In actual work, it is difficult for the naked eye to accurately observe small changes in the color of the indicator's color change point. People visually observe that the process of acid-base indicators changing from one color to another can only occur within a certain pH change range, that is, it can only be achieved when one color is equivalent to ten times the concentration of another color. Identify its color changes. Simultaneously with this color change, the pH of the medium changes from one value to another. When the pH value of the solution is greater than pKHIn, [In-] will be greater than [HIn] and when [In-]/[HIn]=10, the solution will completely show the color of the alkaline component, and the acid color will be covered. This The pH of the solution = pKHIn + 1. In the same way, when the pH value of the solution is less than pKHIn, [In-] will be less than [HIn] and when [In-]/[HIn]=1/10, the solution will completely show the color of the acid component, while the alkaline color is covered, then the pH of the solution = pKHIn - 1.
It can be seen that the color of the solution changes within the range from pH=pKHIn - 1 to pH=pKHIn + 1. This range is called the color changing range of the indicator, that is, the color changing gamut. Within the discoloration range, when the pH value of the solution changes, the ratio of the alkaline component and the acid component changes accordingly, and the color of the indicator also changes. Beyond this range, for example, when pH ≥ pKHIn + 1, only alkaline color is seen; and when pH ≤ pKHIn - 1, only acid color is seen. The color change range of the indicator is therefore approximately 2 pH units. Due to the different sensitivity of human vision to various colors, and the indicator showing mixed colors in the color changing range, the two colors affect each other, so the actual observation results are different from the theoretical values. The color changing range of most indicators is less than 2 pH units.