For clinical and environmental research and monitoring, it is important to be able to measure pH concentrations in small-volume solutions. However, conventional systems require the use of reference electrodes that limit reducing the volume of the solution. We demonstrated the use of Terahertz (THz) chemical microscopy (TCM) to measure the pH of solutions with a volume as small as 16 nL. This method realizes to measure the concentration of ions without any reference electrodes, which enables the measurement of volumes of solution that would be too small for conventional methods. Currently, we attempt the integration for multi-ion sensing for screening of ions in a small amount of blood, so minimally invasive diagnosis can be expected.