You will need to prepare a solution that contains the unknown (use 2 mL of the stock unknown) spiked with Hg at a concentration of 50 ppm (mg/L), diluted to a final volume of 5 mL in 0.1 M KNO3/50 mM HNO3. What is teh dilution of your unknown, so I can calculate back to final concentration? And if 1000 ppm is to be used for the serial dilutions (unknown is close to 1 ppm), what standard concentration should I use for additions?