Did you know that if you add red cabbage leaves into a blender with hot water and blend it until you have a purple-colored cabbage juice, you are actually making your own acid-base indicator? Red cabbage possesses flavin, a type of anthocyanin molecule, which changes color when mixed with an acid or a base. If you add an acid to it, the cabbage juice will turn red, whereas if you add a base to it, it will change from purple to bluish-green!
Pretty interesting, right? Now, let's dive into the world of acid-base indicators!
- This article is about acid-base indicators.
- First, we will define an acid-base indicator.
- Then, we will talk about the limitations of acid-base indicators.
- Next, we will dive into the different types of acid-base indicators and their color changes.
- Lastly, we will take a look at theories involving acid-base indicators.
Definition of an Acid-Base Indicator
First, we need to define what an acid-base indicator is.
An acid-base indicator is a substance that shows a distinct observable change (usually, a color change) when there is a change in the pH of an aqueous solution.
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!
If you are not familiar with acid-base titrations yet, read "Acid-Base Titrations" first, and then come back to this point to keep learning!
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.
The end-point is referred to as the point where the indicator changes color. Usually, it is proximate to the pH of the equivalence point.
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!
You have probably heard of, or even used, a litmus paper in your chemistry lab before. Litmus is an acid-base indicator made from lichen, commonly used to check for the presence of acids and bases. Blue litmus paper is used to show the presence of an acid. If acid is present, the blue litmus paper turns red! Similarly, a red litmus paper turns blue when in presence of a base!
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.
Fig. 2: pH scale - Vaia 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.
Arrhenius acids donate H+ in H2O, increasing the concentration of hydrogen ions in aqueous solution.
Arrhenius bases donate OH- in H2O, increasing the concentration of hydroxide ions.
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.
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.
Fig. 3: Structure Changes to Phenolphthalein - Vaia 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.
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!
Which acid-base indicator would you choose for a strong acid/strong base titration?
In strong acid/strong base titrations, the equivalence point is at pH 7. Therefore, the most appropriate indicator for this type of titration would be bromothymol blue.
If you decided to perform a weak acid/strong base titration, which of the following would you use as an indicator? methyl orange, bromothymol blue, or phenolphthalein?
In weak acid/strong base titrations, the equivalence point has a pH greater than 7. So, we need to choose an indicator with a pH range that is greater than 7.
- Bromothymol blue has a pH range of 6.0 - 7.6
- Methyl orange has a pH range of 3.2 - 4.4
- Phenolphthalein has a pH range of 8.2 - 10
Out of the three indicators, the most appropriate one to use in a weak acid/strong base titration would be phenolphthalein because its entire pH range is greater than 7.
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!
Which color would phenolphthalein have in an ammonia solution?
Ammonia has a pH of around 11-12 . So, by looking at the image above, we know that at that specific pH, phenolphthalein would have a fuchsia pink color!
What would be the color of methyl orange in a solution that has a pH of 2?
According to the image above, at pH 2 methyl orange would have a red color!
Did you know that hydrangea flowers are considered natural indicators? These flowers are blue in color when grown in acidic solids, and pink/red in soils with a basic pH! Other types of natural indicators include red and pink roses, blue iris, and even some fruits and vegetables such as strawberries and beets!
Now, I hope that you feel more confident when choosing acid-base indicators!
Acid-Base Indicators - Key takeaways
- An acid-base indicator is a substance that shows a distinct observable change (usually, a color change) when there is a change in the pH of an aqueous solution.
- In acid-base indicators the color of HIn and conjugate base In- are different.
- Acid-base indicators change color over a specific pH range.
References
- Lawrie, R., & Norris, R. (2014). Cambridge International AS and A Level Chemistry.
- Brown, T. E., LeMay, H. E. H., Bursten, B. E., & Murphy, C. (2014). Chemistry the central science 13th Edition. Prentice Hall.
- Saunders, N. (2020). Supersimple Chemistry: The Ultimate Bitesize Study Guide. London: Dorling Kindersley.
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