Video: Applying Knowledge of the Color of Indicators at pH 7, and in the Presence of a Strong Base

For statements I and II, state for each if they are true or false. I) All indicators have color in solutions at pH 7. II) Indicators lose color only in the presence of a strong base. If both are true, state if II is a correct explanation for I.


Video Transcript

For statements one and two, state for each if they are true or false. 1) All indicators have color in solutions at pH seven. 2) Indicators lose color only in the presence of a strong base. If both are true, state if two is a correct explanation for one.

The phrase indicators in this context means pH indicators. pH indicators are chemicals or mixtures of chemicals that display different colors at different pHs. Most indicators will have two or three different colors. Let’s have a look at methyl orange. At pHs less than or equal to about 3.1, methyl oranges is red. At pHs around 4.4 and above, methyl orange is yellow. In between these pHs, methyl orange will look orange because it will be a mixture of the red and yellow forms. Methyl orange in a solution with pH seven will appear yellow. So, that’s one indicator that does have color in solutions of pH seven.

But, statement one suggests that there are no exceptions to this rule. All indicators have color at pH seven. We can disprove this statement by thinking of one indicator we might use in an acid–base titration. The classic setup has standard sodium hydroxide solution in the burette and an unknown concentration of hydrochloric acid in the Erlenmeyer flask below. The sodium hydroxide is added until all the hydrochloric acid has reacted. Just before the endpoint, each drop generates a swirl of pink. That’s because in this set up, we’ve used an indicator called phenolphthalein.

In solutions with pH between zero and 8.2, phenolphthalein is colorless. At a pH around or above 8.2, phenolphthalein is pink. So, that’s why we see the burst of pink when we add the base sodium hydroxide. At pH seven, phenolphthalein is colorless, which means statement one is false. We do have one indicator, methyl orange, that does have a color at pH seven. But the statement is about all indicators, and phenolphthalein is colorless at pH seven.

Statement two is that indicators lose color only in the presence of a strong base. But we can also disprove this statement using phenolphthalein and the pH scale. Generally speaking, we call seven on the pH scale neutral. Any solution with a pH less than seven is said to be acidic. And any solution with a pH greater than seven is basic. But the pH range, where phenolphthalein has no color whatsoever, spans everywhere from strong acidic to weakly basic. So, for phenolphthalein, we wouldn’t even need to have a strong base or even a strong acid. We could use pure water, and phenolphthalein would have no color whatsoever. So, statement two is false.

Neither statement was true, so we don’t have to address the last part of the question. The eagle-eyed among you may have spotted something interesting about phenolphthalein that I haven’t mentioned up to now. When phenolphthalein is left in strong base, it will start to lose its color again. This is because phenolphthalein starts to degrade and react with the base. In a regular titration, even if we took the pH up to 14, we probably wouldn’t see the color change for quite some time. That’s why phenolphthalein is generally considered to be pink in basic solutions.

And what’s quite cool about phenolphthalein is if you take it down to negative pHs, it’ll turn orangey red. And if you’re really interested, this is what the actual molecule of phenolphthalein looks like in the different pH solutions. Thankfully, you don’t need to remember this. What was important was to identify that phenolphthalein is an example of an indicator that has no color at pH seven and doesn’t need to be in the presence of strong base to have no color.

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