-Sure they can use potassium, some good salt too. Mammals have the same salt-potassium pump as humans. Potassium ascorbate is an option, potassium chloride another.
Arthritis is often related to calcification. I would add some magnesium oil to her food or water. Borax, yes.
My examiner at my scool was a homeopatic veterinarian. She recommended for old dogs with arthritis; A remedy called Rhus Tox, once a week in their water, for the rest of their life. But only if this symptom; difficulty / pain when starting to move, better after moving for a while. This would, according to her, make them relatively pain free their last years.