Measurement of Mass and Surface Stress at One Electrode of a Quartz Oscillator

The theory for the calculation of changes of mass and surface stress from frequency measurements at a quartz oscillator subject to different onesided pressures is outlined. The various contributions to apparent changes of the mass are discussed. Experiments refer to the determination of effective molar masses of potassium ions and fluoride ions during electrostatic adsorption and of chemisorbed lead both on polycrystalline gold by a comparison of masses to charges. Effective molar masses of adsorbed species can also be determined by comparison of mass changes to surface concentrations e.g. determined by the radiotracer method as in the example of the adsorption of chloride at the passivating oxide film on iron. The density of very thin films like the passivating oxide film can be obtained by comparing mass changes to thicknesses from optical measurements. In agreement with theoretical expectation, compressive surface stresses were qualitatively found for the underpotential deposition of lead on gold. In the growing oxide film on passive iron very large compressive lateral pressures were observed.