Phosphinic acid is a monoprotic acid while phosphonic acid is a diprotic acid.
Both are diprotic acids.
Phosphinic acid is a diprotic acid while phosphonic acid is a monoprotic acid.
Both are triprotic acids.
Phosphinic acid is a monoprotic acid as it contains one P−OH bond while phosphonic acid is a diprotic acid as it contains two P−OH bonds.