Different Hydroxyapatite Dissolution Rates Over Short and Long Demineralisation Periods

Methods: HAP discs (d=12.05mm; w=1.25mm; 20 wt% porous) were preconditioned, coated with acid-resistant varnish on all surfaces but one, and sterilised. Demineralising solutions were 0.1% acetic acid and 0.3% citric acid buffered with 1M KOH at pH 2.8, 3.2, 3.6 and 4.0.

LP-RD_HAP: HAP discs were fixed in SMR cells and demineralising solutions were circulated at 0.4 mL.min^-1. The rate of HAP dissolution LP-RDHAP was measured at 34 scanning positions for 5 days at 20°C.

SP-RD_HAP: HAP discs were suspended in a moderately agitated thermostatically controlled reaction vessel at 21°C. SP-RD_HAP was calculated by measuring the rate of consumption of protons to maintain constant pH, over 30 minutes periods. Each measurement was repeated in triplicate.

Results: Citric acid: LP-RD_HAP was 2.13E-4 at pH4, 2.14E-4 at pH 3.6, 3.15E-4 at pH 3.2 and 5.02E-4 at pH 2.8. The SP-RD_HAP was 9.99E-4 at pH4, 2.26 E-3 at pH 3.6, 4.26E-3 at pH 3.2 and 6.80E-3 at pH 2.8.

Acetic acid: LP-RD_HAP was 2.85E-5 at pH4, 4.42E-5 at pH 3.6, 6.51E-5 at pH 3.2 and 1.06E-4 at pH 2.8. The SP-RD_HAP was 1.83E-3 at pH4, 2.55E-3 at pH 3.6, 5.86E-3 at pH 3.2 and 7.57E-3 at pH 2.8.All rates in g.cm^-2.s^-1

Conclusions: The data showed that there is an increase in RD_HAP with decreasing pH over short and long demineralisation periods under both demineralisation conditions. SP-RD_HAP was significantly higher than LP-RD_HAPsuggesting two different physicochemical processes occurring over the two different timescales and flow rates.