Acid-base equilibria
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I'm writing my Chemistry planning exam and have been asked to do a plan for a titration experiment to find out the formula of hydrated sodium carbonate. When coming to deciding the most appropriate indicator I did some research but couldn't find an indicator that gave a definite colour when neutral except universal indicator, bromothymol blue and phenol red. Could you suggest any other properties I should be considering when choosing an appropriate indicator?
Igloo writes ...
The choice of a suitable indicator for acid-base titrations is governed by theory which you won’t be covering until A2, so it is inappropriate to give you a detailed answer now. Here is an outline of the main ideas. You could find out more from an A-level chemistry text book or the web. Try for instance Google “Theory of indicators”.
Contrary to popular belief, the end-point of an acid-base titration does not necessarily occur at pH 7 (neutral) – it all depends on the strength of the acid and base concerned. In addition to this, the pH usually changes very dramatically at or near the end-point, so a range of values is always given for the pH when this stage is reached.
For a strong acid and strong base (e.g. HCl and NaOH) the end-point occurs in a pH range of about 4-10, so from a data book you should be able to see that indicators such as methyl orange, bromophenol blue, bromocresol green, methyl red, bromothymol blue, phenol red and phenolphthalein are suitable ones to choose.
However, if the acid is weak, but the base is strong, the end point is reached at a pH range of about 8-10, so indicators such as phenol red and phenolphthalein are typical choices.
In a situation such as yours [the titration of a strong acid (HCl) and a weak base (Na2CO3)], a pH range of about 3-5 applies at the end-point, so you would need to choose, for example, methyl orange, bromophenol blue, bromocresol green or phenol red.
Universal indicator is never used in titrations, since there are so many different colour changes occurring across the full pH range.
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Risk assessment
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updated: 17 December 2006
