[en] The objectives of this research are to develop and improve methodologies for measurement of diffusion properties in low-permeability sedimentary and crystalline rocks, and to develop methods for measurement of pH in high-ionic-strength aqueous solutions. Four separate projects are described. The first project involved improvement and further development of the radiography method by using a monochromatic Am-241 γ-ray source. The use of monochromatic γ-radiation (γ-RAD) eliminates beam hardening which is a limitation to the precision and accuracy of the X-ray radiography technique. With the elimination of beam hardening, the γ-RAD technique allows for reliable calibration that is essentially independent of background matrix. In the second project, diffusion coefficients for iodide (I-) tracer were measured simultaneously using γ-RAD and through-diffusion on granite. Although only one test was conducted, the results indicate that the γ-RAD method will be a viable alternative to through-diffusion for measurements on very-low-porosity crystalline rocks. The third project focused on the investigation of the effect of partial gas saturation on diffusion coefficients. A method has been developed to generate partial gas saturation in a rock sample by equilibrating the porewater with nitrogen (N₂) gas at high pressure (up to 7000 kPa) and then rapidly lowering the N₂ pressure to atmospheric. The degree of partial saturation is determined by the γ-RAD method. The effective diffusion coefficient (De) for iodide tracer at 100% brine saturation was compared to that at different degrees of partial gas saturation. A preliminary result from Queenston Formation shale indicates a 53% decrease in D_e as a result of 14.6% partial gas saturation. The results indicate good potential for evaluating the effect of partial saturation on diffusion in the low-permeability rocks that contain high salinity porewater. The fourth project focussed on pH measurement in high-ionic-strength brine solutions. Buffers of varying composition and ionic strength were formulated and their pH values were determined by geochemical modelling using the Pitzer ion-interaction approach implemented in the geochemical program PHREEQC. These buffers were used to investigate two methods for pH measurement: potentiometric measurements with glass electrodes, and spectrophotometric measurements using the colorimetric indicator phenol red. The pH electrode response is linear over a range from 1.4 to 9.1 and for ionic strengths up to 8.2 mol/kg. However, there is a systematic offset with increasing ionic strength such that an electrode calibrated with low-ionic-strength buffers will underestimate pH of a high-ionic-strength solution (8.2 mol/kg) by 0.6 to 0.7 pH units. For any given ionic strength, the potentiometric measurement is also sensitive to the ionic composition of the solution. Despite these effects, accurate potentiometric measurements are possible if the composition of the iv calibration buffers is similar to the test solution. The results of spectrophotometric measurements indicate that the disassociation constant (pK'a) of the phenol red indicator is virtually insensitive to the ionic composition of the solution. A maximum error of 0.2 units is possible for pH measured spectrophotometrically if the ionic strength of the buffers does not match the ionic strength of the test solution. However, the measurement range of phenol red is limited to a pH range from ~7 to 9; additional indicators can be used to increase the effective range for the spectrophotometric approach. (author)