Making sodium, potassium and ammonium salts
2 marks · EasyExplain the role of methyl orange indicator in an acid–alkali titration to prepare potassium salts. Why does the indicator colour change at the endpoint? (2 marks)
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Methyl orange is added to show when neutralisation is complete: it is yellow in alkaline solution, turns orange at the endpoint, and red in acid. The colour change occurs at the endpoint because the addition of acid lowers the pH rapidly, and methyl orange changes colour between pH 3.1 and pH 4.4.
Also accept: Methyl orange shows the endpoint: yellow in alkali, orange at endpoint, red in acid. The colour change marks neutralisation, as methyl orange changes in a particular pH range.
Common mistakes: Only describing the colour without explanation of endpoint.
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Marking points
B1 · 1 mark · States the indicator shows when neutralisation has occurred (detects endpoint).
B1 · 1 mark · Explains the colour change at the endpoint is due to rapid change in pH and the indicator’s pH range (yellow to orange to red). Includes colours for acid/alkali.
Worked solution
Methyl orange is used in the titration because it changes colour when the solution goes from alkali to acidic, showing the endpoint. It is yellow in alkali, orange at the endpoint (as neutralisation occurs), and red in acid. The colour change happens because methyl orange changes colour between pH 3.1 and 4.4, signalling the neutralisation point.