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pH Indicators pH indicators are weak acids that exist as natural dyes and indicate the concentration of H+ (H3O+) ions in a solution via color change. A pH value is determined from the negative logarithm of this concentration and is used to indicate the acidic, basic, or neutral character of the substance you are testing.
Introduction
pH indicators exist as liquid dyes and dye-infused paper strips. They are added to various solutions to determine the pH values of those solutions. Whereas the liquid form of pH indicators is usually added directly to solutions, the paper form is dipped into solutions and then removed for comparison against a color/pH key. pH
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Color
Very Acidic Acidic Neutral Basic Very Basic
The Implications of the Indicated pH via the Equation
Recall that the value of pH is related to the concentration of H+ (H3O+) of a substance. pH itself is approximated as the cologarithm or negative logarithm of the H+ ion concentration (Figure 3). pH≈−log[H3O+](3) A pH of 7 indicates a neutral solution like water. A pH less than 7 indicates an acidic solution and a pH greater than 7 indicates a basic solution. Ultimately, the pH value indicates how much H+ has dissociated from molecules within a solution. The lower the pH value, the higher concentration of H+ ions in the solution and the stronger the acid. Likewise, the higher the pH value, the lower the concentration of H+ ions in the solution and the weaker the acid.
How the Color Change of the Indicator Happens
The color change of a pH indicator is caused by the dissociation of the H+ ion from the indicator itself. Recall that pH indicators are not only natural dyes but also weak acids. The dissociation of the weak acid indicator causes the solution to change color. The equation for the dissociation of the H+ ion of the pH indicator is show below (Figure 4).
HIn+H2O⇌
Introduction
pH indicators exist as liquid dyes and dye-infused paper strips. They are added to various solutions to determine the pH values of those solutions. Whereas the liquid form of pH indicators is usually added directly to solutions, the paper form is dipped into solutions and then removed for comparison against a color/pH key. pH
3
4
5
6
7
8
9
10
Color
Very Acidic Acidic Neutral Basic Very Basic
The Implications of the Indicated pH via the Equation
Recall that the value of pH is related to the concentration of H+ (H3O+) of a substance. pH itself is approximated as the cologarithm or negative logarithm of the H+ ion concentration (Figure 3). pH≈−log[H3O+](3) A pH of 7 indicates a neutral solution like water. A pH less than 7 indicates an acidic solution and a pH greater than 7 indicates a basic solution. Ultimately, the pH value indicates how much H+ has dissociated from molecules within a solution. The lower the pH value, the higher concentration of H+ ions in the solution and the stronger the acid. Likewise, the higher the pH value, the lower the concentration of H+ ions in the solution and the weaker the acid.
How the Color Change of the Indicator Happens
The color change of a pH indicator is caused by the dissociation of the H+ ion from the indicator itself. Recall that pH indicators are not only natural dyes but also weak acids. The dissociation of the weak acid indicator causes the solution to change color. The equation for the dissociation of the H+ ion of the pH indicator is show below (Figure 4).
HIn+H2O⇌