I synthesized hydroxyapatite from an agroindustrial waste using the wet precipitation method. However, after synthesis, my material is still basic after several wash with deionised water and centrifugation.
Please all suggestions will be well appreciated
1) You may effectively reduce the pH value of a hydroxyapatite colloid by filtering procedure, but it is accompanied by lowering of surface area of the colloid.
2) Washing a hydroxyapatite colloid with continious decantation provides product with high surface area. However, it may sometimes took over a month of every-day decantation untill the colloid become neutral.
According to our experimental observations (S.A. Ulasevich et al. 2013), the slowest rate of colloid neutralizing is at pH value of the colloid ranging from 7 to 8. One may conclude several explanation of the phenomenon, including buffering properties exhibited by hydroxyapatite (Smičiklas et al., 10.1016/S1383-5866(99)00066-0). In our paper proposed is washing procedure using diluted phosphoric acid, but you should be careful about hydrolitic reactions in your product, when pH value is close to 7-8.
I also found this phenomenon in my investigations. Washing did not help. I do not have any solid experimental proofs but mentally I came to the conclusion, that this might be the property of hydroxyapatite itself because each molecule of Ca10(PO4)6(OH)2 might be considered as 3Ca3(PO4)2*Ca(OH)2. Among them, Ca(OH)2 is a base, which results in basic values of the solution pH. Therefore, elimination of basic Ca(OH)2 from Ca10(PO4)6(OH)2 appears to be the only way to make pH neutral; but this will result in the full transformation of hydroxyapatite to tricalcium phosphate.
When the ratio in the formula is precisely controlled and the pH tested after the precipitation usually three times washing is sufficient
You can employ the method of ion-exchange to make the hydroxyapatite neutral.
https://en.wikipedia.org/wiki/Ion_exchange
- Badges
- Science topic
- Similar topics
- Filing