Question

Cyanide ion can be determined indirectly via atomic absorption based on its ability to dissolve silver while passing through a porous silver membrane, as described by the chemical equation

4 Ag(s) + 8 CN+2 H2O + O2 →4 Ag(CN)2 + 4 OH

A series of silver standards analyzed by atomic absorption spectroscopy gave a linear calibration curve with a slope of 807 meter units per ppm Ag. (The "meter units" are linearly proportional to absorbance, and ppm Ag means µg Ag/mL solution.) An unknown Ag solution passed through the silver membrane gave a signal of 198 meter units.

(a) Calculate the molarity of CN¯in the unknown. [Answer: 4.55 x 10“ M]

(b) Briefly state a data analysis assumption you make to solve the problem. (That is, we can take for granted that all of the reagents are pure and that there is no experimental error that makes the calculations invalid.)

Answer 1

(a) The regression equation is given as

y (meter units) = 807 (meter units/ppm Ag)*x (ppm Ag)

where y denotes the atomic absorption of the sample and x is the concentration of Ag in the sample in units of ppm or µg/mL.

The unknown sample gave an atomic absorption of 198 meter units.

Therefore,

198 meter units = (807 meter units/ppm Ag)*x

=======> x = (198 meter units)/(807 meter units/ppm Ag)

=======> x = 0.2453 ppm Ag

Since 1 ppm Ag = 1 µg Ag/mL, hence, the concentration of the unknown = 0.2453 µg/mL.

The question asks to determine the concentration of Ag in molarity.

Atomic mass of Ag = 107.868 g/mol

Therefore,

0.2453 µg/mL = (0.2453 µg)/(1 mL)

= [(0.2453 µg)*(1 g)/(1.0*10⁶ µg)*(1 mol)/(107.868 g)]/[(1 mL)*(1 L)/(1000 mL)]

= (2.274*10^-9 mol)/(0.001 L)

= 2.274*10^-6 mol/L

≈ 2.27*10^-6 M (ans).

(b) While determining the concentration of Ag in the unknown sample, it must be assumed that the sample is completely free on contaminations and other interfering species. If there are other species present in the sample that can absorb at the same wavelength as Ag, then these species will add to the atomic absorption and given erroneous results.

Moreover, it must be assumed that the reaction between Ag and cyanide converts the Ag to silver cyanide quantitatively so that the entire amount of Ag can be obtained by measuring the atomic absorption.