The operational pH value of a buffering membrane used in an isoelectric trapping separation is determined by installing the membrane as the separation membrane into a multi-compartmental electrolyzer operated in the two-separation compartment configuration. A 3 < pI < 10 carrier ampholyte mixture (feed) is filled into both separation compartments and a binary isoelectric trapping separation is completed. A set of pI markers and a sufficient amount of a pI < 3 auxiliary ampholyte is added to the aliquot collected from the anodic separation compartment of the electrolyzer. The same set of pI markers and a sufficient amount of a 10 < pI ampholyte is added to the aliquot collected from the cathodic separation compartment of the electrolyzer. Both mixtures are then analyzed by CIEF: the pI markers whose pI values fall into the pI range of the carrier ampholytes trapped in the respective separation compartments become separated from each other, yielding two calibration lines. The pI markers whose pI values fall outside the range of the trapped carrier ampholytes appear merged into single bands at the extremes of the pH gradients. The pI values of the merged bands determined from the two calibration lines yield the operational pH value of the buffering membrane.