Acid-Base Indicators - Video Tutorials & Practice Problems

Acid-Base Indicator

Now, an acid based 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 gonna say 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 indicators 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 plus and now it has a weak acid form. So its conjugate form is a different color. Now, best indicator for titration is when the PK A value of the indicator is close to the PH at the equivalent point. Now the indicator has its own PH range and it is also PH equals PK A plus or minus one. 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 equivalent point, we transitioned to a pinkish color. Beyond the equivalent point. Beyond the end point, we have excess strong base or uh 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 end point around the equivalent point of our solution. At this point, we're gonna 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? Um and PK A. Well, we're gonna say here for the purple region, ph equals PK A minus one when we're at the intermediate form, that means we're near the equivalent point near the end point PH equals PK A and beyond the equivalent point and in point ph equals PK A plus one. Now, below here, we have some common types of acid base syndicators. No, you don't need to memorize them here, which is showing you some typical types of indicators. First, we're talking about uh their natural color and then we're talking about their transition when they go to their conjugate form. So for example, we have uh a common one is metal 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. Uh We also have thy all blue. This one here has two types of PH regions because it has more than one site where act as a or 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 uh multiple um chances for us to get conjugate based form or conjugate forms of our original indicator, 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 seven. Now remember the best indicator for titration is when the PK A value of the indicator is close to the Ph at the equivalence point. So here our ph of the equivalence point is near seven. So we want an indicator that has a PK A close to seven as well. If we take a look at, look at our options only option A is close enough to that page of seven. So here option A where we have BROMO Thymol blue 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