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Skin surface pH was determined by colorimetric methods in the early part of the 20th century [2]. This involved the application of an indicator to the skin, recollection thereof in a tube and the color change measured with a colorimeter. Schade and Marchionini [1], credited for the first significant reporting of skin surface pH, used a gas chain bell electrode to measure skin surface pH. In 1930, foil colorimetry, a simplification of the above-mentioned colorimetric method was introduced, in which indicator-impregnated sheets of adsorbing strips were placed on the skin with water [13]. Also, the skin’s barrier capacity was measured with the application of a highly diluted sodium hydroxide solution containing an indicator to the skin [14]. However, the development of glass electrodes responding to H⁺ ions by McInnes and Dole in 1930 and the invention of the “acidimeter” by Beckman in 1934 paved the way for the development and improvements of pH meters as it is known today [15]. The glass electrode with a flat (planar) measuring surface, devised by Ingold, was first used by Schirren (1955) to measure skin surface pH [16]. The planar electrode with one unit containing both the active and reference electrodes (also known as the “combined” electrode or “single glass rod measuring circuit”) is still the most universally method used today to measure skin surface pH (Fig. 1).

Table 1. A summary of factors influencing skin surface pH (adapted from [2, 4, 7, 9–12])

Though the focus of this chapter is on the planar electrode, it is worth mentioning that several other innovative methods for assessing skin surface pH have been developed and include the use of pH-sensitive fluorescent dyes followed by confocal laser microscopy, 2-photon fluorescence lifetime imaging or electron spin resonance imaging [11, 17–21]. A pseudo-pH measurement is possible with in-vivo Raman micro-spectroscopy [11]. However, these methods have not gained general acceptance.