It is a unit that describes the acidity and alkalinity of a solution.
The biochemical processes such as transport of oxygen in the blood, the catalysis of reactions by enzymes and the generation of metabolic energy are strongly affected by the concentration of H+ ion.
Many biological reactions are dependent on the charge on the molecules (+ ve or -ve charge).
The charge on the molecule is determined by the ability of molecule to release or accept a proton, which in turn depends on the pH of the solution. So, in biological experiments it is necessary to measure the concentration of H+ ion.
The H+ ion concentration of most of the biological solutions is very low and is in the range of 10-1 to 10-14 gram ions/L, which is very difficult to measure and express in conventional method of expression. Hence, Sorenson in 1909 introduced the term pH, to express the hydrogen ion concentration in a logarthinmic manne, which is defined as pH = -log of [H+] or log 1 / [H+] . According to this the [H+] concentration of 10-8 g /L will be pH 8.0. The 'p' denotes 'negative logarithm of ','p' also stands for power.pH is the abbreviation of 'power of hydrogen'
The pH scale is the useful way of expressing acidity, which in turn dependent on [H+]. pH is generally in the range of 0 - 14, as the dissociation constant of water at 250C is 10-14.
An acidic solution has a pH below 7 and a basic solution has a pH above 7. Pure water is neither acidic nor basic and is said to be neutral [H+] = [OH-] = 10-7 M
pH is inversely related to hydrogen ion concentration. That is lower the pH the higher the H+ ion concentration.
The pH scale is logarithmic (exponential) not arithmetic (linear). This means that when a solution changes from pH 7 to pH 6, the H+ ion concentration increases by 10 fold. When it goes from pH 7 to pH 5 it increases by 100 fold. The pH of water is 7.0 that means water contains 1x10-7 g of H+ ions/L.
The pH of an aqueous solution can be approximately measured using various indicator dyes, including litmus, phenolphthalein and phenol red, which undergo colour changes as a proton dissociate from the dye molecule.
Accurate determination of pH in chemical and clinical laboratory is made with a glass electrode that is sensitive to H+ ion concentration. Measurement of pH sometimes used in the diagnosis of diseases. The normal pH of human plasma is 7.4, which is referred to as physiological pH. The blood of patients suffering from certain diseases such as diabetes can have a lower pH, a condition called acidosis. The condition in which the pH of the blood is higher than 7 is called alkalosis.
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