Acid-base equilibria
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Measuring limewater concentration by titration with hydrochloric acid. What is the best indicator and why? Assume methyl orange if calcium hydroxide solution is a weak alkali (but is it really?). Web searches generally recommend phenolphthalein but sometimes methyl orange.
Corrie writes ...
The lime (calcium hydroxide) in limewater is not very soluble but that which dissolves dissociates completely into hydroxide ions (and calcium ions) in solution. It can therefore be regarded as a strong base, like sodium hydroxide.
For a titration involving a strong acid and a strong base either phenolphthalein or methyl orange would be suitable as an indicator, but the colour change with phenolphthalein is more dramatic.
For more accurate analytical titrations, especially if the limewater is more dilute than the usual saturated solution and/or also contains some carbonate due to reaction with carbon dioxide in the air, then methyl orange is usually preferred.
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updated: 26 March 2007
