Chemistry Reference
In-Depth Information
A
3 Sulphonamides (see Chapter 3) are a group of antibacterial
drugs widely used for the treatment of infection, although their use
has declined in recent years with the advent of newer antibiotics
such as penicillins. In this assay, a simple dilution cannot be used as
the absorbance spectra of the two drugs overlap. We can, however,
make use of the additive nature of absorbance measurements and
use simultaneous equations to solve the problem. This method of
analysis is used extensively for mixtures of components when the
absorption characteristics of each substance separately are known.
The absorption curves of the two separate compounds are plotted
and from the traces obtained, two suitable analytical wavelengths
are chosen. If possible, these wavelengths should coincide with
reasonably flat regions of the absorption curve to avoid errors due
to inaccurate selection of wavelength by the monochromator.
Usually, the top of an absorbance peak is chosen, but any flat area
(such as a peak shoulder or even a trough) can be used. In this
example, wavelengths of 252 and 280 nm were selected.
The first task in solving the composition of this mixture is
to calculate the
A
1
value of each drug at each wavelength, using
the Beer-Lambert equation. The results obtained from this are
shown below:
Compound
Calculated A
1
values
At 252 nm
At 280 nm
Sulfanilamide
959
136
Sulfathiazole
570
449
If each component of the mixture obeys the Beer-Lambert law,
then the total absorbance at each wavelength is given by the sum
of each drug's contribution, or
A
1
[(
A
1
)
x
c
x
l
]
[(
A
1
)
y
c
y
l
]
(equation 1)
A
2
[(
A
1
)
x
c
x
l
]
[(
A
1
)
y
c
y
l
]
(
equation 2)
where
A
1
is the total absorbance of the mixture at wavelength 1
A
1
x
is the
A
1
of substance
x
A
1
y
is the
A
1
of substance
y
A
2
is the total absorbance of the mixture at wavelength 2
