Acid-Base Indicators - Online Tutor, Practice Problems & Exam Prep

Acid-base indicators are substances that change color at a specific pH during a titration, signaling the end point, which occurs just after the equivalence point where the amount of acid equals the amount of base. The ideal indicator has a pKa value close to the pH at the equivalence point and exhibits a distinct color change when transitioning between its weak acid (or base) form and its conjugate form. The pH range over which an indicator changes color is typically within one pH unit above and below its pKa. For example, methyl orange transitions from red to yellow over a pH range of 3.3 to 4.5. Understanding the relationship between pH, pKa, and the color change of indicators is crucial for accurately determining the end point in acid-base titrations.

Acid-Base Indicator

Now, an acid base indicator is a weak acid or weak base that indicates the pH at the end point of a titration. Now what exactly is the end point? Well, the end point is the point in the titration right after the equivalence point when the indicator changes color and we're going to see that this color change. This is when the weak form of the indicator possesses a different color from its conjugate form.

So let's say your indicator is a weak acid. Once it reaches the end point, it changes colors because that weak acid is transformed into its conjugate base form. Or if you had a weak base as your indicator, after the equivalence point, it's been it's gained in H⁺ and now it has a weak acid form. So it's conjugate form is a different color. Now best indicator for titration is when the P_KA value of the indicator is close to the pH at the equivalence point.

Now the indicator has its own pH range and it is also pH equals P_KA ±1. We've seen this when discussing the buffer range of a typical buffer solution. Now here if we take a look at this image, we have our titration curve, we have color of indicator and then we have the pH involved. So if we take a look here, we can say that at the beginning of our titration we have our purple solution. We are slowly adding base to it when we get to the end point, which is right after the equivalence point.

So the equivalence point will be near it. At the equivalence point we transitioned to a pinkish color. Beyond the equivalence point, beyond the end point, we have excess strong base for. Yeah, excess strong base in this case at that. In that case, it transitions from a pinkish color to a reddish color. So if we take a look at the color of the indicator at each of these steps, we're starting off initially with weak acid, so it's purple here. In this case, the weak acid amount is greater than our conjugate base amount.

Then when we get to the intermediate form, this happens around the endpoint around the equivalence point of our solution. At this point, we're going to say that the weak acid and conjugate base concentrations are equal to one another. And then when we get to the conjugate base, we've transitioned to red. In this case, the weak acid concentration is less than the concentration of our conjugate base. What you should realize also is that we have purple here in the beginning and red here at the end, purple and red mixed together to give us this Violet pinkish color here. So it's kind of like an averaging of the colors here.

Now how does this relate to pH equals and P_KA? Well, we're going to say here for the purple region, pH equals P_KA -1. When we're at the intermediate form, that means we're near the important point. Near the end, point pH equals P_KA and beyond the equivalent point and in point, pH equals P_K +1. Now below here we have some common types of acid base indicators. No, you don't need to memorize them here we're just showing you some typical types of indicators.

First we're talking about their natural color and then we're talking about their transition when they go to their conjugate form. So for example we have a common one is methyl orange. It works best in a range of 3.3 to 4.5. For pH, it starts off as red and as we continue our titration it transitions to a yellow color. We also have bi Mal blue. This one here has two types of pH regions because it has more than one site working act as an asset or a base. So we have 1.2 to 2.8 initially, and then later on 8 to 9.2. For the first region, it goes from red to yellow and then it can go from yellow to blue.

OK, So here we're talking about multiple chances for us to get conjugate base form or conjugate forms of our original indicator. All right. So again, these are just typical types of indicators, the pH ranges in which they work ideally, and then the different types of color transitions that happen as they go from their original form to their conjugate form.

Acid-Base Indicators Example

Here it says to pick the best indicator for the following titration. If we take a look at this titration curve, we can see that the steep drop happens around this region in terms of volume, and dead in the middle of that is our equivalence point. Here our equivalence point looks like it's just hovering just below 7.

Now remember the best indicator for titration is when the pKa value of the indicator is close to the pH at the equivalence point. So here are each of the equivalence point is near 7. So what we want an indicator that has a pKa close to seven as well.

If we take a look at look at our options, only option A is close enough to that pH of seven. So here option A where we have bromothymobilume is the correct answer.

Chemistry student is using an indicator with a pK_a of 4.7 for the titration of a strong acid with strong base. Calculate the pH range at which the indicator will change colors.

a) 2.7–3.5                      b) 10–11                        c) 4.7–8.7                      d) 3.7–5.7                      e) 2.7–5.8

• A. 2.7–3.5
• B. 10–11
• C. 4.7–8.7
• D. 3.7–5.7
• E. 2.7–5.8

Bromophenol blue (pK_a = 4.1) is a common acid-base indicator. It is yellow in its acidic form and blue in conjugate base form. If the solution being titrated has a pH = 4.0, what color would the bromophenol blue indicator possess?

a) yellow                       b) orange                      c) blue                          d) green                      e) purple 

• A. yellow
• B. orange
• C. blue
• D. green
• E. purple

Here’s what students ask on this topic:

What is not true about acid-base indicators?

Acid-base indicators are substances that change color in response to changes in pH, which is a measure of acidity or alkalinity. However, it is not true that all acid-base indicators have a clear and distinct color change at a pH of 7, which is neutral. Each indicator has its own specific pH range over which it changes color, and this range can be above, below, or straddling the neutral pH of 7. Additionally, it is incorrect to assume that acid-base indicators change color at the same pH value for all types of solutions; the ionic strength and composition of a solution can affect the apparent pK_a of an indicator, thereby altering its color change range. Furthermore, not all indicators are suitable for all types of acid-base titrations, as some indicators might not provide a clear color change for certain types of reactions. It's also not true that indicators can precisely determine the pH value; they can only provide an approximate range where the pH lies.

Why do acid-base indicators change color?

Acid-base indicators change color because they are substances that exhibit different colors when in acidic or alkaline environments. These indicators are usually weak acids or bases themselves that dissociate differently depending on the pH of the solution they are in, leading to a change in color.

At a specific pH value, known as the endpoint, the concentration of the indicator in its acidic form equals the concentration in its basic form. This pH corresponds to a particular color observed in the solution. For example, litmus turns red in acidic solutions (pH < 7) and blue in alkaline solutions (pH > 7).

The color change occurs due to structural changes in the indicator molecules. When the pH changes, it can alter the shape or charge distribution within the indicator molecule, which in turn affects the wavelengths of light absorbed and thus the color we perceive. This molecular transformation is reversible, which is why indicators can be used multiple times in different solutions to test their acidity or alkalinity.

How do acid-base indicators work?

Acid-base indicators are substances that change color in response to changes in pH, which is a measure of the acidity or basicity of a solution. These indicators are typically weak acids or bases that exist in two different forms depending on the pH of the environment: the acidic form (HIn) and the basic form (In^-).

In an acidic environment, where the concentration of hydrogen ions (H⁺) is higher, the equilibrium shifts toward the formation of the acidic form of the indicator, which has a certain color. Conversely, in a basic or alkaline environment, where there are fewer hydrogen ions and more hydroxide ions (OH^-), the equilibrium shifts toward the basic form of the indicator, resulting in a different color.

The point at which the color change occurs is known as the endpoint of the indicator. Each indicator has its own pH range over which it changes color, and this range is where the transition from acid to base, or vice versa, is clearly visible. By choosing an appropriate indicator, chemists can visually determine whether a solution is acidic or basic and, in some cases, can approximate the solution's pH.

What are acid/base indicators?

Acid/base indicators are substances that change color in response to changes in pH, making them useful for determining whether a solution is acidic or basic. They are typically weak acids or bases themselves, which dissociate in water to form ions. The color change occurs because the different forms of the indicator—the acid form and its conjugate base—have different colors in solution.

For instance, litmus is a common indicator that turns red in acidic solutions (pH less than 7) and blue in basic solutions (pH greater than 7). Phenolphthalein is another indicator that is colorless in acidic solutions and turns pink in basic solutions, typically starting to change color around a pH of 8.2.

The choice of an indicator for a particular reaction depends on the pH range over which the color change occurs, as different indicators work best at different pH levels. This is crucial during titration experiments, where the endpoint is determined by a noticeable color change of the indicator.