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.