Chemistry Reference
In-Depth Information
The sample container and contents are weighed accurately on an analytical
balance, to four, or sometimes six, decimal places.
2.
Some of the sample is transferred to the reaction flask and the sample container
is re-weighed. Care should be taken not to touch the sample with the fingers, a
spatula, or anything else for that matter. The difference in weight between steps 2
and 3 represents the weight of sample transferred.
3.
This process is repeated until the desired weight has been transferred. If more
than the desired weight of sample is transferred, the sample should be discarded
and the whole procedure begun again.
On no account should excess sample be
returned to the original container
. The
British Pharmacopoeia
allows discretion
of
4.
10% on the stated sample weights.
Approximate titre calculation
The end point of a titration should not come as a surprise to the analyst.
Before a single drop of titrant has been added, an estimate of the endpoint
volume should be carried out. For a simple forward titration, like the citric
acid example above, the approximate titre is given by
Sample weight
---------------
x
mL
Equivalent weight
This calculation makes two assumptions, neither of which is actually valid,
namely that the factor of the titrant to be used in the assay is 1.000, and that
the sample is 100% pure. Neither of these assumptions will be true, but the
factor will be close to 1.000 and the purity will, usually, be close to 100%,
so the estimate is worth doing. The approximate titre calculation is also the
first sign the analyst has that things are going wrong in the assay. If the
approximate titre is estimated as (say) 18 mL, alarm bells should begin to
ring if no end point has been reached after approximately 20 mL. The
stated sample weights in the BP are usually chosen to give titres between 20
and 25 mL. This is because analysts are, by nature, lazy and do not want to
have to refill a 50 mL burette during a titration!
Use of molarities in calculation
Students often prefer to perform simple calculations, like the direct titra-
tion of citric acid, using absolute molarities of titrant instead of deriving
the equivalent and making use of factors. The procedure adopted is to
convert the volume of titrant required to a number of moles and, from the
balanced chemical equation, relate this to the number of moles of reactant
used in the assay. This number is then converted into a weight and the
