U2 S3 L1
Indicators
page 599-600: Neutralization Reactions
Textbook Items p
616: item 8
Upon completion of this lesson, you should be able to:
• define acid/base indicators operationally
• define acid/base indicators theoretically as weak acids and
write net ionic equations to represent the reversible nature of
an acid-base indicator equilibrium system
• use Le Châtelier's principle to predict indicator colour changes
when acid or base is added to a solution containing a specific
indicator species
• determine the pH of a solution using indicator
• An indicator is a weak acid or weak base which
changes color in response to specific change in pH.
– The pH range at which an indicator molecule changes from
the weak acid form to the conjugate base form is a function
of the ease with which the proton can be removed from the
weak acid form.
Example:
• Litmus is an indicator. It has two forms:
– a weak acid form (HLt ) which reflects pink light in low pH
conditions
– and a conjugate base form (Lt - ) which reflects purplishblue light in higher pH conditions.
• phenolphthalein is colourless below pH 8.2 and deep pink above pH 10.0.
– If you add a drop of phenolphthalein solution to a solution and you
see a deep red colour (Ph -) , then you know that the pH of the
solution is something above pH 10.0.
– However, if you see no colour (HPh), then the pH of the solution is
below pH 8.2.
• A slight pink colour tells you that the pH is somewhere between pH 8.2
and 10.0 because similar amounts of both forms of the indicator will be
present in this pH range.
• Phenol red indicator (HPr) has a yellow colour below pH 6.6
and a red colour above pH 8.0.
a. Write the net ionic equation for the reaction between phenol red (HPr)
indicator and water.
b. Predict the position of the indicator equilibrium when a drop of the
indicator is added to a flask containing 50 mL of 0.100 M nitric acid.
c. Suggest a way to shift the position of the indicator equilibrium.
Using the Indicator Table:
• What is the approximate pH range of a solution if it caused
methyl red indicator to turn yellow and phenol red indicator to
turn yellow?
1.
Write a net ionic equation for the reaction of each weak acid form of these indicators with water:
a. bromothymol blue, HBb(aq)
b. congo red, HCr(aq)
c. methyl orange, HMo(aq)
2.
A flask contains 100 mL of 0.100 M ethanoic acid and two drops of bromothymol blue. What form of
the indicator should be favoured in the flask? Predict the changes that should occur when excess
NaOH is added to the flask. Justify your predictions.
3.
A flask contains 100 mL of 0.100 M ammonia and two drops of congo red indicator. What form of the
indicator should be favoured in the flask? Predict the changes that should occur when excess HCl is
added to the flask. Justify your predictions.
4.
Combinations of indicators can be used to determine the approximate pH's of solutions. Predict the pH
range for each solution given these indicator colour results.
a. congo red indicator is red and chlorophenol red indicator is yellow.
b. metacresol purple indicator is yellow and chlorophenol red indicator is red.
c. both thymolphthalein and indigo carmine indicators are blue.
5.
Determine the colour and form of each indicator at the stated pH condition. The formula for each
indicator in the weak acid form is given in parentheses.
a.
b.
c.
d.
e.
f.
g.
h.
methyl orange (HMo) at pH 7.0
alizarin yellow R (HAy) at pH 9.7
litmus (HLi) at pH 8.7
metacresol purple (HMp) at pH 3.4
thymol blue (H2Tb) at pH 3.1
thymol blue (HTb-) at pH 14.0
phenolphthalein (HPh) at pH 9.0
Methyl violet indicator is a yellow colour in a 1.0 mol/L solution. Is the solution a strong acid,
strong base, weak acid, or weak base. Explain.