Ka values of the 1st and 2nd dissociation of phthalic acid (HO2C-C6H4-CO2H) are given below. Estimate the pH value for highest buffer capacity when a buffer is prepared from 1:1 mixture of phthalic acid and sodium hydrogen phthalate (HO2CC6H4- CO2Na) in water? HO2C-C6H4-CO2H <=> H+ +HO2C-C6H4-CO2- Ka1=1.3X10^-3 HO2C-C6H4-CO2- <=> H+ + -O2C-C6H4-CO2- Ka2=3.1X10^-6 so i m wondering that whether the pH (we want to find when buffer capacity is the highest) is just depends on Ka1...? or Ka2 is counted also??