Among the variety of organic substances, there are special compounds that are characterized by color changes in a different environment. Before the advent of modern electronic pH meters, indicators were indispensable "tools" for determining the acid-base parameters of the medium, and continue to be used in laboratory practice as excipients in analytical chemistry, as well as 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 the available methods in the absence of modern 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 informational scale. Thanks to such a variety of chromophores and their low activity among themselves, scientists were able to create universal indicators that are widely used in laboratory and industrial conditions.
Most Used pH Indicators
It is noteworthy that in addition to the identification property, these compounds have good coloring 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 in mixtures with each other, or for specific syntheses and reactions.
Methyl orange
Many dyes were named due to their primary colors in a neutral environment, which is inherent in this chromophore. Methyl orange is an azo dye that has a grouping - N = N - in its composition, which is responsible for the transition of the indicator color to red in an acidic environment, and to yellow in an alkaline one. The azo compounds themselves are not strong bases, but the presence of electrodonor groups (- OH, - NH 2 , - NH (CH 3 ), - N (CH 3 ) 2 , etc.) increases the basicity of one of the nitrogen atoms, which becomes able to attach hydrogen protons by donor-acceptor principle. Therefore, when the concentration of H + ions in the solution changes, one can observe a change in the color of the acid-base indicator.
Read More on Getting Methyl Orange
Methyl orange is obtained in the reaction with diazotization of sulfanilic acid C 6 H 4 (SO 3 H) NH 2 , followed by combination with dimethylaniline C 6 H 5 N (CH 3 ) 2 . Sulfanilic acid is dissolved in a solution of sodium alkali, adding sodium nitrite NaNO 2 , and then cooled with ice to carry out the synthesis at temperatures as close to 0 ° C and added hydrochloric acid HCl. Next, a separate solution of dimethylaniline in HCl is prepared, which is poured chilled into the first solution to obtain a dye. It is further alkalinized, 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 two reagents that are involved in its synthesis. The color of the indicator is notable for a change in its color in an alkaline environment with the acquisition of a raspberry (red-violet, raspberry-red) hue, which discolors when the solution is strongly alkalized. Phenolphthalein can take several forms depending on the pH of the medium, and in strongly acidic environments it has an orange color.
This chromophore is obtained by condensation of phenol and phthalic anhydride in the presence of zinc chloride ZnCl 2 or concentrated sulfuric acid H 2 SO 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 established cumulative properties.
Litmus
This indicator was one of the first reagents used on solid media. Litmus is a complex mixture of natural compounds, which is obtained from some species of lichens. It is used not only as a coloring matter, but also as a means for determining the pH of the medium. This is one of the first indicators that began to be used by humans in chemical practice: it is used in the form of aqueous solutions or strips of filter paper impregnated with it. Solid litmus is a dark powder with a faint ammonia odor. When dissolved in pure water, the color of the indicator takes on a violet color, and when acidified, gives a red color. In an alkaline environment, the litmus turns into blue, which allows it to be used as a universal indicator for the general determination of the environmental indicator.
It is not possible to precisely establish the mechanism and nature of the reaction that occurs when the pH changes in the structures of the litmus components, since it can contain up to 15 different compounds, some of which can be inseparable active substances, 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 been greatly simplified, since now it was not necessary to have ready-made liquid reagents for any field research, which scientists and forensic scientists have been successfully using to this day. So, solutions were replaced by universal indicator papers, which, thanks to a wide spectrum of action, almost completely eliminated the need to use any other acid-base indicators.
The composition of the impregnated strips may differ from different manufacturers, so an approximate list of incoming substances 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 shows the standards of the color scale, allowing to determine the pH of the medium from 0 to 12 (somewhere 14) with an accuracy of one.
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 necessary to choose a universal organic solvent.
Due to its properties, acid-base indicators have found application in many fields of science, and their diversity has allowed the creation of universal mixtures that are sensitive to a wide range of pH indicators.