Indicators

Indicators

A titration is a process for determining, with precision, the concentration of a solution with unknown concentration. The theory behind titrations will be discussed later in this tutorial. An indicator is used to show the scientist carrying out the reaction exactly when the reaction has reached completion.

This experiment looks at the change in colour of an indicator during an acid-base reaction. It is effectively a very rough titration experiment. Principles that can be applied to titrations, such as adding a small volume of acid, then swirling, can be applied here as well. It is important that the learners understand that the pH range that the indicator changes colour in is not always around \(\text{7}\).

Learners are working with a strong acid and a strong base in this reaction. Concentrated, strong acids and bases can cause serious burns. Please remind the learners to be careful and wear the appropriate safety equipment when handling all chemicals, especially concentrated acids and bases. The safety equipment includes gloves, safety glasses, and protective clothing.

Optional Experiment: Indicators

Aim

To investigate the use of an indicator in an acid-base reaction.

Apparatus and materials

  • one volumetric flask, one conical flask, one pipette, a piece of white paper or a white tile

  • A \(\text{1}\) \(\text{mol.dm$^{-3}$}\) solution of sodium hydroxide (\(\text{NaOH}\)), a \(\text{1}\) \(\text{mol.dm$^{-3}$}\) solution of hydrochloric (\(\text{HCl}\)), an indicator

Method

  1. Measure \(\text{20}\) \(\text{ml}\) of the sodium hydroxide solution into a conical flask. Add a few drops of the indicator.

    4be4f1b0f138800fddfdbf57a3e97771.png

  2. In this experiment colour change is very important. So place the conical flask on a piece of white paper or a white tile to make any colour change easier to observe.

  3. Slowly add \(\text{10}\) \(\text{ml}\) of hydrochloric acid. If there is a colour change, then stop. If there is no colour change add another \(\text{5}\) \(\text{ml}\). Continue adding \(\text{5}\) \(\text{ml}\) increments until you notice a colour change.

    c8865efd0720c0b06997d575b9318a8a.png

Observations and discussion

The solution changes colour after a certain amount of hydrochloric acid is added. This is because the solution now contains more acid than base and has therefore become acidic. It can be concluded that the indicator is one colour in a basic solution and a different colour in an acidic solution.

Indicators are chemical compounds that change colour depending on whether they are in an acidic or a basic solution. A titration requires an indicator that will respond to the change in pH with a sensitive and quick colour change. Typical indicators used in titrations are given in the table below.

Titration type

Preferred

indicator

Colour of

acid

Colour of

end-point

Colour of

base

pH range

strong acid +

strong base

bromothymol

blue

yellow

green

blue

\(\text{6.0}\)\(\text{7.6}\)

weak acid +

strong base

phenolphthalein

colourless

faint pink

pink

\(\text{8.3}\)\(\text{10.0}\)

strong acid +

weak base

bromocresol

green

yellow

green

blue

\(\text{3.8}\)\(\text{5.4}\)

Table: Some typical indicators for typical titrations.

Tip:

Revise Grade 11 Acids and Bases for more information on plants and foods that can be used as indicators.

Tip:

Notice that the pH range for colour change of the indicator used should match the approximate pH expected for that type of titration (see table below):

solution

pH

pH range

neutral

\(\text{7}\)

\(\text{6.0}\)\(\text{7.6}\)

weak

basic

\(\text{9}\)

\(\text{8.3}\)\(\text{10.0}\)

weak

acidic

\(\text{5}\)

\(\text{3.8}\)\(\text{5.4}\)

cf05d2872c58f921b1725a4adf14f705.png

Some typical indicators for typcial titrations: bromothymol blue (left), phenolphthalein (centre), and bromocresol green (right).

Indicators change colour (see figure above) according to where the \(\text{H}\) is:

977acdcc6f900b90dfdc1464b60c67b1.png

So, when an acid is added to aqueous bromothymol blue there will be extra \(\text{H}^{+}\) ions. The equilibrium will shift (remember le Chatalier’s principle) to decrease the number of \(\text{H}^{+}\) ions. That is, to the left. If sufficient acid is added, the entire solution will become acidic. This means there will be more HBromothymol blue than bromothymol blue\(^{-}\) and the solution will become yellow.

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This is a lesson from the tutorial, Acid-Base and Redox Reactions and you are encouraged to log in or register, so that you can track your progress.

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