<P> There are three current theories used to describe Acid--base reactions: Arrhenius, Bronsted - Lowry and Lewis when determining pH . </P> <P> The concept of pH was first introduced by the Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909 and revised to the modern pH in 1924 to accommodate definitions and measurements in terms of electrochemical cells . In the first papers, the notation had the "H" as a subscript to the lowercase "p", as so: p . </P> <P> The exact meaning of the "p" in "pH" is disputed, but according to the Carlsberg Foundation, pH stands for "power of hydrogen". It has also been suggested that the "p" stands for the German Potenz (meaning "power"), others refer to French puissance (also meaning "power", based on the fact that the Carlsberg Laboratory was French - speaking). Another suggestion is that the "p" stands for the Latin terms pondus hydrogenii (quantity of hydrogen), potentia hydrogenii (capacity of hydrogen), or potential hydrogen . It is also suggested that Sørensen used the letters "p" and "q" (commonly paired letters in mathematics) simply to label the test solution (p) and the reference solution (q). Currently in chemistry, the p stands for "decimal cologarithm of", and is also used in the term pK, used for acid dissociation constants . </P> <P> Bacteriologist Alice C. Evans, famed for her work's influence on dairying and food safety, credited William Mansfield Clark and colleagues (of whom she was one) with developing pH measuring methods in the 1910s, with wide influence on laboratory and industrial use thereafter . In her memoir, she does not mention how much, or how little, Clark and colleagues knew about Sørensen's work a few years prior . She said: </P>

Importance of ph value in human beings and plants wikipedia