Chemguide – questions
INDICATORS
1. Methyl orange is an indicator commonly used in certain acid-base titrations. It exists in two forms,
one red and one yellow, which are in equilibrium with each other.
H-Meor represents one form, and Meor- the other.
a) Explain why methyl orange turns yellow when you add it to an alkaline solution.
b) Explain why methyl orange turns red when you add it to a sufficiently acidic solution.
c) The pKind for methyl orange is 3.7, and its pH range is 3.1 - 4.4. What does this information tell
you about the indicator?
d) What colour would methyl orange be in a solution with a pH of 7.0? Explain your answer.
e) What colour would methyl orange be in a solution with a pH of 5.0? Explain your answer.
f) What colour would methyl orange be in a solution with a pH of 3.4? Explain your answer.
2. The table shows pKind, the pH range, and the colour changes of three indicators.
Indicator pKind pH range colour at lower pH colour at higher
pH
bromophenol blue 4.0 3.0 - 4.6 yellow blue
methyl red 5.1 4.2 - 6.3 red yellow
phenolphthalein 9.3 8.3 - 10.0 colourless red
What colours would each of these indicators be in solutions with the following pHs?
a) 2.0 e) 6.0
b) 3.0 f) 7.0
c) 4.0 g) 8.0
d) 5.0 h) 9.0
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Chemguide – questions
3. This question is about the use of indicators in various titrations. In each case, I have given you a pH
curve for the reaction. Use these curves to help you to explain whether you could use methyl
orange, phenolphthalein, both, or neither, as the indicator for the titration.
a) Titrating a strong base with a strong
acid:
b) Titrating a strong base with a weak
acid:
c) Titrating a weak base with a strong
acid:
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Chemguide – questions
d) Titrating a weak base with a weak
acid:
e) In the last case, the equivalence point is at a pH of approximately 7.0. Bromothymol blue has a
pKind of 7.0, and a pH range of 6.0 - 7.6. Explain why you couldn't use this as the indicator to give
you an accurate titration result.
4. The pH curve below is for the titration of sodium carbonate solution with dilute hydrochloric acid
with the pH ranges of phenolphthalein superimposed on it.
a) Write the equation for the reaction which phenolphthalein detects the end point of.
b) The point of inflexion which the phenolphthalein detects isn't very different from the one with a
weak acid / weak base in the graph in the last question. What two features of phenolphthalein allow
you to be able to use it successfully in this sodium carbonate case?
c) Could you use bromophenol blue or methyl red (see Q2) to detect the second end point of this
reaction? Explain your answer. If you could use either or both of these indicators, what colour(s)
would you look for to get the most accurate reading for the second end point?
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