INDICATORS in chemistry (Latin indicator the index) — the substances changing the color in the presence of these or those chemical compounds in the studied environment (in solution, in air, in cells, in fabrics), and also at change of pH or redox potential of the environment; are widely applied in biochemical, clinical and sanitary and hygienic laboratories.
And. apply to definition of the end of reaction (a point of equivalence) at titration, to colorimetric determination of the sizes pH or redox potentials, to different detection of substances in these or those studied objects. For all these purposes I. apply in the form of water or spirit solutions or in the form of the indicator pieces of paper representing the strips of filter paper impregnated And.
Depending on appointment and the mechanism of action And. subdivide into a number of groups.
Acid base indicators represent the complex organic compounds changing coloring (two-color And.) or its intensity (one-color And.) depending on pH of the environment. Two-color And. lackmoid is, e.g.: in alkaline condition it has blue coloring, and in acid — red. Example one-color And. phenolphthalein, colourless can serve in acid medium and crimson in alkaline.
According to W. Ostwald's theory acid-base And. represent weak organic to - you or the bases, not dissociated molecules of which have other coloring, than the anions and cations formed by them in solution. Phenolphthalein, e.g., is weak to - that, not dissociated molecules a cut are colourless, and anions paint solutions in crimson color. In solutions I., representing weak to - you, dissociate on the equation
where ON — not dissociated molecules I., H + — hydrogen ions, and A - — anions I.
Ionization constant such And.
Ka = is equal [N + ] • [And - ] / [ON] (2)
(square brackets designated molar concentration of the corresponding particles).
And., the representing mild bases, dissociate on the equation
where — not dissociated molecules I., B+ — cations And., and OH-— hydroxylic ions.
Dissociation constant of these And.
Kb = is equal [B + ] • [OH - ] / [BOH] (4)
2 and 4 follows From the equations that than more size of a dissociation constant, subjects more break up And. on ions and that, therefore, at higher ion concentrations of H + (when P. — weak to - that) or ions of OH - (when And. — mild base) is suppressed its dissociation and there is decolourization. Different And. have various size Ka and Kb., therefore they change the coloring at various pH values of the environment. That interval of pH values, in Krom occurs decolourization this And., call an area of coverage or an interval of transition I. Interval of transition And. it is usually equal to the size pK ± 1 where pK is equal — lgK. Transition point And. call that pH value, at Krom visually change of coloring most clearly is perceived And. The transition point is approximately equal to size rk this And.
Acid-base And. widely apply at titration to - t and alkalis, and also to colorimetric measurement of the size pH biol, liquids, cells, fabrics, etc.
Titration to - t and alkalis shall be finished at the time of achievement of a point of equivalence, i.e. at the moment when to titrable solution to - you (alkali) are added such volume of a titrant, in Krom the equivalent quantity to - you contains (alkalis). For this purpose it is necessary to apply such And., the transition point to-rogo is equal to the size pH of titrable solution in a point of equivalence (see. Neutralizations method ). Are listed in the tab. And., the most used at titration to - t and the bases.
Qualitative test of acidity and alkalinity is made by means of so-called neutral And., which transition point is practically at pH 7,0. Treat them, e.g., the litmus having in acid medium (pH less than 7,0) red, and in alkaline condition (pH more than 7,0) blue color; the neutral red who is painted in acid medium in red color, and in alkaline — in yellow color.
Approximate measurement of the size pH of the environment (with an accuracy of 0,5 — 1,0 units of pH) is usually performed by means of universal (combined) And., representing mix of several And., which intervals of transition are close to each other and cover wide area of pH values.
Add 1 — 2 drop of solution to 0,5 ml of ispytuyemy liquid universal And. and the coloring appearing at the same time is compared to the enclosed color scale, on a cut by the indicated values of pH answering to various colourings of I. Primenyayut also of a strip of filter paper, impregnated universal And.
For more exact (0,1 — 0,5 units of pH) colorimetric determination of the size pH usually use one-color And. a row dinitro-and nitrophenols, offered by L. Michaelis and representing weak to - you, changing coloring from colourless (in acid medium) to yellow (in alkaline). With the same purpose use a row two-color And., offered by Clark (W. M of Clark) and H. A. Lubs, representing sulfoftaleina. Acid and alkaline forms of these And. sharply differ on color, in it their advantage in comparison with L. Michaelis's indicators consists.
Oxidation-reduction, or redoksindikator, represent organic dyes which color in the oxidized and recovered state is various. Such And. apply at oxydimetric titration (see. Oxidimetry ), and also for colorimetric determination of sizes of redox potentials of liquids (see. Redox potential ), separate cells and fabrics in cytochemical, and tsitol, laboratories. The majority of redox indicators at recovery turns into colourless connections, and at oxidation is painted. The oxidized and recovered forms I. are in solutions in a condition of a dynamic equilibrium:
the oxidized form + ne <which -> is got into condition where n — number of electrons.
A ratio between equilibrium concentrations of two forms this And., and consequently, and color of solution, in Krom is And., depend on the size of redox potential of solution. If the size of potential of solution is more than normal redox potential (E0) of this redox indicator, then the most part And. in this solution passes into the oxidized form (usually painted) if the redox potential of the studied environment is less than E0, then And. turns into the got into condition (usually colourless). At equality of values of redox potential of the environment and E0 of the indicator of concentration of the oxidized and recovered forms I. are equal each other. Having a row I. with various E0 values, it is possible on their coloring in this environment to judge the size of redox potential of this environment. The redox indicators offered by Mikhaelis, having the general name «viologena» and representing derivatives gamma and scale '-dipiridilov have small toxicity and are widely used to measurement of redox potentials in biol, systems; at these And. it is painted got into condition.
Normal redox potential of viologen does not depend on the size pH of solution. They differ in it from other redox indicators.
Kompleksonometrichesky indicators (metalindicators) represent well water soluble organic dyes capable to form the painted complex connections with ions of metals. These And. are applied to establishment of a point of equivalence at kompleksonometrichesky titration (see. Kompleksonometriya ).
The adsorptive indicators — these are the organic dyes which are adsorbed on surfaces of rainfall which is formed at titration by a method of sedimentation and the points of equivalence changing the color at achievement. E.g., tropeolin 00 at titration of chlorides solution of caustic silver changes coloring in a point of equivalence with yellow for pink.
Chemiluminescent (fluorescent) indicators — the organic compounds (for example, lyumenol, lyutseginin, siloxen, etc.) having ability to luminesce at natural lighting or at radiation by an ultraviolet light. Intensity and color of a luminescence depend both on the size pH of the environment, and on the size of its redox potential; these And. are applied at titration (at neutralization and oxidimetry) strongly painted or muddy liquids when decolourization usual And. imperceptibly.
And. are used in many biochemical methods applied in a wedge. - biochemical laboratories. The most usable of them are bromtimolovy blue (during the definition of activity of a fruktozodifosfataldolaza in blood serum, activity of acetylcholinesterase and cholinesterase in blood serum according to A. A. Pokrovsky, and also activity of a karboksilesteraza in blood according to A. A. Pokrovsky and L. G. Ponomareva), bromphenol blue (at electrophoretic division of various proteins for coloring elektroforegramm along with amidoblack and acid blue-black), universal And., phenol red (during the definition of activity aspartate - and alaninaminotranspherases in blood serum, activity of cholinesterase in blood serum etc.), phenolphthalein, nitroblue tetrazoliya, used for quality and quantitative standard of activity of various dehydrogenases (see. Dehydrogenases ), etc.
Table. The short characteristic of the indicators which are most often used at titration of acids and the bases
Bibliography: Vinogradova E. H. Methods of definition of concentration of hydrogen ions, M., 1956, bibliogr.; Indicators, under the editorship of E. Bishop and I. N. Marov, lane with English, t. 1 — 2, M., 1976, bibliogr.
V. P. Mishin.