutions of their own composition, but are in process of transformation which is so slow that equilibrium is not attained in most operations.

Tribasic calcium phosphate, Ca₃P₂O₈, may be made by washing precipitated calcium phosphate with ammonia, which dissolves any excess of phosphoric acid above that required to form this compound. It is a white earthy powder which retains water tenaciously, and also adsorbs halides, bicarbonates and hydroxides. It has also been prepared from CaH₄(PO₄)₂ by dissolving this in a large excess of water and adding ammonia free from carbonate with constant stirring and at such a rate that the solution is only faintly alkaline until the end of the operation. The amorphous gelatinous precipitate is washed with water by decantation until the dissolved phosphate is reduced to a minimum. Analysis of a dried sample gave a ratio P₂O₅/CaO = 0.835, the theoretical ratio being 0.845. Ca₃(PO₄)₂ contains 54.2 per cent, of CaO and 45.8 per cent, of P₂O₅. According to the analyses, phosphate rock evidently is more basic than this, and often contains much carbonate. It is considered probable that these rocks contain oxy-apatite, CaO.3Ca₃P₂O₈, hydroxy-apatite, Ca(OH)₂.3Ca₃P₂O₈, or carbonato-apatite, CaCO₃.3Ca₃P₂O₈, with usually a further excess of lime or calcium carbonate. The composition of the solid phases formed by long shaking of CaHPO₄ with Ca(OH)₂ passes the point corresponding to Ca₃P₂O₈, and becomes fixed only at a ratio which corresponds nearly to Ca(OH)₂.Ca₃P₂O₈.

Repeated extraction of Ca₃P₂O₈ with boiling water yields finally Ca(OH)₂.Ca₃P₂O₈. The preparation is more certain in the presence of alkali. The tribasic phosphate is suspended in a large excess of water contained in a pyrex flask and boiled for long periods with alkali free from carbonate, removing the supernatant liquid each day. After several weeks the precipitate reaches a constant composition which undergoes no further change, the ratio of P₂O₅/CaO (in grams) being then 0.76. The hydrolysis is expressed by the equation

10Ca₃(PO₄)₂ + 6H₂O = 3[3Ca₃(PO₄)₂.Ca(OH)₂] + 2H₃PO₄

Both the basic phosphate and Ca₃(PO₄)₂ adsorb definite quantities of Ca(OH)₂. These quantities when plotted as functions of the concentrations of Ca(OH)₂ have the usual form of adsorption isotherms and therefore give no evidence of the formation of definite compounds, 1 gram of the basic calcium phosphate when in equilibrium with a solution containing 1.099 grams of Ca(OH)₂ per litre adsorbs 0.0201 gram of the hydroxide. After 6 months of contact this amount is increased to 0.0243 gram.

A hydrate Ca₃P₂O₈.H₂O has been described as a hygroscopic powder, but this also is not in equilibrium with a definite solution but gives solutions containing greater proportions of phosphoric acid and dissolves with hydrolysis, depositing lime.

Dicalcium phosphate, Ca₂H₂P₂O₈ or CaHPO₄, is the first substance to be precipitated when calcium hydroxide is added to phosphoric acid. It is formed by the interaction of mono- and tri-calcium phosphates, and is then called " reverted phosphate." Thus

Ca₃P₂O₈ + CaH₄P₂O₈ = 2Ca₂H₂P₂O₈

It is also deposited when any aqueous solution of calcium phosphate is evaporated to dryness with hydrochloric acid. It occurs as an anhydrous more soluble form and as a hydrated less soluble crystalline form (monoclinic needles). The solubility product [Ca⁺⁺][HPO₄⁼] is variously given as 4×10^-7 or 3.5 to 12×10^-6. This compound, like the tribasic and indeed all the phosphates of lime, decomposes in contact with water giving a more acid solution and a more basic solid. The original solid compound is in equilibrium only with a solution containing a higher ratio of PO₄/CaO.