A quantity called the pH is a scale that is used to compare the relative strength of acids and bases. Generally speaking the pH of a substance can range from anywhere between 0 and 14.
History
The concept of pH was first introduced by Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909 and revised to modern pH in 1924 after it became apparent that electromotive force in cells depended on activity rather than concentration of hydrogen ions. In the first papers, the notation had the H as a subscript like so: ph.
