第七届青年地学论坛

Two examples are given here to illustrate simple gravimetric measurements can yield useful scientific results. The 1^st one is about the determination of permeability of Au, Pt and AgPd to hydrogen at 2 kb and between 450 and 812 ºC by using the double-capsule oxygen buffer technique (Chou, 1986, Am. J. Sci., 286, 638-658). The experimental approach was based on the fact that for one mole of H₂ transferred out of a metal capsule at a fixed pressure (P)-temperature (T) condition under a fixed H₂ fugacity (fH₂) gradient, there was a loss of one mole of H₂O from the originally loaded H₂O in the capsule, and this mass loss can be easily and accurately determined. Results showed that the mass loss was a linear function of experimental duration for a particular experimental setup (i.e., P, T, and fH₂ gradient), indicating a steady state had been established, and the permeability constants were then calculated from the measured rates of H₂ transfer across the cylindrical metal membrane at a steady state. The results of these experiments have been widely used to provide guidelines for the design as well as the required experimental duration for reaching equilibrium in hydrothermal experiments related to redox reactions.    
The 2^nd example is about the determination of stability fields of hydrous metal sulfate minerals in the relative humidity (RH)-T space under 1 atmosphere pressure by using the humidity buffer technique (Chou et al., 2002, Am. Mineral., 87, 108-114). This method was based on the isopiestic technique, in which a mixture of about the same mass of hydrous metal sulfates with two different H₂O contents (e.g., epsomite (MgSO₄ . 7H₂O) + hexahydrite (MgSO₄ . 6H₂O)) was exposed to a vapor phase with its humidity buffered by a salt-saturated aqueous solution at a fixed temperature (e.g., 75.30 % RH at 25.0 ºC for NaCl-saturated solution) for a certain period of time, and the mass difference between the initial and final sample was used to indicate which hydrous metal sulfate was stable at the buffered RH-T condition. By varying temperature of the sample-RH buffer assemblage, the equilibrium RH-T condition, at which no mass change could be detected, could be easily determined with high precision. This experimental method has been widely used to determine the stability fields of hydrous metal sulfates, containing Fe, Cu, Mg, Ni, Co, Cd and Zn metals, and their thermodynamic properties (i.e., Gibbs free energy, enthalpy and entropy of reaction; summarized in Chou et al., 2013, J. Asian Earth Sci., 62, 734-758 and references therein). Results have been applied to the studies related to the acid mine drainage problems on Earth and mineralogy in Mars and Europa.
These two examples illustrated that, in some cases, sophisticated experimental facilities and complicated experimental procedures are not required for obtaining useful scientific results.

gravimetric measurement, permeability of H2 in precious metals, humidity buffer, metal sulfate