SIMULTANEOUS EQUATION METHOD OR VIERODT'S METHOD
If a sample contains two absorbing drugs (X and Y) each of which
absorbs at the λmax different from the other, it may be possible to
determine both drugs by the technique of simultaneous equations
(Vierodt's method), provided certain criterias apply. The information required is (a) The aborptivities of X at and λ1 and λ2 are ax1 and ax2 respectively (b) The aborptivities of Y at and λ1 and λ2 are
ay1 and ay2 respectively. (c) The absorbances of the diluted sample
at λ1 and λ2 are A1 and A2 respectively. Let Cx and Cy be the
concentrations of X and Y respectively in the diluted sample. Two equations are constructed based upon the fact that at λ1 and λ2, the
absorbance of the mixture is the sum of the individual absorbance of
X and Y.
At λ1 A1 = a X1 b Cx + aY1b Cy ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ (1)
At λ2 A2 = a X2 b Cx + aY2 b Cy‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ (2)
For measurements in 1 cm cells b=1
Rearrange eq. (2)
A2 ‐ a X2 b Cx Cy = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
aY2
