The Effect Of Citrate, Ferric, And Aluminium Ions On The Crystallisation Of Phosphates At Different pH Values (6.5; 7; 8)

Abstract

The calcic phosphates represent a complex family. Their conditions of crystallisation and inhibition were studied at various pH values. Phosphate precipitates obtained from artificial urine in the pH range (6.5; 7; 8) were identified using FTIR and XRD. At pH = 6.5 brushite accompanied by carbonated amorphous phosphate complexes precipitated while at pH = 7, the coexistence of brushite, struvite and amorphous phosphate complexes carbonated were observed. On the other hand carbapatite and struvite were formed exclusively at pH = 8. The complete disappearance of brushite crystals was obtained after addition of 1mM of citrate ion at pH = 6.5 to artificial urine. Its effect on struvite was more important at pH=7 as at this pH value inhibition was complete. Total inhibition of brushite was only observed for a concentration of citrate ion of 4 mM. At lower concentrations inhibition was partial. At the same pH, pentahydrated octocalcic phosphates (POP) and amorphous carbonated calcium phosphates (ACCP) were formed as identified by FTIR spectroscopy. In the presence of aluminium ion, the inhibition of brushite growth and aggregation increased. In fact at a concentration of 0.05 mM the formation of bushite crystals was halted but ACC P persisted. The addition of up to a concentration of 1 mM of ion aluminium resulted in total inhibition and crystalline transformation of the ACCP into withlockite. On the other hand, the effect of ferric ion was less important than that of aluminium ion at pH = 6.5. However, inhibition was total with a concentration of 0.1 mM ferric ion where ACCP and POP were the only compounds formed. The total disappearance of the crystalline forms was attained with a ferric ion concentration of 0.5 mM, while amorphous carbonated calcium phosphate and the carbapatite precipitated. For the iron III ion, total disappearance of brushite was reached at 0.25 mM, only the ACCP and POP were present. The inhibition effect of citrate, aluminium, and ferric ions on the aggregation and size of struvite crystals was significant at pH=8 but inhibition and transformation of this crystalline species were not complete.