Selecting an Indicator — Overview & Important Components

We're beginning to study acid-base titrations. In titrations, we add a known concentration of acid or base to an unknown. Our goal is to find the equivalence point. It's the moment when moles of acid equals moles of base. But, how do we know if we're at equivalence? In our previous lesson, we investigated indicator solutions. As pH changes, they change color. They reflect light in the visible region. So, we can see that our solution's pH has changed! If we choose the right indicator, it can tell us we're near the equivalence point!

Using Indicators

Ideally, an indicator would change color right at the equivalence point. But, the equivalence point is the steepest region of the titration curve. So, tiny titrant additions can skyrocket the pH. What can we do? Thankfully, the equivalence region is almost vertical. So, the plot is linear. We only need an indicator to approximate the region's start-point and end-point. With two points, we plot a linear line. The equivalence point is the mid-point of our line.

Strong Acid and Strong Base Titration

The easiest titration is a strong acid with a strong base. Strong acids and bases completely neutralize! so, the equivalence point occurs when pH=7. So, we use two indicator solutions. The first is methyl-orange. It changes color in the pH range of 3.2-4.4. It will give us an excellent approximation of where the equivalence region starts. For the end-point, we can use phenolphthalein. It has a pH range of 8.2-10.0. So, when we see pink, we know we've passed our equivalence point.

Adding Strong Base to a Weak Acid

There are a few differences between strong acid titrations and weak acid titrations. What happens when we start with a weak acid? First, weak acids have a higher initial pH. For example, a weak acid could have an initial pH of 3.5. So, methyl orange is not a useable indicator. It's nowhere near the equivalence region. Second, the weak acid has some buffer capacity. So, the initial change is slow. Most importantly, the equivalence point has a basic pH. Why? Remember, the equivalence point is when moles of acid equals moles of base. So, at the equivalence point, the acid is completely neutralized. What's remaining? Only the conjugate base! So, we expect our equivalence point to be in the pH range of 8-10. What's a good indicator? Phenolphthalein!

Adding Strong Acid to a Weak Base

What happens when we titrate a weak base? When we start with a weak base, our titration curve flips. So, the pH starts very high. Our equivalence point is acidic because of the conjugate acid. The most common range is 4-6. So, we can't use phenolphthalein. Instead, methyl red is a perfect choice.

Polyprotic Acid Titrations

What about polyprotic acid titrations? What does the titration curve look like for polyprotic acids? It's multiple titration curves put together! So, there are equivalence points for each dissociation. For example, let's consider phosphoric acid. It's a triprotic Arrhenius acid! Let's write the balanced chemical equations for its reaction with sodium hydroxide.

$H_{3}PO_{4} + OH^{-} \rightarrow H_{2}PO_{4}^{-} + H^{+}$

$H_{2}PO_{4}^{-} + OH^{-} \rightarrow HPO_{4}^{2-} + H^{+}$

$HPO_{4}^{2-} + OH^{-} \rightarrow PO_{4}^{3-} + H^{+}$

Each of the three reactions will be a titration. So, overall, we'll have three equivalence points. Remember, $pH=pK_{a}$ at the half equivalence point. So, we can use the dissociation constants to help determine our indicator. The $pK_{a}$ values are 2.2, 7.2, 12.3. So, we could guess that the equivalence points would be in the ranges 4-6, 8-10, and 13-14. What indicators should we use? For the first equivalence points, I'd use methyl red. For the second, phenolphthalein would work well. Often, we don't care about the third equivalence point. It's at too high a pH. But, if we had to indicate it, I'd use Universal Indicator for the whole reaction.

The Best Indicator: Universal Indicator

The best indicator is Universal Indicator. It's a mixture of many indicator solutions. So it changes color across the whole pH scale. With it, you can collect a lot of information. You'll indicate many different pH transitions. The table below provides you with the colors and pH range.

If you ever get a chance, I recommend trying Universal Indicator. It's genuinely just plain fun! You get to see the solution change from red to violet. It makes you feel like a real chemist! Sadly, most teachers don't want titrations to be too easy. So, they don't let you use Universal Indicator. It's not even that expensive! I found some at Walmart for ten dollars!

\begin{array}{c|c} pH & Color \\ \hline <3 & Red \\ 3-6 & Orange \\ 7 & Green \\ 8-11 & Blue \\ >11 & Indigo \end{array}

Chemistry in the World: Auto-Titrators

Unfortunately, as students, you'll have to do all your titrations by hand. Indicators can be challenging to use. As a freshman in college, I remember titrating with phenolphthalein. As I approached the equivalence point, I would start to see flashes of pink. But, with one more measurable addition, my solution would turn bright pink. I had passed the equivalence point! Do you know the worst part? My professor based my grade on how accurately I found the equivalence point. It was so frustrating!

The summer after my sophomore year, I worked in an analytical lab. I was amazed! They had an auto-titration instrument! They're exceptionally accurate and precise. The pH probe records constant measurements. It also adds minuscule volumes of the titrant. So, it keeps track of titrant volume and pH in real-time. The instrument connects to software that gives you a titration curve. I was resentful about the poor conditions in my freshman chemistry lab. Students never get quality equipment!