Hydroxyapatite columns as an in vitro system to study demineralization processes

Acetate buffers were passed through small hydroxyapatite columns and the column effluents were analysed for calcium and phosphate. Changes in the pH of the column effluents were also measured. As long as the pH of the column effluent was higher than about 5.5, only calcium was eluted from the column. However, when the pH fell below 5.5, the rate of calcium elution decreased and phosphate elution increased until calcium and phosphate elution reached constant values. Pretreating the hydroxyapatite with sodium fluoride or 1-hydroxyethylidene-1,1-bisphosphonate accelerated the drop of the pH and reduced the initial Ca/P ratio in the column effluent. We suggest that these results are due to the formation of a pH gradient along the progressing buffer fron with calcium and phosphate liberated from the hydroxyapatite reprecipitating to form calcium-poor minerals.