A robust method for evaluating the pH of e-liquids
The measurement of pH is one of the simplest and most common analytical methods practiced in the laboratory. The pH-meter is calibrated using certified buffers, the electrode is placed in the solution, and if a reading is obtained, the value is used. In reality, pH is much more complicated and the equivalency of readings from aqueous and non-aqueous solutions cannot be taken for granted. There are a number of physical chemical reasons for this which will be described. Besides equivalency, another issue with non-aqueous solvents is that small changes in water concentration can lead to large changes in the pH readings. With e-liquids, not only are the solvents non-aqueous, but they are also hygroscopic. The water content can vary depending on the environment in which they are stored or used. With glycerin, for example, the equilibrium water content can vary from 6% to 27% as relative humidity varies from 20% to 60%. The storage and measurement environment can lead to high variability in the pH measurement even for a consistent initial formulation of the e-liquid. Experiments were performed using simulated and commercial e-liquids to find a water dilution level for pH measurements that would be minimally influenced by variations in water content. At a 5:1 (w:w) dilution of the e-liquids with water, the measured pH becomes insensitive to the initial water content yet still provides information regarding the acid-base character. The diluted aqueous solution is also more equivalent to the aqueous buffers used for calibration and could potentially be more physiologically relevant to e-liquid aerosol deposited in the moist respiratory tract.