How does the indicator change color 5, how does the indicator change color

<> indicator should produce different structures when the acid-base, coordination and oxidation potential are different, and different structures will have different colors.

Simply think: acetic acid is CH3COOH without dissociation in strong acids, and CH3COO- in alkalis, but the conjugated chain on its COO- is too small and absorbs in the ultraviolet region, and you can't see it.

Of course, there is an equilibrium between the structures, and individual concentrations can be calculated using the equilibrium constant. It is the species with high concentration that color can be seen.

Like phenolphthalein, there is ka between the structures, and you calculate it like a high school chemistry calculating acidity and alkali.

Among the organic matter, which can have color, it should be absorbed in the visible light area. The long chain conjugation can only reach this area, and the other short chains may absorb light above the energy of ultraviolet light. (You can think of a particle in a one dimensional box.)

The longer the box, the smaller the energy step difference. To put it simply: ropes with similar tension, density, and material, the longer the frequency, the lower the frequency).

Looking at the figure above, taking phenolphthalein as an example, looking at pH0-8, except for the benzene ring conjugation, there is no other long chain, and the benzene ring is absorbed in the ultraviolet light region, so it is colorless. : It is obvious that the three rings can form a large conjugate that can absorb about blue-green light (find its absorption spectrum in detail), and you will see some colors.

What is an indicator?!

The theoretical discoloration point of the indicator is determined by the negative logarithm of the ionization equilibrium constant of the upper indicator。The theoretical discoloration point of the acid-base indicator is the discoloration range of the acid-base indicator. Acid-base indicators, also known as pH indicators and hydrogen ion concentration indicators, are chemical reagents used to test pH values.

They themselves are weak acids or bases, and contain pigments, which, when dropped into the solution, combine with hydrogen ions or hydroxide ions and convert into the corresponding acid or base formula, thus showing different shades.

The role of the indicatorIndicators are a class of chemical reagents. Under certain medium conditions, its color can change, turbidity or precipitation, and fluorescence. It is often used to test the acidity and alkalinity of the solution; It is used in titration analysis to indicate the titration endpoint; Detection of hazardous substances in environmental testing.

Generally, it is divided into acid-base indicators, redox indicators, metal indicators, adsorption indicators, etc.

The discoloration range of the acid-base indicator is pH. When the pH of the solution is acidic, the methyl orange is red, when the pH is >, the methyl orange is yellow, and the mixed color orange of red and yellow appears.

Methyl orange is a commonly used acid-base titration indicator in analytical chemistry, and is not suitable as an indicator for the titration of organic acids. Its concentration is the aqueous solution pH is red) yellow), suitable for titration between strong acid, strong alkali and weak alkali. It is also used spectrophotometrically for the determination of chlorine, bromine, and bromine ions, and is used for biological staining, among other things.

Notes:

The amount of two-color indicator has an effect on the color change, and too much or too little will make the color change not sharp.

The amount of monochromatic indicator has little effect on the color change, but it affects the range and end point of the color change.

The indicators themselves are all weak acids or bases, and they are also involved in acid-base reactions.

Temperature: As the temperature changes, both the indicator constant and the ionic product of the water change, and the discoloration range of the indicator also changes.

Fluorescent yellowIndicator, the color change of the indicator from yellow to red.

Fluorescent yellow is used as an indicator for the titration of Agno3 CL-, and the color change of the indicator from yellow to red is due to the electroneutral crying of AGCL precipitation on fluorescent yellow anions.

adsorption. Positively charged.

Adsorption of fluorescent yellow anions by the colloidal precipitate of AGCL. Adsorption of fluorescent yellow molecules by negatively charged agCl colloidal precipitates. The structure of the agCl precipitated crystal form has changed.

The adsorption indicators used in fluorescent xanguine titration are a class of organic dyes used for precipitation titration. When it is adsorbed on the surface of the particle, it may be due to the formation of a certain compound that causes a change in the molecular structure of the indicator, which causes a change in color. In precipitation titration, this property can be used to indicate the end point of the titration.

Adsorption indicators can be divided into two categories, one is acid dyes, such as fluorescent yellow and its derivatives, which are organic weak acids.

It can dissociate the indicator anion. The other type is basic dyes, such as methyl violet and other celery, which are organic weak bases that can dissociate indicator cations.

Basic Information. Fluorescent indicators are fluorescent indicators that can be used to titrate and determine the pH of turbid liquids and colored liquids. Fluorescence color change during titration is not affected by the color of the liquid and its transparency, so it is often used.

The discoloration range of the acid-base indicator is pH. Since the acid-base indicators are all organic weak acids or organic weak bases, they have different degrees of ionization in different acid-base solutions, so they will show different colors. pH test strips are made of a mixture of indicators, usually golden yellow, pH 5.

It can be seen that the indicator mixture has been adjusted to weak acidity, which is not neutral, which is to reduce the influence of CO2 in the air on the determination. In addition, when neutralizing the reaction, only 2 or 3 drops of acid-base indicators can be used, also because acid-base indicators are organic acids or organic bases, and using too much will increase the error.

Influencing factors. In practice, there are two main factors that affect the discoloration range of acid-base indicators: one is the factors that affect the constant khln of the indicator, including temperature, solvent, ionic strength of the solution, etc., among which the influence of temperature is greater.

The other is the factors that affect the width of the discoloration range, such as the amount of indicator, titration procedure, etc., which are discussed as follows.

1. When the temperature changes, the constant khln of the indicator and the ion product kw of the water are changed, so the discoloration range of the indicator also changes. For example, at 18, methyl orange has a discoloration range of , and at 100, it becomes. Therefore, titration should be performed at room temperature.

If heating is necessary, the solution should be cooled before titration.

2. The PKHLN value of solvent indicators is different in different solvents, so the discoloration range in different solvents is different. For example, methyl orange pkhln= in aqueous solution and pkhln= in methanol.

3. The presence of neutral electrolyte in neutral electrolyte solution increases the ionic strength of the solution, changes the indicator constant, and affects the discoloration range of the indicator. In addition, some electrolytes have the property of absorbing light waves of different wavelengths, which can cause changes in the color depth, tone, and color-changing sensitivity of the indicator. Therefore, a large number of salts should not be present in the titration solution.

<>1. Litmu: The color change range is PH5.8 to 8.0, from red to blue.

2. Methyl orange: the discoloration range is PH3.1 to 4.4, from red to yellow.

3. Phenolphthalein: the discoloration range is pH 8.2 to 10.0, from colorless to red.

4. Methyl red: the discoloration range is PH4.4 to 6.2, from red to yellow.

5. Bromothymol blue: the discoloration range is PH 6.2 to 7.6, from yellow to blue.