In the example below, an acid and an alkali react to make sodium chloride.
hydrochloric acid + sodium hydroxide sodium chloride + water.
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
The burette is filled with hydrochloric acid.A known quantity of alkali (say 50 cm3 sodium hydroxide)is released from a pipette into the conical flask.The tap on the burette is turned opento allow the acid to be added drop by drop into the alkali.The alkali contains an indicator (phenolphthalein)which is pink in an alkali and colourless in an acid.
When enough acid has been added to neutralise the alkalithe indicator changes from pink to colourless.
The titration can be repeated using the same amounts of acid and alkalibut without the indicator.Pure salt crystals which are free from indicatorcan then be crystallised from the neutral solution.
Alternatively a pH meter can be used to find the end point.At neutralisation the pH is 7.
Indicators used for weak Acids or Alkalis.
Colour in Acid
Colour in Alkali
Strong Acid+Weak Alkali
Strong Alkali+Weak Acid
Either Indicator can be used for a Strong Alkali + Strong Acid.
Universal indicator is not usually used for a titrationbecause it changes gradually giving different colours for a different pH.Methyl orange or phenolphthalein are usedbecause they give a sudden change in colour at neutralisationwhich makes it easier to see the end point of the titration
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