There are some crystals formed on the surface of the glass-ceramic substrate (mainly composed of Al, Si, Ca, O) after being treated with PBS (pH> ~10). I know that buffer salt crystalization is quite common (and might happen in the solution itself too) and I assume that the crystals are phosphate. But I want to know which sort of phosphates are more likely to form here! I would also appreciate any further information regarding the phosphate buffer crystalization.
Dear Ali,
If you have a ceramic surface, it is naturally composed of Al, Si, and Ca.
In soil chemistry, at this higher pH, phosphate would be mainly forming Ca-phosphate, from pH about 7 up to about 10, CaHPO4. To obtain Ca3(PO4)2, much higher pH is necessary, near 14. Aluminum phosphate would be stable at pH about 3.5 up to 7. Once the ceramic surface is partially composed of surface OH's, with the formed buffered system H2PO4-/HPO42-, some acid-base reaction will occur.
Here, speaking in terms of extraction of Ca from your ceramic surface.
If no extraction condition is achieved, the primary phosphate ion expected in your solution would be H2PO4- up to pH about 8 and HPO42- at pH about 12-13, reaching near pH 14 the PO43- ion. In such a case, the crystals would be of some counter ion from your solution like extracted Ca, as initially stated or some other metal (in solution) phosphate, since no H3PO4 nor H2PO4- are expected et al.
So, if extraction conditions are achieved, CaHPO4 is the more reasonable; otherwise, solution metals that are stable at pH above 10 may form crystals at the ceramic surface, acting as nucleation medium.
Best regards,
WNM.
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