When Synthesis and characterization of HA and CHA Why are the carbonate and phos

Question

When Synthesis and characterization of HA and CHA

Why are the carbonate and phosphate sources mixed together before adding to the calcium solution? What would happen if you were to mix the calcium and carbonate source together and then add it to the phosphate solution?

Prepare two solutions, 100 mL each: (1) 0.5 M Ca2 solution (from Ca(NO3)2 4H20) with pH adjusted to 11 using 1 M NaOH; (2) 0.3 M solution of PO3 ions (from Na2HPOs). For the synthesis of CHA add the carbonate source (NaHCO3) to the phosphate solution such that the PO3: CO32 ratio is 5:1. Dropwise add the phosphate and carbonate solutions over the course of 10 minutes to the calcium solution. Heat and mix the solution to 100 °C for 2 hour adjusting the pH to 11 if needed by adding 1 M NaOH. Cool it down and vacuum filter your precipitate, wash three times with DI water. Dry the powder in air until the next lab period

Explanation / Answer

Among the starting material salts given, all are freely soluble in water, making the entire reaction occur in a single phase, if the procedure is followed properly. If however, the calcium and carbonate source were to be mixed prior to phosphate addition, the reaction of the first two ions would almost immediately result in the formation of a highly water-insouble precipitate - calcium carbonate. As the calcium carbonate formed will remain as a precipitate even under heat, it would trap all the carbonate and calcium source for the reaction and thus prevent the formation of the desired product.

Therefore, it should be noted that procedure takes extreme caution to ensure that the activity of the carbonate and calcium ions are not reduced by procedural error and thus decrease the yield of CHA by transforming the reaction into a heterogenous equilibrium instead of a homogenous, aqueous one.

It should also be noted that even while addition of the carbonate - phosphate solution to the calcium solution, there exists a chance of ion exchange that might form calcium carbonate and to avoid this completely, the procedure calls for dropwise addition, as then the formation of the precipitate species would be low enough to have a majority dissolved in water, thus not greatly affecting the yield of CHA desired.

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