11 Li was determined by the method of standard additions usi

11. Li was determined by the method of standard additions using atomic emission. From the data in the following table, calculate the concentration of Li in the unknown solution. The Li standard contained 2.250 ppm Li. (12 pts). Unknown, ml 10.00 10.00 10.00 10.00 10.00 Std, ml Final Vol, ml Emission 0.00 100.0 309 5.00 100.0 452 10.00 100.0 600 15.00 100.0 765 20.00 100.0 906 ---> 12. You will perform a photometric titration at 435 nm. A solution is prepared by taking 1.534 grams of a powder containing an unknown amount of cobalt, digesting it in concentrated HNO3, and diluting it to 250 mL. A 10 mL aliquot of this solution is then titrated with a color reagent R that is at a concentration of 4.0 x 104 M. The color reagent reacts with the Co in the following reaction: Co + 2R

Solution

11)

With no standard added the solution gave an emiision intensity = 309

with 5 ml of standard in 100 ml solution gave an emission intensity = 452

Emission due to standard = 452 - 309

= 143

concentration of 5 ml of standard in 100 ml solution = (2.250 x 5/1000) x (5/100)

= 5.6 x 10^-4 mg

So concentration of Li in the unknown 100 ml solution = 5.6 x 10^-4 x 309/143

= 12.10x 10^-4 mg

concentration of Li in 10 ml solution = 12.10 x 10^-4 x 100/10

= 12.1x 10^-3 mg

Concentration of Li in the unknown (ppm) = 12.1 ppm