Chemistry Reference
In-Depth Information
hydrochloric acid will not appear anywhere in the calculation.
Volume of 1
M
NaOH (
f
1.012) in blank titration
49.65 mL
In this case, the volume of 1
M
NaOH reacting with chalk is given by
(Volume of blank titration
volume of back titration)
factor of NaOH
The NaOH factor is used because both of
these volumes are NaOH
volumes. That is,
(49.65
18.5)
1.012 mL 1
M
NaOH
Since from the equivalent
0.05005 g CaCO
3
then the weight of calcium carbonate in the sample is
1 mL 1
M
NaOH
(49.65
18.50)
1.012
0.05005 g
1.5778 g CaCO
3
However, 1.5931 g of
chalk was weighed, so the percentage purity of
calcium carbonate is
1.5778
———
100
99.0% w/w
1.5931
The calculation involving the blank should be more accurate than the back
titration on its own since the NaOH has, in effect, been standardised during
the course of the assay.
These two procedures should be studied closely since there is a subtle
difference in calculation. In the back titration, the volume of acid was
multiplied by the factor of the acid, and the volume of base was multiplied
by the factor of the base. In the blank titration, neither the volume nor the
factor of the reagent added in excess is required and the volume of titrant
equivalent to the chalk is given by the expression (blank volume
test
volume)
factor of titrant.
Assay of unit-dose medicines
Unit-dose medicines are preparations that contain doses designed to be
taken separately. Examples of this type of preparation include tablets,
capsules, suppositories or pessaries. To determine the purity of unit-dose
medicines, the calculations outlined above need to be modified, in order to
determine how much drug is present in each individual dosage form. The
