determine the concentrations of common anions (F-, Cl- and SO42-) in an unknown water sample using the external calibration and standard addition methods. Experimental Prepare the eluents 1 (carbonate/bicarbonate buffer) and 2 (sulphuric acid) – to conserve reagents, this need not be prepared if it had already been prepared by a previous student. Prepare standards mixtures that contain 1.00, 2.00, 5.00 and 10.00 ppm of F-, Cl- and SO42-. Use micropipette to transfer 1000 ppm stock solutions into 25 or 50 mL volumetric flask. Use ultrapure water to prepare all solutions. Preparation of mixtures of anions for determination using the external calibration method Standard Solution # Volume transferred from stock solution, µL Concentration, ppm 1 2 3 4 1.00 2.00 5.00 10.00 Volumetric flask: mL; concentration of stock solution: ppm Prepare a calibration curve from the stock solutions prepared. Inject unknown sample and calculate the concentration of F-, Cl- and SO42- found in the water sample. Report the average and standard deviation. Comment on the quality of your calibration curve. The standard addition method can be carried out by spiking in series known amounts and concentrations of standard solution to the unknown sample. The peak area obtained is plotted against the concentration of spiked solution. Preparation of solutions for determination using the standard addition method Solution # Volume transferred from stock solution, µL Concentration spiked, ppm 5 6 7 1.00 2.00 5.00 10.00 Volumetric flask: mL; concentration of stock solution: ppm Calculate the concentration of the anions from standard addition method. Questions: i. Suggest an alternative analytical method that can be used to determine the concentration of these anions. ii. Suggest how the quantitative analysis of these anions can be carried out at ultra-trace levels (low ppm).
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