Various solvent systems like distilled water, 0.1 N HCl,
phosphate buffer pH 6.8 and pH 7.4, methanol, distilled
water: methanol (1:1), 0.1 N HCl: methanol (1:1),
phosphate buffer pH 6.8-methanol (1:1) and phosphate
buffer pH 7.4-methanol (1:1) were tried to select an
appropriate solvent with good suitability and stability. A
solvent system, 0.1 N HCl: methanol (1:1) was selected
for the determination of aspirin and paracetamol, since
both drugs were soluble in it. In this solvent, minimal
interference in the absorbance of both drugs was
observed. The standards of aspirin and paracetamol
were scanned in the range of 200 to 400 nm against
water as blank for obtaining overlain spectra. The overlain
UV spectra are shown in Figure 2. The absorbance
and absorptivities of serial standard solutions of aspirin
and paracetamol were carried out at selected wave
lengths λ1 265 and λ2 257 nm, respectively. The UV
absorption of aspirin and paracetamol interfered with
each other, therefore law of additivity was employed for
the determination of both drugs in mixture. The method
employed solving of simultaneous equations using
Cramer’s rule and matrices. The absorptivity (a) is
extinction coefficient which was calculated using equation