The hydrogen index (pH, from Latin pondus Hydrogenii — "weight of hydrogen") is a measure of the acidity of aqueous solutions. It is a way of expressing the activity of hydrogen cations in solutions. Opposite in sign and equal in modulus to the decimal logarithm of the activity (a) of hydrogen cations (H+) expressed in moles per liter, which in highly dilute solutions can be considered equal to their equilibrium molar concentration ([H+]):

pH = -lg(aH+) approximately -lg([H+]).

For aqueous solutions (under standard conditions):

pH < 7 corresponds to an acidic solution;

pH = 7 corresponds to a neutral solution, sometimes referred to as acidic;

pH > 7 corresponds to the basic solution, go to the section "pH values in solutions of different acidity".

The hydrogen index can be determined using acid-base indicators, measured by a potentiometric pH meter.

Accurate measurement and regulation of pH is necessary in various branches of chemistry, biology, materials sciences, technology, medicine and agronomic chemistry.

This concept was introduced in 1909 by the Danish chemist Sørensen. The indicator is called pH, according to the first letters of the Latin words potentia hydrogenii — the strength of hydrogen, or pondus hydrogenii — the weight of hydrogen. In general, in chemistry, the combination of pX is used to denote a value equal to −lg X. For example, the strength of acids is often expressed as pKa = −lg Ka.

In the case of pH, the letter H denotes the concentration of hydrogen ions (H+), or, more precisely, the thermodynamic activity of hydroxonium ions.

Several techniques are widely used to determine the pH of solutions. The hydrogen index can be estimated approximately using indicators, measured accurately with a pH meter, or determined analytically by acid-base titration.

For a rough assessment of the concentration of hydrogen ions, acid-base indicators are widely used - organic dyes whose colour depends on the pH of the medium. The best known indicators are litmus, phenolphthalein, methyl orange and others. Indicators can exist in two different colour forms - either acidic or basic. The colour change of each indicator occurs within its own acidity range, usually 1-2 units.

To extend the working range of pH measurement, a so-called universal indicator is used, which is a mixture of several indicators. The universal indicator changes colour continuously from red to yellow, green, blue and violet as it moves from the acidic to the basic range. pH measurement using the indicator method is difficult in turbid or coloured solutions.

Using a special instrument - a pH meter - allows you to measure the pH over a wider range and more accurately than using indicators. The ionometric method of pH determination is based on the measurement of a millivoltmeter ionometer EMF of a galvanic circuit with a special glass electrode, the potential of which depends on the concentration of H+ ions in the surrounding solution. The method is convenient and highly accurate, especially after the calibration of the indicator electrode in the selected pH range, allows the measurement of the pH of opaque and coloured solutions and is therefore widely used.

The analytical volumetric method - acid-base titration - also gives accurate results for determining the acidity of solutions. A solution of known concentration (titrant) is added drop by drop to the test solution. When they mix, a chemical reaction takes place. The equivalence point - the point at which the titrant is just enough to complete the reaction - is determined using an indicator. In addition, the acidity of the solution is calculated by knowing the concentration and volume of the titrant solution added.

In the absence of instrumental means to determine pH, aqueous extracts of anthocyanins - plant pigments that colour flowers, fruits, leaves and stems - can be used. Their structure is based on a flavyl cation in which the oxygen in the pyran ring is free. For example, cyanidin has a reddish-purple colour, but the colour changes with pH: solutions are red at pH <3, purple at pH 7-8 and blue at pH >11. Typically, anthocyanins have a red colour of varying intensity and shades in acid, and a blue colour in alkaline. Such changes in anthocyanin colour can be observed by adding acid or alkali to the coloured juice of currants, cherries, beetroot or red cabbage.

The acidity of the medium is important in a variety of chemical processes and the ability to proceed or the outcome of a particular reaction is often dependent on the pH of the medium. Buffers are used to maintain a certain pH in a reaction system during laboratory tests or in production, allowing a near constant pH to be maintained when diluting or adding small amounts of acid or alkali to the solution.

The Hydrogen pH Index is widely used to characterise the acid-base properties of various biological media.

The acidity of the reaction medium is of particular importance for biochemical reactions in living systems. The concentration of hydrogen ions in solution often affects the physicochemical properties and biological activity of proteins and nucleic acids, making the maintenance of acid-base homeostasis a task of paramount importance for the normal functioning of the body. The dynamic maintenance of the optimal pH of biological fluids is achieved through the action of the body's buffering systems.