الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
Selenium is an essential element for the proper functioning of the human body. It plays a crucial role in metabolism and thyroid function and helps protect the body from oxidative stress damage. Therefore, the highly sensitive assessment of selenium in waters and food samples is of great importance for environmental, biochemical and health concerns. Selenium assessment based on its catalytic effect of the reduction of methylene blue by sulfide ion has been reported using the fixed time and induction period monitoring methods. However, the very poor precision and relatively limited sensitivity prompted us to develop a new highly sensitive, precise and validated initial rate kinetic method for selenium assessment based on the methylene blue- sulfide ion reaction. The progress of reaction was traced spectrophotometrically and with a digital camera. Various reaction variables affecting the methylene blue decolorization were thoroughly investigated, including: reagents concentrations, order of addition, mixing and standing times, pH, temperature, ionic strength, and the surfactant type and concentration. A mono-variate optimization procedure was followed for the optimization of reaction variables and the optimized conditions have been included in the recommended procedure. Cetyltrimethyl-ammonium bromide (CTAB) greatly enhanced the selenohydrosulfide reduction of methylene blue near micellar concentrations, lowered the linear range of Se determination and boosted the analytical sensitivity of the developed method. The concentration effect of polysulfide or selenohydrosulfide on the micellar surface caused micellar catalysis, i.e., an increase in reaction rate. Procedures for dealing with various possible interfering species were investigated. The findings of this investigation allowed for a higher sensitivity, higher selectivity, lowered detection and quantification limits, and improved precision. The developed method has been successfully applied to natural, wastewaters, and food samples with excellent recovery results and excellent harmony with the standard AOAC method. Statistical treatment of analytical data showed the absence of any systematic errors and revealed the high accuracy and precision of the developed method.