The point of zero charge of hydroxyapatite and fluorapatite in aqueous solutions
Journal of Colloid and Interface Science, Volume 42, Issue 2, February 1973, Pages 250-261
L. C. Bell, A. M. Posner, J. P. Quirk
http://www.sciencedirect.com/science?_ob=ArticleListURL&_method=list&_ArticleListID=792415840&_sort=d&view=c&_acct=C000053059&_version=1&_urlVersion=0&_userid=1489329&md5=6ac2d866950b11f4c3bec4cbf4601b90