As the pH reaches 5.7, the onset of the second deprotonation is reached (pK A2 = 7.21 for H 2 PO - 4 ), leading to a second H + neutralization step during which Al(OH) 3 is continuously formed. In addition, the formation of a calcium phosphate phase (such as hydroxylapatite Ca 5 (PO 4 ) 3 (OH) as modeled here -see Figure ) is also expected above pH 5.8. Note that this modeled pH value is consistent with the experimental plateau value. The overall dissolution reaction between CA and H 3 PO 4 can therefore be written as: