How to carry out a titration?
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The right diagrams show the typical apparatus (1)-(6) used in manipulating liquids and on
the left a brief three stage description of titrating an acid with an alkali:
1. An accurate volume of acid is pipetted into the conical flasks using a suction bulb
and pipette for health and safety reasons. Universal indicator is then added, which
turns red in the acid.
2. The alkali, of known accurate concentration, is put in the burette and you can
conveniently level off the reading to zero (the meniscus on the liquid surface should
rest on the zero -- graduation mark).
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Note in stage 2. other possibilities are:
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A small amount of accurately weighed solid acid is dissolved in
water and titrated with alkali.
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A small amount of accurately weighed solid alkali is dissolved in
water and titrated with acid.
After this, the method is essentially the same as described below.
3. The alkali is then carefully added by running it out of the burette in small
quantities, controlling the flow with the tap, until the indicator seems to be going
yellow-pale green.
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The conical flask should be carefully swirled after each addition of alkali to
ensure all the alkali reacts.
4. Near the end of the titration, the alkali should added drop-wise until the universal
indicator goes green.
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This is called the end-point of the titration and the green means that all the
acid has been neutralised.
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The volume of alkali needed to titrate-neutralise the acid is read off from
burette scale, again reading the volume value on the underside of the
meniscus.
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The calculation can then be done to work out the concentration of the alkali.
5. Universal indicator, and most other acid-base indicators, work for strong acid and
alkali titrations, but universal indicator is a somewhat crude indicator for other acidalkali titrations because it gives such a range of colours for different pH's. Examples
of more accurate and 'specialised' indicators are:
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titrating a strong alkali with a strong acid (or vice versa):
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e.g. for sodium hydroxide (NaOH) - hydrochloric/sulphuric acid
(HCl/H2SO4) titrations, use ...
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phenolphthalein indicator (pink in alkali, colourless in acid-neutral
solutions), the end-point is the pink <==> colourless change.
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Litmus works too, the end point is the red <==> purple/blue colour
change.
titrating a weak alkali with a strong acid:
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e.g. for titrating ammonia (NH3) with hydrochloric/sulfuric acid
(HCl/H2SO4), use ...
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methyl orange indicator (red in acid, yellowish-orange in neutralacid), the end-point is an 'orange' colour, not easy to see accurately.
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screened methyl orange indicator is a slightly different dye-indicator
mixture that is reckoned to be easier to see than methyl orange, the
end-point is a sort of 'greyish orange', but still not easy to do
accurately.
titrating a weak acid with a strong alkali:
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e.g. for titrating ethanoic acid (CH3COOH) with sodium hydroxide
(NaOH), use ...
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phenolphthalein indicator (pink in alkali, colourless in acid-neutral
solutions, pink in alkali), the end-point is the first permanent pink.
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methyl red indicator (red in acid, yellow in neutral-alkaline), the
end-point is 'orange'.
titrating a weak acid with a weak alkali (or vice versa):
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These are NOT practical titrations because the pH changes at the
end-point are not great enough to give a sharp colour change with
any indicator.