Among the variety of organic substances, there are special compounds that are characterized by color changes in different environments. Before the advent of modern electronic pH meters, indicators were indispensable "tools" for determining the acid-base indicators of the environment, and continue to be used in laboratory practice as auxiliary substances in analytical chemistry, and also in the absence of the necessary equipment.
What are indicators for?
Initially, the property of these compounds to change color in various media was widely used to visually determine the acid-base properties of substances in solution, which helped to determine not only the nature of the medium, but also to draw a conclusion about the resulting reaction products. Indicator solutions continue to be used in laboratory practice to determine the concentration of substances by titration and allow you to learn how to use improvised methods for lack ofmodern pH meters.
There are several dozens of such substances, each of which is sensitive to a rather narrow area: usually it does not exceed 3 points on the informativeness scale. Thanks to such a variety of chromophores and their low activity among themselves, scientists managed to create universal indicators that are widely used in laboratory and production conditions.
Most used pH indicators
It is noteworthy that in addition to the identification property, these compounds have a good dyeing ability, which allows them to be used for dyeing fabrics in the textile industry. Of the large number of color indicators in chemistry, the most famous and used are methyl orange (methyl orange) and phenolphthalein. Most other chromophores are currently used mixed with each other, or for specific syntheses and reactions.
Methyl orange
Many dyes are named for their primary colors in a neutral environment, which is also true for this chromophore. Methyl orange is an azo dye having a grouping - N=N - in its composition, which is responsible for the transition of the color of the indicator to red in an acidic environment, and to yellow in an alkaline one. Azo compounds themselves are not strong bases, however, the presence of electron donor groups (‒ OH, ‒ NH2, ‒ NH (CH3), ‒ N (CH 3)2 and others) increases the basicity of one of the nitrogen atoms,which becomes capable of attaching hydrogen protons according to the donor-acceptor principle. Therefore, when changing the concentrations of H+ ions in the solution, a change in the color of the acid-base indicator can be observed.
More on making methyl orange
Get methyl orange by reaction with diazotization of sulfanilic acid C6H4(SO3 H)NH2 followed by a combination with dimethylaniline C6H5N(CH 3)2. Sulfanilic acid is dissolved in a sodium alkali solution by adding sodium nitrite NaNO2, and then cooled with ice to carry out the synthesis at temperatures as close as possible to 0°C and hydrochloric acid HCl is added. Next, a separate solution of dimethylaniline in HCl is prepared, which is poured into the first solution when cooled, obtaining a dye. It is further alkalized, and dark orange crystals precipitate from the solution, which, after several hours, are filtered off and dried in a water bath.
Phenolphthalein
This chromophore got its name from the addition of the names of the two reagents involved in its synthesis. The color of the indicator is notable for its change in color in an alkaline medium with the acquisition of a raspberry (red-violet, raspberry-red) hue, which becomes colorless when the solution is strongly alkalized. Phenolphthalein can take several forms depending on the pH of the environment, and in strongly acidic environments it has an orange color.
This chromophore is produced by the condensation of phenol and phthalic anhydride in the presence of zinc chloride ZnCl2 or concentrated sulfuric acid H2SO 4. In the solid state, phenolphthalein molecules are colorless crystals.
Previously, phenolphthalein was actively used in the creation of laxatives, but gradually its use was significantly reduced due to the established cumulative properties.
Litmus
This indicator was one of the first reagents used on solid media. Litmus is a complex mixture of natural compounds that is obtained from certain types of lichens. It is used not only as a coloring agent, but also as a means to determine the pH of the medium. This is one of the first indicators that began to be used by man in chemical practice: it is used in the form of aqueous solutions or strips of filter paper impregnated with it. Litmus in the solid state is a dark powder with a slight ammonia odor. When dissolved in pure water, the color of the indicator becomes purple, and when acidified, it turns red. In an alkaline medium, litmus turns blue, which makes it possible to use it as a universal indicator for the general determination of the medium indicator.
It is not possible to accurately establish the mechanism and nature of the reaction that occurs when pH changes in the structures of litmus components, since it can include up to 15 different compounds, some of whichthey may be inseparable active ingredients, which complicates their individual studies of chemical and physical properties.
Universal indicator paper
With the development of science and the advent of indicator papers, the establishment of environmental indicators has become much simpler, since now it was not necessary to have ready-made liquid reagents for any field research, which scientists and forensic scientists still successfully use. So, solutions were replaced by universal indicator papers, which, due to their wide spectrum of action, almost completely eliminated the need to use any other acid-base indicators.
The composition of the impregnated strips may vary from manufacturer to manufacturer, so an approximate list of ingredients may be as follows:
- phenolphthalein (0-3, 0 and 8, 2-11);
- (di)methyl yellow (2, 9–4, 0);
- methyl orange (3, 1–4, 4);
- methyl red (4, 2–6, 2);
- bromothymol blue (6, 0–7, 8);
- α‒naphtholphthalein (7, 3–8, 7);
- thymol blue (8, 0–9, 6);
- cresolphthalein (8, 2–9, 8).
The packaging necessarily contains color scale standards that allow you to determine the pH of the medium from 0 to 12 (about 14) with an accuracy of one integer.
Among other things, these compounds can be used together in aqueous and aqueous-alcoholic solutions, which makes the use of such mixtures very convenient. However, some of these substances may be poorly soluble in water, so it is necessaryselect universal organic solvent.
Due to their properties, acid-base indicators have found their application in many fields of science, and their diversity has made it possible to create universal mixtures that are sensitive to a wide range of pH indicators.
