Preparation of alkaline earth phosphates with sol containing sodium alginate and sodium diphosphate

Abstract

Magnesium hydrogen phosphate, calcium hydroxyapatite, and strontium hydroxyapatite were successfully prepared from sol consisting of sodium alginate and Na 4P 2O 7 with Mg 2+, Ca 2+, and Sr 2+ in the corresponding nitrates, respectively. It is revealed that the order of the addition of those substrates and the role of sodium alginate are important factors for the preparation of desired phosphate compounds. According to the previous paper on the preparation of calcium hydroxyapatite, sodium alginate was mixed with aqueous Na 4P 2O 7, followed by the addition of the aqueous divalent cations, resulting in the poor formation of the target phosphates. However, as a revised sol–gel technique, sodium alginate was added to the mixture of Na 4P 2O 7 and aqueous Mg 2+ and Sr 2+, resulting in a rather favorable formation of MgHPO 4 and strontium hydroxyapatite, respectively, while the sol thus obtained was stable within a few days. However for aqueous Ca 2+, calcium hydroxyapatite could not be obtained through the revised sol–gel technique. In the preparation of magnesium hydrogen phosphate, sodium alginate contributes mainly to the sol formation of the precursor. The ion exchange between Na + in sodium alginate and aqueous Ca 2+ was important for the preparation of calcium hydroxyapatite. In contrast, the reaction of sodium alginate with the mixture of Na 4P 2O 7 and aqueous Sr 2+ afforded strontium hydroxyapatite at the specific ratio of those three substrates. The structure of calcium and strontium phosphates prepared from the revised sol–gel process evidently depended on the amount of sodium alginate introduced into the mixture of Na 4P 2O 7 and the corresponding divalent cations.