Chemical quantities (AS)
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I am planning my chemistry AS level coursework and have a dilemma. My planning sheet recommends that when doing a titration between sulphuric acid and sodium carbonate solution the best indicator is methyl orange. However I have found that the end point of methyl orange is pH 3.7, thus it will not change colour until this point. So why has it been recommended when it will only tell me when the solution is acidic and not neutral?
Igloo writes
Your question brings up two issues: the choice of indicators generally, and the selection of methyl orange in this particular case. The theory behind the use of indicators is dealt with in the Nuffield Chemistry Students’ Book – Topic 14 (an A2 topic), and there you will find that for each titration there is a set of rules to follow when choosing an appropriate indicator. If you are interested, and if it will help to improve the quality of your coursework, I suggest that you try to do a little research on this theory.
Fortunately I can explain the use of methyl orange in this case, in a fairly straightforward fashion. The complete neutralization of sodium carbonate by sulphuric acid produces sodium sulphate, carbon dioxide and water:
Na2CO3(aq) + H2SO4(aq) -> Na2SO4(aq) + H2O(l) + CO2(g)
A solution of sodium sulphate is neutral and has a pH of 7, but the carbon dioxide also formed is a weak acid, and some of this gas remains dissolved in the solution, producing what is sometimes referred to as carbonic acid (H2CO3). This causes the mixture to be weakly acidic at the end point, leading to the use of methyl orange, which, as you rightly point out, changes colour at about pH 4. Actually, in more precise work, the use of the indicator bromocresol green is preferred, since the relevant pH being measured is closer to 4.5. Also many chemists find this colour change easier to follow (yellow in acid, blue in alkali, green at the 'end point').
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updated: 02 December 2003
