Differential spectrophotometric determination of phenylephrine in eye‐drops: the ΔAr method

In the classical spectrophotometric method for the determination of phenylephrine, the absorbance A in 0 . 1 ~ sodium hydroxide is related to the concentration C by an equation A = a, + b,C, where a, is the ‘blank’ absorbance and b, is the absorptivity. In the relative absorbance method (Pall, Svehla & Erdey, 1964; Svehla, 1966; Belekov, 1970), in which the absorbance of the solution of unknown concentration C is compared with that of a reference solution of known concentration Cr and similar composition, the relative absorbance Ar is related to C and Cr by an equation Ar = b2(C Cr) = a2 + b2C. Another modification, which also partially compensates for interference by other components of the eye-drops, is to measure the difference between the absorbance of two solutions of the same concentration, one in 0 . 1 ~ sodium hydroxide and the other in 0 . 1 ~ sulphuric acid. The absorbance difference AA is attributed solely to the phenylephrine and it is assumed that the irrelevant absorption is independent of the pH (Aulin-Erdtman, 1955; Junejo & Glenn, 1956). This leads to an equation AA = a3 + b3C. It is now proposed to use a combination of the Ar and AA methods, so as to gain the benefits of both methods. The relative absorbance difference AAr is related to C by an equation AAr = a4 + b4C. Experiments were made with concentrations of phenylephrine leading to absorbances or absorbance differences in the range 0.15 to 0.9, and with a reference solution giving ab absorbance of about 0.4. The measurements were made in 0 . 1 ~ sodium hydroxide at the maximum at 238 nm, and in 0 . 1 ~ sulphuric acid at the maximum at 274 nm. The relation between absorbance or absorbance difference and concentration (C, ,ug per ml) were found to be as follows: A = -0.0076 + 0.0534C Ar = -0.4293 + 0.0530C AA = 0.0028 + 0.0421C AAr = -0.3344 + 0.0419C