Standard addition. The U.S. Environmental Protection Agency sets guidelines for the level of contaminants in drinking water below which no adverse health effects are likely to occur. The limit for thallium is 0.5 μg/L0.5 μg/L.
Thallium (Tl+)(Tl⁺) was measured using anodic stripping voltammetry, a very sensitive electro-chemical method. The blank-corrected current at -0.68 V-0.68 is proportional to the concentration of thallium. Repeated additions of 5.0 μL 5.0 μL of a 100-gg/L Tl+ 100-gg/L Tl⁺ standard were made to a 5.00-mL 5.00-mL sample solution.

a. Measure the responses in the figure and prepare a graph to find out how much Tl+Tl⁺. was in the unspiked sample. Is the calibration linear?
b. Determine the amount of Tl+(μg/L)Tl⁺ (μg/L) in the unspiked sample.
c. Find the 95%95% confidence interval for Tl+Tl⁺ in the sample.

Blank subtracted current (A) 0.08 0.06 0.04 0.02 0.00 -0.80 20-L spike 15-L spike 10-L spike 5-L spike No spike -0.75 -0.70 -0.65 Voltage (V versus Ag | AgCl reference) -0.60 -0.55