Figure 19.2: A pipette
An indicator solution is added to the sample. This will indicate the end point of the
titration by a change of colour. Once the colour has changed, no more titrant is added.
The volume of the titrant used is determined by subtracting the initial volume in the
burette from the final volume. Titrations are repeated at least three times to ensure
accurate, consistent results. If one result is inconsistent, more titrations will be required to
provide a minimum of three consistent results. Consistent results are averaged together,
to be used in calculations.
The calculations are very similar to those you have already seen in this course.
Doing accurate, precise titrations at such a small scale requires a great deal of laboratory
skill. Computers and automated titrations that work with colorimetry are often used
in industry now, though there is always a need for titrations and skilled laboratory
technicians in some applications.
The end point is the amount (the final drop) of titrant
at which the colour of the indicator changes. The equivalence point occurs when
the number of moles of H
ions equals the number of moles of OH
ions. In order to
perform an accurate titration, the endpoint and the equivalence point must be reached
simultaneously. The indicator must be carefully selected, based on the identities of the
acid and the base. If the wrong indicator is chosen, the titration solution will change
colour at some pH other than the proper equivalence point.
A strong acid/strong base neutralization produces a neutral salt. The indicator used
should change colour at (or very near) pH 7. A strong acid/weak base titration produces
an acidic salt. The indicator used should change colour below pH 7 at approximately
pH 5 or 6. A weak acid/strong base titration produces a basic salt. The indicator used
should change colour above pH 7, at around pH 8 or 9. The following table is used to help
chemists select appropriate indicators when the salt produced during a titration will be
acidic or basic.
Other indicators exist besides the ones in Table 19.3, for higher and lower pH equivalency
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