A multi-angle continuous-flow injection analysis turbidimetric method for the determination of diphenhydramine...

Diphenhydramine hydrochloride was determined through a newly developed turbidimetric method on a home-designed continuous-flow injection analysis (CFIA) system. The principle of the method is based on the precipitation-induced signal enhancement, using potassium hexacyanoferrate (III) as a selective precipitating agent. The CFIA manifold uses a flow-through detection cell that consists of 12 white light-emitting diodes and three solar cells placed at 0, 90 and 180 degrees in order to track cumulative light scattering over a range of angles. The systematically optimized chemical and instrumental parameters were the reagent concentration, flow rate and sample volume to attain optimal sensitivity and reproducibility. The technique had good linearity (correlation coefficient, R = 0.9975) and per cent linearity (R ²% = 99.49). The limit of detection was 255.325 ng/sample, calculated from serial dilution within the calibration range. Very high accuracy was achieved with a relative standard deviation (r.s.d.% < 0.4) of six replicates at 5 and 12 mmol l^-1. Diphenhydramine hydrochloride in pharmaceutical tablets was successfully determined using the method. The method was found to be accurate, reliable and suitable for pharmaceutical quality control, as demonstrated by statistical comparisons with a conventional spectrophotometric technique using a paired t-test and a one-way ANOVA at the 95% confidence level.