Calculate the concentrations of nickel:

Question:

A  mixture  of  Ni²⁺    and  Co²⁺    is  analyzed  by  forming  the  salicyaldoxime complexes in chloroform solvent. The molar absorptivities for Ni and Co ions at 500  nm  are,  10  and  1000  respectively.  However,  at  400  nm  both  molar absorptivities are 5.0 × 10³ cm^-1mol ^-1dm³. The absorbances were recorded using 1 cm cuvette and were found to be 0.091 at 500 nm and 0.615at 400 nm. Calculate the concentrations of Ni²⁺ and Co²⁺ in the given mixture.

Answer:

We know in which for concurrent determination of two absorbing species the absorbance is measured at two different wavelengths. For measurements in a cuvette of 1 cm path length, the absorbances, concentrations and the extinction coefficients are associated as:

A_λ1  = C₁ (ε₁ ) λ₁+ C₂ (ε ₂ ) λ₁

A_λ2   = C₁ (ε₁ )λ₂+ C₂ (ε ₂ )λ₂

With given data we can write as:

A_{500 nm}   =   10 × [Ni²⁺]  + 1000 × [Co ²⁺]  = 0.091   ... (A)

A_{400 nm}   =   5.0 × 10³ × [ Ni²⁺] + 5.0 × 10³ × [Co ²⁺]  = 0.615

Multiplying (A) through 5 and subtracting from (B) we get,

4950 [ Ni²⁺] = 0.615 - 0.455 = 0.160

[Ni²⁺] = 0.160 / 4950 = 3.2 × 10^-5   mol dm^-3

Substituting it in (A) or (B) and solving we get

[Co ²⁺]  = 9.1 × 10^-5 mol dm^-3