Physiocochemical investigations on urea-nitrate calcium phosphate system

Abstract

The experimental results obtained in the present investigation clearly demonstrate that when urea nitrate is ground with tribasic calcium orthophosphate, a reaction takes place in the solid-state even at laboratory temperature (25–30°C). Protonation occurs and calcium hydrogen phosphate is formed (CaHPO?). As a result of this protonation, the insoluble tricalcium phosphate becomes water-soluble. Similar protonation of the phosphate is observed with hydroxyapatite, fluorapatite and rock phosphate. A brief account of the characteristic features of the solid-solid reaction and the scope of the present study are presented in the first part of the thesis. A variety of physicochemical techniques have been employed to procure experimental evidence to establish that the reaction takes place in the solid-state. Infrared spectrum, X-ray powder technique, thermoanalytical methods, solid-state electrical conductivity, optical and electron microscopy are the major instrumental techniques employed for this study. The instruments are checked for their satisfactory performance with standard substances wherever possible before making any measurements with the test samples. The physicochemical behaviour of the individual reactant has also been examined in greater detail to gather information about the nature of these chemical entities. The reactants used for the experiments have been prepared under standard conditions and in some cases, experimental conditions have been standardised to obtain pure samples. It is known that the salts of nitric acid (both inorganic and organic) are usually hygroscopic. Urea nitrate, however, is least hygroscopic as observed in the present study. It requires more than relative humidity to make urea nitrate take up moisture. The solubility of urea nitrate in water is quite appreciable at 25°C whereas in alcohol and nitric acid, it is very low. It undergoes easy dissociation in water to urea and nitric acid. The aqueous solutions are highly acidic. Urea nitrate has a peculiar layer lattice structure. It is reasonable to expect the mobility of proton in such crystal. This is clearly revealed by the measurement of the electrical conductivity of urea nitrate in the solid-state. Infrared spectrum of the solid maintained at different temperatures confirms such a conclusion. Urea nitrate is thermally stable up to 150°C. Beyond this temperature, the solid undergoes decomposition with a violent burst of nitrous oxide leaving behind ammonium nitrate and urea as the major components of the residue. Only traces of CO? and H?O are produced. In the absence of moisture, crystals of urea nitrate remain stable over long periods up to a temperature of 90°C. Different orthophosphates of calcium have characteristic crystal structures which influence their physical properties and chemical reactivity. The infrared spectra and the differences in their thermal behaviour have been successfully employed for their identification in the present study. The insoluble tribasic calcium orthophosphate is rendered completely water-soluble when it is mixed with urea nitrate and ground. The minimum amount of urea nitrate required for this purpose is found to be 3.15 moles for every mole of Ca?(PO?)?. In case of hydroxyapatite, at least 6.5 moles of urea nitrate are required to make it completely water-soluble. Still greater amount of urea nitrate is needed to completely dissolve fluorapatite in water. 8.5 moles of urea nitrate will have to be ground with every mole of fluorapatite for this purpose. It is of interest to point out that the amount of calcium phosphate rendered soluble when ground with urea nitrate is slightly more than the amount of calcium phosphate that goes into solution in the presence of nitric acid of corresponding concentration or a solution of urea nitrate of the same strength. It is easy to infer from the results that there could be interaction between urea nitrate and calcium phosphate in the solid phase. Calcium nitrate would be one of the products of such solid-state reaction. Calcium nitrate is highly soluble in absolute alcohol whereas none of the phosphates of calcium is even sparingly soluble. Calcium nitrate could easily be extracted from such a mixture, if there could be any solid-state reaction. Alcohol extraction experiments indicate that one mole of calcium nitrate is formed for every mole of Ca?(PO?)? taken along with urea nitrate. Similarly, two moles of calcium nitrate are formed per every mole of hydroxyapatite or fluorapatite. Extraction experiments show that only a part of the calcium is replaced from the phosphate and this may correspond to the first stage of protonation of phosphate forming calcium hydrogen phosphate (CaHPO?). The infrared spectra, X-ray powder pattern and thermal analysis of such samples indicate the presence of CaHPO?. It is possible to detect the presence of calcium nitrate and urea also in such mixtures. Besides the identification of the products, it is possible by these physicochemical methods to establish the stoichiometry of this solid-state reaction. Two moles of urea nitrate react with one mole of tribasic calcium orthophosphate according to the equation: Ca?(PO?)? + 2O?(NO?)? + HNO? ? 2CaHPO? + Ca(NO?)? + 2NH?CONH? The urea and nitrate contents are found to be the same before and after grinding urea nitrate with calcium phosphate, indicating that both urea and nitrate do not undergo any decomposition during the operation. It may be pointed out that CaHPO? is insoluble in water. But the presence of excess of urea nitrate contributes to the easy dissolution of the entire phosphate. In the case of hydroxyapatite and fluorapatite, two moles of calcium nitrate are formed per mole of the phosphate. Every mole of apatite consumes four moles of urea nitrate giving rise to CaHPO?. The reaction with Ca?(PO?)? is faster than that with hydroxyapatite or fluorapatite. The CaHPO? isolated from the reaction mixture exhibits peculiar surface properties. The presence of calcium nitrate in the products of the reaction makes the mixture hygroscopic. This tendency is greatly reduced by suitable addition agents. After having procured unimpeachable experimental evidence in support of the solid-state reaction, it is of interest to work out the mechanism of this solid-solid reaction. A detailed investigation of this aspect has been restricted to Ca?(PO?)? in order to avoid other complications that might arise in the case of hydroxyapatite or fluorapatite.