Question

Copper was determined in a river sample using spectrometry and the method of standard additions. For the addition, 100.0 μL of a 1000.0 ppm Cu standard was added to 100.0 mL of river water.

Absorbance of river water: 0.572

Absorbance of river water with standard: 1.122

Calculate the concentration of copper in the river water.

Answer 1

The Beer-Lambert Law, states that:

A=ϵlc                                      -(1)

Where

· ϵ is the molar attenuation coefficient or absorptivity of the attenuating species

· l is the optical path length

· c is the concentration of the attenuating species

· A is absorbance

Given

· Absorbance of river water, Af= 0.572

· Absorbance of river water with standard, Astd = 1.122

· Concentration of standard Copper solution .Cstd=1000ppm

· Volume of standard copper solution added to river water, Vstd=100μL

· Volume of river water taken for test,Vw=100 mL=100*103 μL

Let Concentration of river water be Cw ,

Now, Concentration of solution so formed after mixing standard solution with river water, Cf =

Cf =

  =                 -(2)

Now,from eq 1

Astd=ϵlCstd      -(3)

and

Af=ϵlCf          -(4)

Dividing eq (4) and (3)

Cf =509.8 ppm

Also we know ,from eq (2)

Cf=      

          

Cw+1 =       

Cw+1=510.313

Cw=510.313-1

Cw=509.313ppm