Hi,

What is the best way to dissolve pure CaWO4?

I tried:

- conc. Aqua Regia

- conc. Aqua Regia + conc. HF

- conc. HNO3 + conc. HF

- 7N HNO3

- 7N HNO3 + conc HF.

Every experiment was for 55 minutes in the microwave @200 °C. At the experiments with HF I added H3BO3 afterwards and placed it for another 25 Minutes @180 °C in the microwave.

In every test H2WO4 precipitates (yellow residue), except conc. HNO3 + conc. HF, where I got first a clear solution, but over the weekend the PP-Tube turned yellow at the bottom (no residue!).

Low amount of CaWO4 (