Acid Base Indicators

Acid-Base Indicators

Many substances, including litmus, the one dye almost everyone associates with acids and bases, change color in response to acid or base. The pigment in red cabbage is another natural substance very commonly used to show color change. Phenolphthalein is one of the most common indicators used for beginning chemistry, because its color change is very obvious which makes it easy to use. There are many other indicators that change colors at different pH's, and so are useful for different purposes. pH paper commonly contains a mixture of different indicators that change colors at different pH's. The mixture is applied to paper, and then compared to a color chart to see what the pH of a solution is, approximately.

Acid-Base indicators are dyes that are themselves weak acids and bases. However, the conjugate acid-base forms of the dye have different colors. The actual chemical structures of the dyes is often quite complex; however, we can use the generic symbol for the indicator as HIn. The Brönsted-Lowry equation for the indicator is:

HIn + H₂O H₃O⁺ + In^-

where the color of the letters is used to show the differently colored forms of the dyes.

Suppose that we increase the concentration of H₃O⁺. Then if we apply LeChatelier's principle, the result should be

HIn + H₂O H₃O⁺ + In^-

There will be more H₃O⁺ (because we added more), but the system will respond to:

use up some of the added H₃O⁺, at the same time
removing some of the In^-, and
making more HIn (and H₂O, though that is pretty irrelevant since the reaction takes place in lots of water anyways).

Thus we will see the color of the HIn form.

Now suppose that we decrease the concentration of H₃O⁺(which we could do by adding more OH^-). Now applying LeChatelier's principle, the result should be

HIn + H₂O H₃O⁺ + In^-

There will be less H₃O⁺ (because we removed it), but the system will respond to try and:

make some more H₃O⁺, at the same time
removing some of the HIn, (and some of the water, but there is so much present that this is pretty irrelevant) and
making more In^-.

Thus we will see the color of the In^- form.

In summary: at a low pH, an indicator is almost entirely in the HIn form. As the pH increases, the intensity of the colour of In^- increases as the equilibrium shifts to the right.

Different dyes will change color at different pH's (the value can be calculated from the equilibrium constant for the indicator).. Here is a small sample of some common acid-base indicators, and the range at which their pH changes color. One of the difficulties with giving a range of colors is that different person's eyes are not all equally sensitive (also different monitors will display colors differently), so these colors are only approximations.

When conducting a titration, one must select the proper indicator so that its pH range will match the equivalence point of the titration. Also, you must use an indicator that changes color obviously, so that it can be detected easily. This is why phenolphthalein is so often used for strong acid-strong base titrations.

The equivalence point is the point at which the amount of H₃O⁺ and OH^- are equal. It is important to select an indicator which changes color -- whose endpoint -- is near the equivalence point.

Check Water To Wine for an excellent and more detailed discussion of how many substances change color in response to acids and bases.