Analysis of Phosphate Content in Natural Waters Background The United States Environmental Protection Agency sets the maximum lawful amount of phosphorous in streams, lakes, and reservoirs. For example, a limit of 0.1 mg/L is allowed in streams that do not discharge into lakes or reservoirs. The analysis of natural water samples will be accomplished here by a spectrophotometric determination, similar to the analysis we employed for the determination of Fe in an ore sample earlier this quarter. Our initial task will be to construct external calibration data and correlate absorbance with phosphate concentration. Having confirmed that our procedure is appropriate for the determination of phosphate concentration in aqueous samples, we can then consider a standard addition procedure and the analysis of natural water samples. A potential source of difficulty when determining phosphate is the presence of interferences. Competitive complexation from ions such as arsenate, aluminum, or chloride (especially in marine and estuarine waters) is an issue to be aware of. Also, the presence of soap used for cleaning glassware is another potential interference as phosphates are used as additives in the majority of detergents. It may be prudent to thoroughly and repeatedly rinse your glassware in distilled water prior to its use. ______________________________________________________________________ Reagents A. A 2.5% ammonium molybdate solution, prepared by dissolving 2.5g of (NH4)6 Mo7 O24, 4H2O in 100mL of distilled water. Sulfuric acid, 6N concentration of H2SO4. C. A 10% ascorbic acid (vitamin C) solution, prepared by dissolving 10g in 100ml of distilled water. This may be stored under refrigeration at 2-4 degree C and will be stable for about 7 weeks. REAGENT I. Mix 1 volume of 6N sulfuric acid with 2 volumes of distilled water and 1 volume of 2.5% ammonium molybdate, then add 1 volume of 10% ascorbic acid and mix well. Prepare fresh each day. Procedure In a beaker, place 10ml of the sample to be analyzed. This may be a blank, a standard, or an ?unknown? sample for analysis. Add 10ml of REAGENT I. Heat sample for 30 minutes at 45°C and measure the absorbance at 820nm. Effect of Time and Temperature. The color development at room temperature is incomplete even in 3 hours but is complete in 1 hour at 37° C and in 30 min at 45°C. Depending on time constraints, it may be useful to wait until next class period before measuring absorbance values. Standard Addition Procedure Having calculated an approximate concentration of phosphate in a given sample by use of external standards, it is possible to use the method of standard addition to re-determine the concentration of phosphate in these same samples. The optimum concentration of the added standard solution should be one that increases the absorbance for a factor of 1.5 ? 3.0. Keep the added volume of the standard small relative to that of the original solution, e.g. near 1 percent. After having added the standard solution to the unknown solution, measure the absorbance of the combined concentration. Plot the Absorbance of the solution as a function of the standard Concentration. A plot of signal intensities of the solutions versus the added concentrations should yield a straight line. Determine the concentration of the analyte from the point at which the extrapolated line crosses the concentration axis at zero signal. PAGE PAGE 1
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