Did you know that you can make your own acid-base indicator using red cabbage? All you need to do is blend red cabbage leaves with hot water until you have a purple-colored juice. Red cabbage contains flavin, which is a molecule that changes color when mixed with an acid or a base. If you add an acid to the juice, it turns red, and if you add a base, it changes to bluish-green.

Now, let's explore the world of acid-base indicators! An acid-base indicator is a substance that changes color depending on whether it's mixed with an acid or a base. However, there are limitations to using acid-base indicators. For instance, they can only tell you whether a substance is acidic or basic, but not how acidic or basic it is.

There are different types of acid-base indicators, and each has its own color changes. For example, phenolphthalein is colorless in acids but turns pink in bases. Bromothymol blue is yellow in acids, green in neutral solutions, and blue in bases.

Finally, there are theories about how acid-base indicators work. Some theories suggest that the indicator molecule donates or accepts protons, while others propose that the indicator molecule undergoes a structural change when exposed to an acid or a base.

In conclusion, acid-base indicators are fascinating substances that can tell us a lot about the acidity or basicity of a solution. By understanding how they work and their limitations, we can use them to conduct experiments and better understand the chemical world around us.

Definition of an Acid-Base Indicator

Let's start by defining what an acid-base indicator is. It's a substance that changes color when there is a change in the pH of an aqueous solution. Indicators are weak acids or bases that partially dissociate in water to form H+ ions and their conjugate base, In-. The cool thing is that the color of the indicator depends on the relative concentrations of HIn and In-. If most of the indicator molecules exist in the form HIn, the solution will have the color of the lower pH form, and if most of the indicator molecules exist in the form In-, the solution will have the color of In-.

Indicators have different uses: they can be used to signal the end-point of an acid-base titration and estimate the pH of a solution. The end-point is the point where the indicator changes color, which is proximate to the pH of the equivalence point. Phenolphthalein is an indicator commonly used in a weak acid/strong base titration because it changes color at around a pH of 8.2. 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.

Litmus paper is a commonly used acid-base indicator that can check for the presence of acids and bases. Blue litmus paper turns red in the presence of an acid, and red litmus paper turns blue when in the presence of a base. If we want to find the approximate pH of a solution, we can use a universal indicator made up of a mixture of several indicators. For example, bleach has a basic/alkaline pH of around 12, so you should expect bleach to turn purple with the addition of a universal indicator.

In conclusion, acid-base indicators are substances that can tell us a lot about the pH of a solution. By understanding how they work and their different uses, we can use them in experiments and better understand the chemical world around us.

Theories of Acid-Base Indicators

Now, let's take a closer look at the two theories behind acid-base indicators. The first theory is Ostwald's theory, which was proposed in 1891 and is based on the Arrhenius acid and base theory. According to this 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 form have. For example, phenolphthalein is colorless in its unionized form and pink in its conjugate base form.

The second theory is the Quinonoid theory, which 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. The color change occurs due to the conversion from one tautomer to another as the pH of the solution is changed. Phenolphthalein, for example, has a benzenoid form in acidic solution and a quinonoid form in basic solution.

Understanding these theories can help us better understand how acid-base indicators work and how they can be used in experiments. By using indicators, we can gain insights into the pH of a solution, which can help us conduct experiments and understand the chemical world around us. 

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

It's important to choose the appropriate indicator for each titration to get accurate results. In strong acid/strong base titrations, where the equivalence point is at pH 7, the most appropriate indicator would be bromothymol blue. On the other hand, in weak acid/strong base titrations, where the equivalence point has a pH greater than 7, an indicator with a pH range greater than 7 should be used. Phenolphthalein is a good indicator for this type of titration because its entire pH range is greater than 7. By choosing the right indicator, we can ensure that we get accurate results and a better understanding of the chemical reaction at hand.

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 takeawaysAn 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.

ReferencesLawrie, 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.

Acid-Base Indicators

What is an acid-base indicator?

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.

What are the uses of acid-base indicators?

Indicators can can be used to signal the end-point of an acid-base titration, and they are also used by chemists to estimate the pH of a solution!

How to find the pka of an acid base indicator?

The pKa of an acid-base indicators will be equal to the pH of the solution at the end-point of the titration.

What are natural indicators of acids and bases?

Natural indicators are types of plants and fruits that change color in the presence of acids or bases. An example of a natural indicator is hydrangea. These flowers are blue in color when grown in acidic solids, and pink/red in in soils with a basic pH

What is an acid base indicator example?

An example of an acid-base indicator is Methyl red. Methyl red is an indicator that changes color between pH 4.2 and 6.3.

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