In 1904, H. Friedenthal recommended that the hydrogen ion concentration be used to characterize solutions. He also pointed out that alkaline (modern word = basic) solutions could also be characterized this way since the hydroxyl concentration was always (1 x 10-14 / the hydrogen ion concentration). Many consider this to be the introduction of the pH scale. The concept of pH was itself introduced in 1909 by the Danish chemist Søren Sørensen as a convenient way of expressing acidity, the negative logarithm of hydrogen ion concentration.
In 1909, Robert Andrews Millikan began a series of experiments to determine the electric charge carried by a single electron. He began by measuring the course of charged water droplets in an electrical field. The results suggested that the charge on the droplets is a multiple of the elementary electric charge, but the experiment was not accurate enough to be convincing. He obtained more precise results in 1910 with his famous oil-drop experiment in which he replaced water (which tended to evaporate too quickly) with oil.
In 1911 Frederick Donnan studied the conditions under which equilibrium is established between two electrolytic solutions separated by a semipermeable membrane or a membrane through which the solvent and some, but not all, of the dissolved ions can pass. In the absence of such a membrane, the solvent and every species of dissolved ion will diffuse freely from each solution into the other, until the composition of the two solutions becomes the same. The semipermeable membrane, however, prevents the transfer of at least one ionic species, and the preservation of electrical neutrality limits the diffusion of that species' oppositely charged partner. The nature of the equilibrium and the existence of the potential have both become associated with Donnan's name.
Jaroslav Heyrovský, Nobel laureate, eliminated the tedious weighing required by previous analytical techniques using the differential precipitation on mercury step by monitoring drop-time. In the previous method a voltage was applied to a dropping mercury electrode and a reference electrode was immersed in a test solution. After 50 drops of mercury were collected, they were dried and weighed. The applied voltage was varied and the experiment repeated. Measured weight was plotted vs. applied voltage to obtain the curve. In 1921, he had the idea of measuring the current flowing through the cell instead of just studying drop-time.
On February 10, 1922, the "polarograph" was born as Heyrovský recorded the current-voltage curve for a solution of 1 M NaOH. Heyrovskýcorrectly interpreted the current increase between -1.9 and -2.0 V as being due to deposition of Na+ ions, forming an amalgam. Shortly thereafter, with his Japanese colleague, Masuzo Shikata, he constructed the first instrument for the automatic recording of polarographic curves, which became world famous later as the polarograph.
In 1923, within several months of each other, Johannes Nicolaus Brønsted (Denmark) and Thomas Martin Lowry (England) published essentially the same theory about how acids and bases behave. An acid was defined as a compound tending to give up a proton (or hydrogen ion), while a base was one tending to combine with a proton. This new view accounted for all the facts already satisfactorily accounted for by the old view of Svante Arrhenius. In addition it represented a greater flexibility that made it possible to extend acid-base notions into areas in which the old view was inadequate.
Alexander Naumovich Frumkin and his co-workers made extensive measurements of the adsorption of organic compounds on mercury, for which he proposed a quantitative model based on a semi-empirical adsorption isotherm that is universally known as the Frumkin isotherm. He summarized his work on the double layer on mercury in a comprehensive review in the series Ergebnisse Exakten Naturwissenschaft in 1928. Following studies on liquid gallium and amalgams he proposed explicitly the concept of potential of zero charge (PZC) as a characteristic property of the metal closely related to the Volta potential. During the 1920’s he also began work on porous solids, such as platinum black, charcoal, and silver iodide.