text: When cumulative constants have been refined it is often useful to derive stepwise constants from them. The general procedure is to write down the defining expressions for all the constants involved and then to equate concentrations. For example, suppose that one wishes to derive the pKa for removing one proton from a tribasic acid, LH, such as citric acid.
  The stepwise association constant for formation of LH is given by
  Substitute the expressions for the concentrations of LH and  into this equation
  whence
  and since  its value is given by
  Note the reverse numbering for pK and log β. When calculating the error on the stepwise constant, the fact that the cumulative constants are correlated must accounted for. By error propagation
  and