Dissolved-away materials measurement

Irrespective of how the abnormal stress concentration arises, it is possible that the material at the crack apex is locally stressed into the plastic deformation region of the stress-strain curve. What is the result of the yielding of the metal near the crack apex The result is that as anodic dissolution dissolves away the kinky surface, further plastic yielding creates a fresh kinky surface inside the crack, and thus the yielding helps the metal dissolution along at a rate (e.g., of millimeters per hour) that turns out to be far greater than what would be expected at the overpotential concerned from measurements on the normal surface (Hoar and West, 1976). 

In the gas saturation method, a carrier gas such as helium or air is bubbled slowly through the sample, coming to equilibrium with it. The material exiting the apparatus is collected, and the amount of sample dissolved can be measured at the end of the experiment. Knowledge of the number of moles carried away, the number of moles of carrier gas used, and the pressure of equilibration allows calculation of the partial pressure of the sample at equilibrium, which is approximately its vapor pressure. Care must be taken to ensure that all of the carrier gas is equilibrated with the sample and to prevent premature condensation before the solute is collected. This method is typically applied for vapor pressures below about 3 kPa, and may achieve 1% accuracy with careful use. 

In a typical experiment, a small volume of an insoluble surface-active material (dissolved in a water-insoluble solvent such as benzene) is placed atop a clean water surface. As the solvent evaporates away, a film remains and the moving barrier can be adjusted so that the surface film exerts pressure on the mica float. A calibrated torsion balance is used to measure the force that the film exerts on the float. That force divided by the length of the float is the force per unit length or the surface pressure. For studies of lipolysis kinetics , a Langmuir trough can be constructed so that one can measure lipase action under first-order and zero-order conditions. 

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