We made CHA nanospheres and nanoneedles, just wondering if you would know this q

Question

We made CHA nanospheres and nanoneedles, just wondering if you would know this question? Thanks. Why is the carbonate and phosphate source mixed together before addition to the calcium source? What would happen if you were to mix the calcium and carbonate source together and then add it to the phosphate solution?

Procedure is below:

Synthesis of CHA nanospheres:

You will need to prepare 120 mL of two solutions: an aqueous & an acetone solution. Make sure that the molar ratio of Ca2+ : PO43- : CO32- = 1.67 : 1.00 : 0.50 respectively. Use concentrations so that your theoretical yield will be 2.0 g. An aqueous solution is to contain the phosphate & carbonate source, while the acetone solution should contain the calcium source. Adjust the pH of the aqueous solution to 11 using 1.0 M sodium hydroxide. Quickly combine & mix the two solutions for 1 minute using a magnetic stirrer. Vacuum filter your precipitate & wash three times with DI water. Obtained slurry is to be dried in an oven set to 75 °C for ~1H, or until dry* (record the weight of your product for % yield calculation). Obtain an IR of your product at the UIC. In a small porcelain crucible submit ~half of your sample to the TA for calcination at 900 °C for 4 h in air.

*Note: One group will be selected to have a portion of their sample freeze dried. As a result IR of freeze dried HA will be performed during day 2.

Synthesis of CHA nanoneedles:

Prepare two solutions in the same manner as you did for the CHA nanosphere synthesis EXCEPT both solutions are to be aqueous (no acetone). Adjust the pH of the calcium solution to 11 using 1.0 M sodium hydroxide. Drop wise add the phosphate & carbonate solution over the course of 10 minutes to the calcium solution. Heat & mix the solution to 80 °C for 2 hours. Vacuum filter your precipitate & wash three times with DI water. Obtained slurry is to be air dried until day 2.

Note: One group will be selected to have a portion of their sample freeze dried.

Explanation / Answer

Here phosphate ions and added NaOH act as a buffer system and control the pH to 11. Under these conditions, calcium ions react with carbonate ions to form nanoparticles. If calcium ions and phosphate ions were directly mixed without phosphate buffer, then the precipitated calcium carbonate will have much higher particle size and it will no longer be nanoparticle.

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