<P> The pH scale is traceable to a set of standard solutions whose pH is established by international agreement . Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode . The pH of aqueous solutions can be measured with a glass electrode and a pH meter, or an indicator . </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> pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, a +, in a solution . </P>

The ph of pure water is 6.5 its temperature is