Blue, is neutral. The key point to remember

Blue, orange, yellow, and red are just some of the colors you may see when testing a solution using an acid-base indicator. Continue reading to learn about acid-base indicators and how the process works.

Acid-Base Indicator: Introduction

Have you ever heard of cabbage juice? Yes I said that correctly, cabbage juice.

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Specifically I am referring to the red cabbage juice taken from soaked red cabbage leaves. Now this might sound odd, but red cabbage juice can be used as an acid-base indicator. But what is an acid-base indicator?Acid-base indicators are substances that respond to a change in the hydrogen ion concentration of a solution.

You will be able to recognize this change most often through a color change. Keep in mind that acid-base indicators are typically weak acids. A weak acid is a compound that partially dissociates, or breaks apart, in solution. As we will see shortly, the role of a weak acid in the acid-base indicator process is very important.

When discussing acid-base indicators with fellow scientists, don’t be alarmed if someone refers to this method as a pH indicator. These terms are interchangeable.Now, acid-base indicators come in many different flavors depending on the color change. For example, phenolphthalein indicates if the solution is an acid, which is colorless, or base, which is pink in color. Bromophenol blue has a color of yellow for acids and blue-violet for bases.The beauty of seeing a color change is the ability to relate this visual to the pH of the solution.

pH refers to a numerical scale that tells you whether a solution is acidic or basic. A pH value less than 7 is considered acidic, while pH values greater than 7 are basic. A pH equal to 7 is neutral.

The key point to remember is that an indicator’s color change corresponds to a pH range. This is illustrated in this image:

Example of Correlation Between Acid-Base Indicator, Color Change, and pH
acids bases

Depending on the color, the pH of the solution may be low (less than 7), neutral (pH=7), or high (greater than 7). Now that we know what an acid-base indicator is, and the principles behind this chemical method, let’s explore the process of running this test.

The Process

Before we look at an example of using acid-base indicators, we need to understand how the process works chemically.

Earlier we learned that acid-base indicators are commonly weak acids. We also learned that these indicators respond to changes in hydrogen ion concentration. When an acid-base indicator mixes with an unknown solution, a change in equilibrium to the weak acid and its conjugate base occurs. It is this change that causes a change in pH. A conjugate base is a compound that accepts a hydrogen ion from a weak acid.

In a perfect world we would love to keep everything in equilibrium. However, when the hydrogen ion concentration changes, a shift in equilibrium happens. When this happens, the acid-base indicator responds by giving off a color change. Relating this to pH range, recall that a color change corresponds to a pH value. Let’s look at the following example.

Say that we add 5 drops of Thymol blue to an aqueous solution. We’ll call this solution our acid-base indicator. It is a weak acid, it’s yellow in color, that changes to blue when a base is present in solution.

We add an unknown solution to the acid-base indicator. We notice a color change of the solution to blue. Using our pH scale, we also see that this color change corresponds to a pH value of 9. Using what we learned about acid-base indicators, we can conclude the unknown solution is basic. Now, what happened chemically following this color change? An equilibrium shift occurred.

Essentially, the concentration of H3O+ (i.e., the base) was high in the unknown solution. This caused a shift in equilibrium to the right in order to compensate for the high concentration. By shifting to the right, we were able to see the solution change colors from yellow, which is an acid, to blue, which is a base. We were also able to determine the pH value to be 9.

Let’s go back to our red cabbage juice. Why do you think this would be a great acid-base indicator, given what you have learned? It responds to changes in hydrogen ion concentration. That response, we see, is a color change in the juice depending on the solution it is mixed with. One cool fact to remember is that red cabbage juice is a natural acid-base indicator.

Lesson Summary

Again, acid-base indicators are substances that respond to changes in hydrogen ion concentration. Also known as pH indicators, they are commonly weak acids. Through the help of a color change and pH range, these indicators are used to determine whether a solution is basic or acidic.

There are different types of acid-base indicators, including thymol blue, bromophenol blue, and phenolphthalein. During this process, an equilibrium shift between the weak acid-conjugate base must occur. It is this shift that supports the response from an acid-base indicator. This color change, in solution, corresponds to the pH value.


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