What is the principle behind the color change of acid-base indicators?

This is the most basic knowledge: I. The principle of action of indicators 1. Acid-base indicators: These are generally certain organic weak acids or weak bases, or organic substances that are amphoteric; they can also participate in proton transfer reactions during acid-base titrations. Changes in their molecular structure cause a change in their color, and this color change associated with the structural change is reversible. For example, phenolphthalein, methyl orange. 2. Classification: Monochromatic indicators: Indicators in which only one of the acid or base forms is colored. Such as phenolphthalein, a bifunctional indicator: an indicator that is colored in both its acidic and basic forms. Such as methyl orange. II. pH range for indicator color change: 1. Principle of color change: The indicator is denoted as HIn. HIn = H+ + In-; the acid form and the base form. Ka is the dissociation constant of the indicator. (1) The color of the solution is determined by the ratio of H+/In-, and it changes as the pH of the solution changes. ⑵1. When pH ≤ 1/10 and PH ≤ PKa–1, it is an acidic color; when 10 > pH > 1/10 and PH is between PKa–1 and PKa+1, it is a mixed color with a gradual change in hue; when pH ≥ 10 and PH ≥ PKa+1, it is an alkaline color. 2. Color change range: The pH range within which the indicator changes from an acidic color to a mixed color and then to an alkaline color can be observed when the pH of the solution changes from PKa–1 to PKa+1 (or vice versa). This pH range, namely pH = PKa ± 1, is referred to as the indicator’s color change range. 3. Theoretical color change point: When the concentrations of the acidic and basic forms of the indicator are equal, that is, / == 1, the pH of the solution equals pKa; this is known as the theoretical color change point of the indicator. III. Factors affecting the color change range of indicators: 1. Amount of indicator used: For dual-color indicators, the color change range is not affected by the amount used. However, the color change of the indicator also consumes a certain amount of titrant, thereby introducing errors. Monochromatic indicator: As the amount of the monochromatic indicator increases, its color change range shifts in the direction of lower pH. The amount used should be appropriate when applying it. 2. Temperature: Changes in temperature cause changes in the dissociation constant of indicators and the proton transfer constant of water; as a result, the color change range of indicators also changes. This effect is more pronounced for basic indicators than for acidic indicators. 3. Neutral electrolytes: 4. Solvents: Different solvents have varying dielectric constants and acidity levels, which in turn affect the dissociation constants and color change ranges of the indicators. IV: Mixed indicators: 1. Mixed indicators take advantage of the complementary effect between colors; they have a very narrow range of color change, and exhibit a sharp color shift at the titration endpoint. 2. Preparation method: It is prepared using a dye whose color does not change with the H concentration in the solution (referred to as an inert dye) and an indicator ； Select two (or more) indicators with PK values that are relatively close to each other, and use them in combination in a certain proportion.

This post was last edited by qugd on 2015-8-21 at 13:31. The principle behind the color change of the indicator is that when the degree of matching changes within a certain range, the molecular structure of the indicator changes, which in turn alters its light-absorption properties; this results in a change in its color. This shows the effect of pH on the color of phenolphthalein. http://pic.baike.soso.com/p/20130306/20130306215648-796470937.jpg This shows the color change of methyl orange. From the perspective of the color change mechanism, it is the form of the molecular conjugate bonds that changes under different acid-base conditions.

A large portion of acid-base indicators are organic weak acids. They can partially ionize in solution into the ions of the indicator and hydrogen ions (or hydroxide ions), and due to structural changes, their molecules and ions have different colors, thus exhibiting different colors in solutions with varying pH levels.