The concentrations of Fe^3+ and Cu^2+ in a mixture can be determined following t

Question

The concentrations of Fe^3+ and Cu^2+ in a mixture can be determined following their reaction with hexacyanoruthenate (II), Ru(CN)_6^4-, which forms purple-blue complex with Fe^3+ (lambda_max = 550 nm) and a pale-green complex with Cu^2+ (lambda_max = 396 nm). The molar absorptivities (M^-1 cm^-1) for the metal complexes at the two wavelengths are summarized in the following table. When a sample containing Fe^3+ and Cu^2+ is analyzed in a cell with a path-length of 1.00 cm, the absorbance at 550 nm is 0.183 and the absorbance at 396 nm is 0.109. What are the molar concentrations of Fe^3+ and Cu^2+ in the sample?

Explanation / Answer

Using Lambert law equation:

A = e*l*C

Here, l = 1 cm

For the given mixture:

A550 = e550(Fe)*CFe + e550(Cu)*CCu.

Putting values we get:

0.183 = 9970*CFe + 34*CCu. ----(I)

A396 = e396(Fe)*CFe + e396(Cu)*CCu

Putting values we get:

0.109 = 84*CFe + 856*CCu. ----(II)

From (I) and (II), we get:

CFe = 1.79*10-5 M

CCu = 1.25*10-4 M

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