Color changes of indicators in solutions. Plant indicators in the school laboratory. Color change of indicators in solutions

Color change of indicators depending on pH

Acid-base indicators are compounds whose color changes depending on the acidity of the medium.

For example, litmus is red in an acidic environment and blue in an alkaline environment. This property can be used to quickly evaluate the pH of solutions.

Acid-base indicators are widely used in chemistry. It is known, for example, that many reactions proceed differently in acidic and alkaline media. By adjusting the pH, the direction of the reaction can be changed. Indicators can be used not only for qualitative, but also for quantitative assessment of the acid content in a solution (acid-base titration method).

The use of indicators is not limited to "pure" chemistry. The acidity of the environment must be controlled in many production processes, when assessing the quality of food products, in medicine, etc.

IN table 1 the most "popular" indicators are indicated and their color in neutral, acidic and alkaline media is noted.

Methyl orange

Phenolphthalein

In fact, each indicator is characterized by its own pH interval in which the color change occurs (transition interval). The change in color occurs due to the transformation of one form of the indicator (molecular) into another (ionic). As the acidity of the medium decreases (with an increase in pH), the concentration of the ionic form increases, and that of the molecular form decreases. Table 2 lists some acid-base indicators and their respective transition ranges.

There are various methods for determining the concentration (more precisely, activity) of hydrogen ions (and, accordingly, the concentration of hydroxide ions). One of the simplest (colorimetric) is based on the use acid-base indicators. Many organic acids and bases, which change their color in a certain narrow range of pH values, can serve as such indicators.

Indicators are weak acids or bases that have different colors in their undissociated and dissociated (ionic) forms.

Example.

1.Phenolphthalein is an acid which in molecular form (HJnd) is colorless at pH8.1. Phenolphthalein anions (Jnd -) at pH9.6 have a red-violet color:

H Jnd  H + + Jnd -

Colorless  red-violet

pH8.1 pH9.6

With a decrease in the concentration of H + ions and an increase in the concentration of OH ions, the molecular form of phenolphthalein becomes anionic due to the detachment of the hydrogen ion from the molecules and its binding to the hydroxide ion in water. Therefore, at pH9.6, the solution in the presence of phenolphthalein acquires a red-violet color. On the contrary, in acidic solutions at pH  8.1, the equilibrium shifts towards the molecular form of the indicator, which has no color.

2.Methyl orange is a weak base JndOH , which in molecular form at pH 4.4 has a yellow color. Jnd + cations at pH3.0 color the solution red:

JndOH  Jnd + + OH -

yellow  red

pH4.4 pH3.0

acid form indicator is called the form that prevails in acid solutions, and basic form - the one that exists in basic (alkaline) solutions. In a certain range of pH values ​​in the solution, a certain amount of both forms of the indicator can be simultaneously in equilibrium, as a result of which a transitional color of the indicator occurs - this is the pH range of the indicator color transition, or simply indicator transition interval.

Table 1 shows the transition intervals of some commonly used indicators.

Table 1

Acid-base indicators

For a quick determination of pH, it is also convenient to use a solution of a universal indicator, which is a mixture of various indicators and has a large transition range (pH values ​​from 1 to 10). On the basis of a universal indicator, the industry produces special paper tapes for determining the pH of solutions by comparison with a special scale for changing their color under the action of the test solution.

In the colorimetric method, standard buffer solutions are used to accurately determine pH, the pH value of which is precisely known and constant.

Buffer solutions are mixtures of weak acids or bases with their salts. Such mixtures maintain a certain pH value both when diluted and when small amounts of strong acids or alkalis are added.

Substances that change color when the reaction of the medium changes are indicators - most often complex organic compounds - weak acids or weak bases. Schematically, the composition of indicators can be expressed by the formulas НInd or IndOH, where Ind is a complex organic anion or indicator cation.

In practice, indicators have been used for a long time, but the first attempt to explain their action was made in 1894 by Ostwald, who created the so-called ionic theory. According to this theory, undissociated indicator molecules and its Ind ions have different colors in solution, and the color of the solution changes depending on the position of indicator dissociation equilibrium. For example, phenolphthalein (an acid indicator) has colorless molecules and crimson anions; methyl orange (main indicator) - yellow molecules and red cations.

phenolphthalein methyl orange

HIndH + + Ind–IndOH
Ind + +OH-

colorless raspberries. yellow red

A change in accordance with Le Chatelier's principle leads to a shift in equilibrium to the right or to the left.

According to the chromophore theory (Hanch), which appeared later, the change in the color of indicators is associated with a reversible rearrangement of atoms in the molecule of an organic compound. Such a reversible rearrangement in organic chemistry is called tautomerism. If, as a result of a tautomeric change in the structure, special groups called chromophores appear in the molecule of an organic compound, then the organic substance acquires a color. Chromophores are groups of atoms that contain one or more multiple bonds that cause selective absorption of electromagnetic vibrations in the UV region. Groupings of atoms and bonds, such as −N=N− , =C=S , −N=O, quinoid structures, etc., can act as chromophore groups.

When a tautomeric transformation leads to a change in the structure of the chromophore, the color changes; if, after rearrangement, the molecule no longer contains a chromophore, the color will disappear.

Modern ideas are based on the ion-chromophoric theory, according to which the change in the color of the indicators is due to the transition from the ionic form to the molecular one, and vice versa, accompanied by a change in the structure of the indicators. Thus, one and the same indicator can exist in two forms with different molecular structures, and these forms can transform into one another, and an equilibrium is established between them in solution.

As an example, we can consider structural changes in the molecules of typical acid-base indicators - phenolphthalein and methyl orange under the action of alkali and acid solutions (at different pH values).

The reaction, as a result of which, due to the tautomeric rearrangement of the structure of the phenolphthalein molecule, a chromophore group appears in it, which causes the appearance of color, proceeds according to the following equation:

colorless colorless colorless

crimson

Indicators, as weak electrolytes, have small dissociation constants. For example, K d of phenolphthalein is 2 ∙ 10 -10 and in neutral media it is found mainly in the form of its molecules due to a very low concentration of ions, which is why it remains colorless. When alkali is added, H + -ions of phenolphthalein bind, “contract” with OH - alkali ions, forming water molecules, and the equilibrium position of indicator dissociation shifts to the right - towards an increase in the concentration of Ind - ions. In an alkaline medium, a disodium salt is formed, which has a quinoid structure, which causes the color of the indicator. The shift in equilibrium between tautomeric forms occurs gradually. Therefore, the color of the indicator does not change immediately, but passes through a mixed color to the color of the anions. When an acid is added to the same solution simultaneously with the neutralization of an alkali - at a sufficient concentration of H + -ions - the equilibrium position of the dissociation of the indicator shifts to the left, towards molarization, the solution becomes discolored again.

Similarly, the color of methyl orange changes: neutral molecules of methyl orange give the solution a yellow color, which, as a result of protonation, turns into red, corresponding to the quinoid structure. This transition is observed in the pH range 4.4–3.1:

yellow Red

Thus, the color of the indicators depends on the pH environment. The color intensity of such indicators is quite high and is clearly visible even with the introduction of a small amount of the indicator, which is not able to significantly affect the pH of the solution.

A solution containing an indicator changes color continuously as the pH changes. The human eye, however, is not very sensitive to such changes. The range in which the color change of the indicator is observed is determined by the physiological limits of color perception by the human eye. With normal vision, the eye is able to distinguish the presence of one color in a mixture of it with another color only if there is at least some threshold density of the first color: a change in the color of the indicator is perceived only in the area where there is a 5-10-fold excess of one form in relation to another. Taking HInd as an example and characterizing the state of equilibrium

Hind
H + + Ind-

corresponding constant

,

it can be written that the indicator shows its purely acid color, usually captured by the observer, when

,

and a purely alkaline color at

Within the interval determined by these values, a mixed color of the indicator appears.

Thus, the eye of the observer distinguishes a change in color only when the reaction of the medium changes in the range of about 2 pH units. For example, in phenolphthalein, this pH range is from 8.2 to 10.5: at pH = 8.2, the eye observes the beginning of the appearance of a pink color, which intensifies to pH = 10.5, and at pH = 10.5, an increase in red color already invisible. This range of pH values, in which the eye distinguishes a change in the color of the indicator, is called the transition interval of the color of the indicator. For methyl orange, K D = 1.65 10 -4 and pK = 3.8. This means that at pH = 3.8, the neutral and dissociated forms are in equilibrium in approximately equal concentrations.

The specified pH range of approximately 2 units for various indicators does not fall in the same region of the pH scale, since its position depends on the specific value of the dissociation constant of each indicator: the stronger the acid HInd , the more acidic the transition interval of the indicator is . In table. 18 shows the transition intervals and colors of the most common acid-base indicators.

To more accurately determine the pH value of solutions, a complex mixture of several indicators applied to filter paper (the so-called "Kolthoff universal indicator") is used. A strip of indicator paper is dipped in the test solution, placed on a white waterproof substrate, and the color of the strip is quickly compared with the reference scale for pH.

Table 18

Transition intervals and coloring in various media

the most common acid-base indicators

In an acidic pH solution< 7, в нейтральной среде рН = 7, в щелочной рН >7. The lower the pH, the greater the acidity of the solution. At pH values ​​> 7, one speaks of the alkalinity of the solution.

There are various methods for determining the pH of a solution. Qualitatively, the nature of the solution medium is determined using indicators. Indicators are substances that reversibly change their color depending on the medium of the solution. In practice, litmus, methyl orange, phenolphthalein, and a universal indicator are most often used (Table 2).

table 2

Coloring of indicators in various solution media

The hydrogen index is very important for medicine, its deviation from normal values ​​even by 0.01 units indicates pathological processes in the body. With normal acidity, gastric juice has pH = 1.7; human blood has pH = 7.4; saliva - pH = 6.9.

Ion exchange reactions and conditions for their occurrence

Since electrolyte molecules in solutions decompose into ions, reactions in electrolyte solutions proceed between ions. Ion exchange reactions- these are reactions between ions formed as a result of the dissociation of electrolytes. The essence of such reactions is the binding of ions through the formation of a weak electrolyte. In other words, the ion exchange reaction makes sense and proceeds almost to the end if weak electrolytes (precipitate, gas, H 2 O, etc.) are formed as a result of it. If there are no ions in the solution that can bind to each other to form a weak electrolyte, then the reaction is reversible; equations for such exchange reactions are not written.

When recording ion exchange reactions, molecular, full ionic, and abbreviated ionic forms are used. An example of recording an ion exchange reaction in three forms:

K 2 SO 4 + BaCl 2 \u003d BaSO 4 + 2KCl,

2K + + SO 4 2– + Ba 2+ + 2Cl – = BaSO 4 + 2K + + 2Cl – ,

Ba 2+ + SO 4 2– \u003d BaSO 4.

Rules for compiling equations of ionic reactions

1. Formulas of weak electrolytes are written in molecular form, strong ones in ionic form.

2. For the reaction, solutions of substances are taken, therefore, even poorly soluble substances in the case of reagents are recorded in the form of ions.

3. If a poorly soluble substance is formed as a result of a reaction, then when writing the ionic equation, it is considered insoluble.

4. The sum of the charges of the ions on the left side of the equation must be equal to the sum of the charges of the ions on the right side.

Test on the topic “Theory of electrolytic dissociation. Ion exchange reactions»

1. The reaction that occurs when magnesium hydroxide is dissolved in sulfuric acid is described by the reduced ionic equation:

a) Mg 2+ + SO 4 2– = MgSO 4;

b) H + + OH - = H 2 O;

c) Mg(OH) 2 + 2H + = Mg 2+ + 2H 2 O;

d) Mg(OH) 2 + SO 4 2– = MgSO 4 + 2OH –.

2. Four vessels contain one liter of 1M solutions of the following substances. Which solution contains the most ions?

a) Potassium sulfate; b) potassium hydroxide;

c) phosphoric acid; d) ethyl alcohol.

3. The degree of dissociation does not depend on:

a) the volume of the solution; b) the nature of the electrolyte;

c) solvent; d) concentration.

4. Reduced ionic equation

Al 3+ + 3OH - \u003d Al (OH) 3

corresponds to the interaction:

a) aluminum chloride with water;

b) aluminum chloride with potassium hydroxide;

c) aluminum with water;

d) aluminum with potassium hydroxide.

5. An electrolyte that does not dissociate stepwise is:

a) magnesium hydroxide; b) phosphoric acid;

c) potassium hydroxide; d) sodium sulfate.

6. Weak electrolyte is:

a) barium hydroxide;

b) aluminum hydroxide;

c) hydrofluoric acid;

d) hydroiodic acid.

7. The sum of the coefficients in the brief ionic equation for the interaction of barite water and carbon dioxide is:

a) 6; b) 4; at 7; d) 8.

8. The following pairs of substances cannot be in a solution:

a) copper chloride and sodium hydroxide;

b) potassium chloride and sodium hydroxide;

c) hydrochloric acid and sodium hydroxide;

d) sulfuric acid and barium chloride.

9. A substance whose addition to water will not change its electrical conductivity is:

a) acetic acid; b) silver chloride;

c) sulfuric acid; d) potassium chloride.

10. How will the graph of the dependence of the incandescence of an electric bulb included in the circuit on time look like if the electrodes are immersed in a solution of lime water through which carbon dioxide is passed for a long time?

a) Linear increase;

b) linear decrease;

c) first decrease, then increase;

d) first increase, then decrease.

When carrying out a chemical process, it is extremely important to monitor the conditions for the course of the reaction or to establish the achievement of its completion. Sometimes this can be observed by some external signs: the cessation of the evolution of gas bubbles, a change in the color of the solution, precipitation, or, conversely, the transition of one of the reaction components into the solution, etc. In most cases, auxiliary reagents are used to determine the end of the reaction, so called indicators, which are usually introduced into the analyzed solution in small quantities.

indicators called chemical compounds that can change the color of the solution depending on environmental conditions, without directly affecting the test solution and the direction of the reaction. So, acid-base indicators change color depending on the pH of the medium; redox indicators - from the potential of the environment; adsorption indicators - on the degree of adsorption, etc.

Indicators are especially widely used in analytical practice for titrimetric analysis. They also serve as the most important tool for the control of technological processes in the chemical, metallurgical, textile, food and other industries. In agriculture, with the help of indicators, analysis and classification of soils are carried out, the nature of fertilizers and the required amount of them to be applied to the soil are established.

Distinguish acid-base, fluorescent, redox, adsorption and chemiluminescent indicators.

ACID-BASE (PH) INDICATORS

As is known from the theory of electrolytic dissociation, chemical compounds dissolved in water dissociate into positively charged ions - cations and negatively charged - anions. Water also dissociates to a very small extent into positively charged hydrogen ions and negatively charged hydroxyl ions:

The concentration of hydrogen ions in a solution is denoted by the symbol .

If the concentration of hydrogen and hydroxide ions in the solution is the same, then such solutions are neutral and pH = 7. At a concentration of hydrogen ions corresponding to pH from 7 to 0, the solution is acidic, but if the concentration of hydroxide ions is higher (pH = from 7 to 14), the solution alkaline.

Various methods are used to measure the pH value. Qualitatively, the reaction of the solution can be determined using special indicators that change their color depending on the concentration of hydrogen ions. Such indicators are acid-base indicators that respond to changes in the pH of the medium.

The vast majority of acid-base indicators are dyes or other organic compounds, the molecules of which undergo structural changes depending on the reaction of the medium. They are used in titrimetric analysis in neutralization reactions, as well as for colorimetric determination of pH.

If it is necessary to improve the accuracy of pH measurement, then mixed indicators are used. To do this, two indicators are selected with close pH intervals of the color transition, having additional colors in this interval. With this mixed indicator, determinations can be made with an accuracy of 0.2 pH units.

Widely used are also universal indicators that can repeatedly change color in a wide range of pH values. Although the accuracy of determination by such indicators does not exceed 1.0 pH units, they allow determinations in a wide pH range: from 1.0 to 10.0. Universal indicators are usually a combination of four to seven two-color or single-color indicators with different color transition pH ranges, designed in such a way that when the pH of the medium changes, a noticeable color change occurs.

For example, the commercially available universal indicator PKC is a mixture of seven indicators: bromocresol purple, bromocresol green, methyl orange, tropeolin 00, phenolphthalein, thymol blue, and bromothymol blue.

This indicator, depending on pH, has the following color: at pH = 1 - raspberry, pH = 2 - pinkish-orange, pH = 3 - orange, pH = 4 - yellow-orange, pH = 5 yellow, pH = 6 - greenish yellow, pH = 7 - yellow-green,. pH = 8 - green, pH = 9 - blue-green, pH = 10 - grayish blue.

Individual, mixed and universal acid-base indicators are usually dissolved in ethanol and added a few drops to the test solution. By changing the color of the solution, the pH value is judged. In addition to alcohol-soluble indicators, water-soluble forms are also produced, which are ammonium or sodium salts of these indicators.

In many cases, it is more convenient to use not indicator solutions, but indicator papers. The latter are prepared as follows: the filter paper is passed through a standard indicator solution, the paper is squeezed out of the excess solution, dried, cut into narrow strips and bookleted. To carry out the test, an indicator paper is dipped into the test solution or one drop of the solution is placed on a strip of indicator paper and a change in its color is observed.

FLUORESCENT INDICATORS

Some chemical compounds, when exposed to ultraviolet rays, have the ability, at a certain pH value, to cause the solution to fluoresce or change its color or shade.

This property is used for acid-base titration of oils, turbid and strongly colored solutions, since conventional indicators are unsuitable for these purposes.

Work with fluorescent indicators is carried out by illuminating the test solution with ultraviolet light.

REDOX INDICATORS

Redox indicators- chemical compounds that change the color of the solution depending on the value of the redox potential. They are used in titrimetric methods of analysis, as well as in biological research for the colorimetric determination of redox potential.

ADSORPTION INDICATORS

Adsorption indicators- substances in the presence of which the color of the precipitate formed during titration by the precipitation method changes. Many acid-base indicators, some dyes and other chemical compounds are able to change the color of the precipitate at a certain pH value, which makes them suitable for use as adsorption indicators.

CHEMILUMINESCENT INDICATORS

This group of indicators includes substances capable of emitting visible light at certain pH values. Chemiluminescent indicators are convenient to use when working with dark liquids, since in this case a glow appears at the end point of the titration.