[Henderson-Hasselbalch]

Essentially, I want to run a scan (ATR-IR) in alkaline conditions where I remove any adsorbed CO₃^2- onto the hydroxylapatite. If the peaks are not present during this, then the peaks can be attributed to carbonate incorporated into the apatite. This way I know that when I have run acid over the film, the decrease in the bands is due to the dissolution of the apatite.

Anywho, what could I use to remove the CO₃^2- under alkaline conditions?

[DevaDevil]

removal of trace carbonate from alkaline electrolyte is very tricky. When I work with alkaline solutions of high purity, I usually end the experiment when I see telltale signs of carbonate.

What you would like to try first of all is start with ultra-high-purity alkaline electrolyte, keeping it de-aerated to avoid CO₂ from the air to dissolve in the electrolyte. When I work with these chemicals in this way I can usually keep carbonate out for a few hours.

The removal of carbonates from NaOH solution is usually done by concentrating the NaOH to 50 wt-%, then waiting for the less soluble Na₂CO₃ to precipitate, which is very time consuming. A quick literature seach shows me that a group recently claimed to have an other method, which requires 8M NaOH, with added CaO and stirred overnight. But this in either case is not in-situ removal!


better to start without carbonates, and trying to keep them out.

[Henderson-Hasselbalch]

I read that to get carbonate to desorb from TiO₂ films, a NaOH solution with the concentration of 10^-4 mol L^-1 is run over the film for 5 to 10 minutes. Firstly, how would his cause desorption on a TiO₂ film. Secondly, would it likely result in carbonate desorption on a hydroxylapatite film?

[DevaDevil]

I would not know if that would work, the carbonate will have replaced one of the anions in the crystal structure and will not be merely adsorbed...
My advice is to do a literature search yourself, as you are getting into more advanced matter there outside of my expertise. I do not nearly have the time to do the full search for you, but a quick check tells me that there are plenty article available concerning HAp and carbonate. You should be able to find more details there.