Method development for pH analysis of ends aerosol and e-liquid
E-liquids are hygroscopic, non-aqueous liquids containing a small amount of water (~3-15%). When the pH of a non-aqueous liquid is measured using conventional methods and calibration buffers, the value can only be regarded as an "apparent pH". This is because the ionization constant of the acid or base, the dielectric constant of the medium, the liquid-junction potential, and the hydrogen-ion response of the glass electrode are all changed (paraphrased from USP 39 <791>). In addition, the hygroscopic nature of e-liquids results in uncontrolled variation in water content depending on the storage and measurement environment. This leads to an uncontrolled variation in the "apparent pH" value. Methodology was developed to measure the pH of e-liquids diluted with water to provide a more robust measurement which was compatible with conventional pH buffers and methods. To validate the dilution level, the pH of 25 commercial e-liquids was measured at dilution levels from 1:1 to 20:1 (w:w) water : e-liquid. The commercial samples covered a wide range of nicotine levels and glycerin to propylene glycol ratios. The most robust dilution level was found to be in the 7:1 to 9:1 dilution range. For aerosol measurement, an 8:1 dilution range was found to give more stable pH results. It was found that glass-fiber Cambridge filter pads (CFPs) used for aerosol collection could alter pH to higher values. Whatman QM-A quartz-fiber filter pads were found to have minimal influence on pH.