Samples of surface water, groundwater and drinking water were obtained from different locations of Uruguay. For arsenic, the calibration curve in the range 2.0–15 lg l-1 was prepared by accurately measuring 0.05-, 0.1-, 0.2- and 0.3-ml of the 0.6-mg l-1 intermediate standard solution, followed by the addition of 10 ml of water, 0.2 ml of 7% (v/v) sulfuric acid and
0.4 ml of 5% (w/v) potassium persulfate. The mixture was heated on a hot plate for 1 h at gentle boiling and
then cooled down at roomtemperature. Afterward, 1 ml of 7% (w/v) sodium iodide and 1 ml of concentrated
hydrochloric acid were added to carry out the prereduction of As(V) to As(III), followed by dilution to 15 ml with water.