From Big Medical Encyclopedia

HYDROGEN ION EXPONENT (pH) — the quantitative characteristic of active reaction of the environment.

Accusative is of great importance since the speed of enzymatic reactions and consequently, processes of a metabolism and fiziol, functions substantially depend on reaction of the environment. Constancy of pH biol, liquids is a necessary condition of a normal course of vital processes (see. Acid-base equilibrium ). Accusative is in number equal to a negative decimal logarithm of hydrogen ion activity: pH = — lga H + . In dilute solutions the size of activity can be replaced with concentration; in this case rn = — lgC H + .

The scale of acidity and alkalinity of aqueous solutions is constructed on the basis of the following reasons. Pure water dissociates on ions:

H 2 O ⇆ H + + OH - .

Applying the mass action law (see. the Operating masses the law ), it is possible to write:

K D = ([H + ] [OH - ]) / [H 2 O]

Since extent of water dissociation insignificant (at t ° 18 ° αH 2 O = 1,4×10 - 9 ), concentration of not dissociated water molecules can be considered a constant and to combine it with K D in one size K W

K W = K D • [H 2 O] = [H + ] • [OH - ].

K W call ionic product of water. This size is a constant for this temperature; at t°22 °K W = 10 - 14 g ion/l.

As at dissociation of water molecules the identical number of hydrogen and hydroxylic ions, in pure water is formed [H + ] = [OH - ] = 10 - 7 g ion/l.

Therefore, pure pH is equal to 7. Aqueous solutions with such pH are neutral. Solutions with >pH7 are alkaline, and with <rn7 — acid.

Blood of the person normal has alkalescent reaction. Little changes of pH of blood can lead to serious consequences (see. Alkalosis ,, Acidosis ). PH values nek-ry biol, liquids of a human body are included below at a standard temperature of a body (see the tab.).

Definition of pH is carried out by usually colorimetric and electrometric methods.

Colorimetric methods of measurement of pH are based that coloring of acid-base indicators (see) depends on the size of hydrogen ion concentration. In the beginning carry out rough definition of pH by means of the universal indicator or an indicator piece of paper. On the basis of roughly certain pH value select the indicator so that its transition point (rk) differed from pH of the studied liquid as little as possible. After that carry out exact definition of pH by one of following methods.

Buffer method: the identical quantity of drops of solution of the picked-up indicator is added to the studied solution and a series of buffered solutions with the known pH values (see. Buffered solutions ) also compare character or intensity of coloring.

At an unbuffered method compare intensity of coloring of the studied solution, the indicator, and solution is added to Krom, in Krom the indicator is dissociated completely.

The following shortcomings are inherent in colorimetric methods of definition of pH: small accuracy (to 0,2 units of pH), presence of proteins and salts often changes an area of coverage of the indicator (proteinaceous and salt mistakes), the method is of little use for the painted and muddy solutions.

The electrometric method of definition pH is based on measurement e d from the galvanic cells made of a reference electrode, i.e.

an electrode, potential to-rogo is known and constant, napr, calomel electrode (see), and indicator — an electrode, potential to-rogo depends on hydrogen ion concentration, napr, hydrogen electrode (see), quinhydrone electrode (see) and glass electrode (see). Accuracy of electrometric definition of pH is in limits of 0,01 — 0,005 units of pH.

In medical practice broad application for definition of pH was found by Astrup's device allowing to define except a true value of pH of blood, value of active reaction of the blood saturated with carbon dioxide gas. The device represents the temperature-controlled measuring camera (t ° 38 °), the cut is entered into a cavity a glass electrode of pH-meter. The device of the camera allows to pass carbon dioxide gas through the studied liquid.

See also Active reaction of the environment .


Bibliography: Bates R. Definition of pH, the theory and practice, the lane with English, L., 1968; Schwab K. Bases of the technology of measurement of pH, the lane with it., M., 1962, bibliogr.

V. A. Popkov.