Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

250 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSin some instances the adsorption properties resemble physical adsorption ratherthan chemisorption. Adsorption of electrolytes may involve ion exchange or nonexchangeprocesses. The kinetics of the adsorption process in solutions has notbeen widely studied. Many adsorptions are extremely fast and only equilibriumphenomena are observed and in many other instances mass transfer is the ratecontrolling step. Mass transfer in liquids is treated in Section 3.2.Information on the kinetics of adsorption has been obtained from electrical measurementson electrolyte solutions. The electrocapillarity effect'. 31 l can be used tomeasure the charge on the adsorbed layer, As the potential at the surface is changed,the desorption of ions or molecules produces a maximum in the differential capacity/ooltuge curve. The dropping mercury electrode is most commonly used, but similarresults have been obtained for solid metals312. With an impedance bridge as usedby Grahame31 ', the differential capacity can be measured directly. A slow uniformdropping rate is established for the mercury and the time interval from the fallingof a drop to the attainment of a minimum in the audio frequency hum is measured.After estimating the surface area of the drop, the differential capacity can becalculated. As the frequency is increased, the height of the maxima on the differentialcapacity/voltage curve changes, because there is insufficient time for theadsorption equilibrium to be established at higher frequencies. Theoretical analysesof the chan'ges have been devel~ped~'~*~'~ and it is concluded that adsorptionrates of lo-' mole. cm-' . sec-' can be measured. Because the impedance ofthe double layer should be comparable with the resistance of the cell, frequenciesof 0.1 to 1 megacycle are required.IKFig. 40. Block diagram of measuring circuit. P, potentiometer; L, load inductance; R, smallresistance; D, dual scoper; 0, audio frequency oscillator; A, sawtooth wave generator; B, tunedamplifier; K, oscilloscope equipped with amplifiers; Q, gain control of cathode follower to read Qvalue of resonance circuit. From ref. 316.