They are not so much more expensive that using potassium rather than sodium is inadvisable. At 100C, NaF has a solubility in water of 5.05 g/ 100 mL,
and 4.04 g/ 100 mL at 20C. KF, on the other hand, has a solubility of about 350 g/ 100 mL at only 18C. KOH is more expensive than NaOH, but only by a
few dollars. K2CO3 and KHCO3 are quite a bit more expensive than their sodium counterparts, (and, interestingly, they are also more expensive than
KOH, at least on the places I saw them sold, so KOH is probably a better idea) but the increased yield of fluoride would still justify it.
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