HH equation: pH = pKa2 + log([Ma(2-)]/[HMa-]) rearranging: pKa2 = pH + log([HMa-]/[Ma(2-)]) using calculator: pKa2 = 6.03 + log(0.25/0.5) = 5.73 To not look further pKa1/2 for malonic acid taken from BATE database are 2.847 and 5.696. That's almost perfect fit....