What is the pH of Acid

What actually is the pH value? General information on acids and bases

In the course of the historical development of chemistry, the terms “acids” and “bases” have undergone profound changes several times. The term "acid" has been used for many centuries. It was first used for acidic vegetable juices. BOYLE (1663) was the first to introduce a clear phenomenological definition for acids:

An acid is a substance that is fluffed up with chalk, sulfur is precipitated from sulfur liver and certain vegetable pigments redden. Solutions that taste unpleasantly bitter, have a soapy filling, turn litmus blue and can neutralize their effects when combined with acids, were called alkaline (al kali Arabic = vegetable ash).

It was later discovered that salts can be obtained by combining acidic and alkaline solutions, and the soluble substances contained in the alkaline solutions were called bases (base gr. = Basis (namely for the production of a salt)).

In the course of time, more precise definitions have been coined and the properties of acids and bases have been related to their composition and structure.

The ion theory established in 1883, according to which freely movable ions are present in solutions of electrolytes, formed a milestone in the development of modern chemistry and also enabled for the first time a deeper, albeit imperfect, picture of the properties of acids and bases. According to the terminology of ARRHENIUS, all hydrogen compounds that give rise to H3O + ions in aqueous solution are to be referred to as acids; Bases are hydroxy compounds that form hydroxide ions (OH ions) when dissolved in water. The formation of these ions was thought to be the “dissociation” of molecules.

Numerous reactions in aqueous solutions can be made understandable with the terms; It was also possible for the first time to precisely determine the strength of acids and bases by means of a material constant - the equilibrium constant of the respective dissociation equilibrium.

In 1923, we independently developed a broader concept for acids and bases. According to this, an acid is a substance that can donate protons (proton donor); a substance that can absorb protons is called a base (proton acceptor). According to this theory, substances are not characterized, but a specific function is described: the potential ability to release or absorb protons. According to this, an acid-base reaction consists in the transfer of protons from the acid to the base.

The last major milestone was introduced in 1938 by an extended acid-base term. According to this, a base (a nucleophilic substance) provides a pair of electrons to form a covalent bond and thus acts as an electron pair donor. The acid (an electrophilic substance) acts as an electron pair acceptor.
However, there are also substances that can react either as an acid or as a base. They are called amphoteric. Many solvents are amphoteric. For example, water reacts as a base in the presence of acids and as an acid in relation to bases.

The pH

If the conductivity of water is checked with very sensitive measuring instruments, it can be observed that even the purest, repeatedly distilled water has a conductivity that is, however, minimal. This phenomenon proves that ions must also be present in low concentrations in pure water. These are created by a process known as autoprotolysis of the water:

As the low value of the electrical conductivity shows, the equilibrium lies far to the left of the equation. The equilibrium constant, which can be calculated for every homogeneous equilibrium reaction, is obtained according to the law of mass action for autoprotolysis:

At 25 ° C, Kwater = 1.008 * 10 minor. Since the same number of HO and OH ions are present in water, their concentration according to the ion product at 25 C is:

In about 555 million water molecules there is only one HO and one OH ion.

The ability to associate acidic solutions with a value goes back to the Danish chemist SOREN SORENSEN, who in 1909 formulated the pH value of a solution as the negative logarithm of the HO ion concentration.

pH is an abbreviation and means "pondus hydrogenii" in Latin, which means something like "power of water". This conversion gives you small, more practical numbers. The pH value of a solution indicates whether a solution is acidic, neutral or alkaline. The pH value scale ranges from 1–14 (see illustration).

How can the pH value be measured?

The pH value determines the course of many reactions, e.g. B. all reactions in aqueous solutions in which acids and bases are involved, and especially the course of many biochemical reactions. A simple and quick pH measurement is therefore of great importance. It is carried out either potentiometrically (with pH measuring devices and an accuracy of +/- 0.01 pH units) or with indicators. Indicators or more precisely “pH indicators” are dyes that can change their color depending on the pH value. Examples of indicators are plant dyes that BOYLE already used, such as litmus or red cabbage. If the transition points of the indicators are known, the pH of various solutions can be determined with an accuracy of 0.1-0.2 pH units. In practice, either paper strips impregnated with indicators (“indicator papers”) or mixtures of different indicator solutions are used, the transition areas of which are selected so that a different color occurs at each pH value (universal indicators).

SWEETS (2005) issue 1-2