Ka and Kb from pH and Conductivity Measurements: A General Chemistry Laboratory Exercise

The acid ionization constant, K a, of acetic acid and the base ionization constant, K b, of ammonia are determined easily and rapidly using a datalogger, a pH sensor, and a conductivity sensor. To decrease sample preparation time and to minimize waste, sequential aliquots of a concentrated standard are added to a known volume of water and measurements are made after each addition. In this laboratory exercise, students show that K a and K b are constants, compare and contrast pH and conductivity approaches, and evaluate how solution components (weak acid alone and weak acid plus conjugate base) affect the results.