simultaneous spectrophotometric determination of thiamine and pyridoxine in multivitamin dosage forms using h-point standard addition and vierodt᾿s methods

Two simple, reliable and accurate spectrophotometric methods were described for the simultaneous determination of thiamine hydrochloride (thc) and pyridoxine hydrochloride (pyh) in pharmaceutical formulation without any pre-separation procedures. these methods are based on the difference in the color observed from diazotization coupling methods of thc and pyh. when the diazonium salts formed from the reaction of hno2 with p-nitroaniline (pna), they were coupled with thc and pyh in sodium hydroxide medium and the formed dyes had maximum absorption bands at 452 nm for thc and 482 nm for pyh. all experimental parameters were optimized to reduce experimental error. the first method was based on the vierodt᾿s method, in which the two wavelengths 452 and 482 nm λmax of thc and pyh, respectively, were selected for the formation of simultaneous equation. in the second method the h-point standard addition method (hpsam) was applied, when absorbances at wavelengths pair, 452 and 500 nm, were monitored with the addition of a standard solution of thc to a solution containing a fixed concentration of thc and pyh. the results from applying vierodt᾿s and hpsa methods showed that thc and pyh can be determined simultaneously in the concentration range 1.5–11.0 μg ml−1 (lod = 0.48 μg ml−1) and 0.4–5.0 μg ml−1 (lod = 0.23 μg ml−1), respectively. the methods were successfully applied for simultaneous determination of thc and pyh in different synthetic and pharmaceutical products. to assess the obtained vitamins (thc and pyh) analysis by both methods (vierodt᾿s and hpsam) in the several pharmaceutical formulations, the results are compared with that obtained by hplc method as a reference, and good agreements between the results indicate the reliability of the proposed methods.