While it is possible to quantitatively measure
the pH of a solution using a device known
as a pH meter,
there are also ways to determine pH qualitatively.
In this lesson, we shall learn about indicators,
which help us "indicate" the pH of a solution.
An indicator changes colour depending on the
pH of the solution analysed.
Because of this, they are also known as pH
indicators.
Universal indicator is very versatile as it
can measure pH across the entire scale from
0 to 14.
It is not a single indicator, but rather,
a combination of various indicators.
If you look at the colour changes of universal
indicator according to the pH of a solution,
you will find that it resembles a rainbow.
If universal indicator solution turns deep
red in a solution,
it means that the solution is strongly acidic
with a pH 1-2.
If the universal indicator solution turns
green,
it means that the solution is neutral with
a pH of 7.
If the universal indicator solution turns
dark blue or purple,
it means that the solution is strongly alkaline.
Universal indicator paper works in exactly
the same manner.
Litmus is another indicator commonly found
absorbed onto paper as "litmus paper".
Blue litmus paper turns red under acidic conditions,
pH less than 7.
Red litmus paper turns blue under alkaline
conditions, pH greater than 7.
Phenolphthalein is an indicator commonly used
to determine the endpoint of
a strong acid-strong alkali or weak acid-strong
alkali titration.
It goes from colourless to fuchsia pink at
a pH of 8.2.
Methyl orange is used to determine the endpoint
of either
a strong acid-strong alkali or strong acid-weak
alkali titration.
It is red at pH lower than 3.1, and yellow
at pH higher than 4.4.
In conclusion,
indicators are substances that change colour
at a certain pH.
The point at which an indicator changes colour
is known as its 'end point', which can be
at any pH.
Common indicators that you will encounter
are universal indicator, litmus in the form
of litmus paper, phenolphthalein, and methyl
orange.
