Investigation of penbutolol—iron(III) complex and its spectrophotometric determination in tablets

It has been established that penbutolol reacts with iron(III) chloride in the presence of ammonium thiocyanate to form a pink complex (2:1) that is soluble in chloroform with a maximum absorbance at 478 nm. By application of the methods of Sommer and Job involving non-equimolar solutions, the conditional stability constant (log k′) of the complex at the optimum pH of 1.5±0.02 and an ionic strength of (μ) 0.14 M, was found to be 5.769. The molar absorptivity at 478 nm was 136 l mol^?1 cm^?1 at pH 1.5±0.02. The validity of Beer's law has been tested in the concentration range 3–18 × 10^?4 M; the relative standard deviation (n = 8) was 1.52–3.21%. The proposed method was found to be suitable for the accurate, simple and rapid analysis of penbutolol in the bulk drug and in tablets.