Acid-Base Indicators

Definition of an Acid-Base Indicator

First, we need to define what an acid-base indicator is.

Indicators are weak acids or bases. An indicator (HIn) partially dissociates in water to form H⁺ ions and its conjugate form In^- .

$$HIn_{(aq)}\rightleftharpoons H^{+}_{(aq)}+In^{-}_{(aq)}$$

Where,

• HIn is the unionized indicator, possessing a color.
• In^- is the conjugate base of HIn that has a different color compared to HIn

What's pretty cool about indicators is that the color of HIn _(aq) (i.e. the lower pH form) and conjugate base In^-_(aq) (i.e. the higher pH form) are different.

Basically, the color of the indicator depends on the relative concentrations of HIn and In^-. A solution will have the color of the lower pH form, HIn, if most of the indicator molecules exist in the form HIn. Similarly, a solution will have the color of In^- if most of the indicator molecules exist in the form In^-.

• If the solution contains roughly equal amounts of the weak acid and conjugate base form, then the color will be a mixture of the two!

Indicators have different uses: they can be used to signal the end-point of an acid-base titration, and they also estimate the pH of a solution! Acid-base indicators help chemists know when the end-point of titration has been reached. Ideally, the indicator should change color at the equivalence point. However, the pH changes are rapid near the equivalence point, and one extra drop of titrant can change the pH by several units.

Let's look at an example involving a weak acid/strong base titration.

In a weak acid/strong base titration, we know that the equivalence point will be a bit higher than pH 7. So, to be able to detect when the equivalence point has been reached, we can use an indicator that changes color around that pH range. Phenolphthalein is an indicator commonly used in weak acid/strong base titrations because it changes color at around a pH of 8.2.

• When first added to a weak acid solution, phenolphthalein is colorless.
• Then NaOH is added and when the pH is the solution becomes around 8.2, phenolphthalein changes the color of the solution from colorless to light pink!
• If more NaOH is added, the solution becomes a darker pink/purple.

Let's think about another indicator. Methyl red is an indicator that changes color between pH 4.2 and 6.3. Methyl red is a great indicator when dealing with the titration of a weak base using a strong acid because the pH of a weak base/strong acid titration at the equivalence point is slightly less than 7, so the equivalence point falls within the pH interval!

To find the approximate pH of a solution, we can use a universal indicator, made up of a mixture of several indicators. For example, if we wanted to determine the pH of bleach, we could add a couple of drops of universal indicator to a sample of bleach and note the color change. Then, we could compare the color change with a pH scale to find the approximate pH.

• Bleach has a basic/alkaline pH of around 12. So, you should expect bleach to turn into a purple color with the addition of a universal indicator!

Fig. 2: pH scale - StudySmarter Originals.

Theories of Acid-Base Indicators

Now that we know what acid-base indicators are, let's look at some theories involving them! There are two theories that you should be familiar with: Ostwald's theory and the Quinonoid theory.

Ostwald's theory was proposed in 1891, and it is based on the Arrhenius acid and base theory.

According to Ostwald's theory, an acid-base indicator can be either a weak organic acid or a weak organic base. The color change of an indicator occurs due to the partial ionization of the indicator and the different colors that the unionized form and the conjugate base (ionized) form has.

• For example, Phenolphthalein (HPh) partially dissociates to form H⁺ and Ph^- ions. HPh is colorless, whereas Ph^- is pink.

The second theory, the Quinonoid theory, is based on the idea that acid-base indicators exist in two tautomeric forms (benzenoid and quinonoid), and these two forms have different colors. One tautomer exists in an acidic medium, while the other exists in a basic medium. So, the color change occurs due to the conversion from one tautomer to another as the pH of the solution is changed.

• Phenolphthalein has a benzenoid form in acidic solution and a quinonoid form in basic solution.

Fig. 3: Structure Changes to Phenolphthalein - StudySmarter Originals.

Limitations of Acid-Base Indicators

As we learned above, indicators have different colors depending on their form (HIn or In^-). The limitation of acid-base indicators is their pH range. Indicators only change colors over a specific pH range, so you need to be familiar with their pH ranges to be able to choose the appropriate indicator for an acid-base titration.

• For strong acid/strong base titrations, the acid-base indicators commonly used are those that change color at pH 7.
• For weak acid/strong base titrations, the acid-base indicators commonly used are those that change color at pH > 7.
• For weak base/ strong acid titrations, the acid-base indicators commonly used are those that change color at pH < 7.

Acid-Base Indicator Chart

Let's take a look at the pH ranges of some indicators and the end-point pH. Remember that the color at lower pH is the color of the weak acid or base, while the color at higher pH is the color of its conjugate form.

• The end-point is the pH in which the indicator changes into a color between the two colors.

For example, bromothymol blue has a yellow color at lower pH (HIn) and blue at higher pH (In^-). However, at end-point, the color formed is some kind of teal color!

Let's solve some problems!

Colors of Acid-Base Indicators

Each indicator has a different pH range and different colors involved! First, let's look at the image below, showing the colors of some common indicators.

We can use this image to solve some problems! Let's look at some examples!

Now, I hope that you feel more confident when choosing acid-base indicators!