Colorimetric estimation of copper

COLORIMETRIC DETERMINATION OF COPPER
Principle:
When a monochromatic light of intensity I0 is incident on a transparent medium, a part, Ia of it is absorbed, a part, Ir is reflected and the remaining part, It is transmitted.
I0 = Ia + Ir + It
For a glass-air interface Ir is negligible and therefore,
I0 = Ia + It
It / I0 = T called the transmittance, log 1/T = log I0 / It is called the absorbance or optical density. The relation between absorbance, A, concentration, c (expressed in mol/dm3) and path length, t (expressed in cm) is given by Beer-Lambert's law,
A = log I0 / It = ?ct
where ? is the molar extinction coefficient, t is the path length and is a constant for a given substance at a given wavelength. If t, the path length is kept constant, then, A ? C. Hence a plot of absorbance against concentration gives a straight line.

A series of standard solutions of copper sulphate penta hydrate is treated with ammonia to get blue cuprammonium complex, and is diluted to a definite volume. The absorbance of each of these solutions is measured at 590 nm since the complex shows maximum absorbance at this wavelength. The absorbance values are plotted against concentration to get a calibration curve.

The given test solution is treated with strong ammonia and diluted to the same volume as above. The absorbance of this solution at 590 nm is measured and its concentration is determined from the calibration curve.

Procedure: (Note: use distilled water only)
Transfer the given copper sulphate solution (stock solution) to a burette and draw out 2, 4, 6, 8 and 10 cm3 of the solution into 50 cm3 volumetric flasks. Now collect the test solution from the teacher / examiner, add 5 cm3 of ammonia solution to each of them, shake well and then dilute up to the mark with ion exchange water. Stopper the flasks and mix the solutions well. To the test solution given in a 50 cm3 measuring flask, also add 5 cm3 of ammonia solution, shake well and then dilute up to the mark with distilled water and mix well. Prepare a blank solution by diluting only 5 cm3 of ammonia solution in a 50 cm3 measuring flask up to the mark with distilled water and mixing well. After 10 minutes, measure the absorbance of the solutions against blank at 590 nm using a photoelectric colorimeter. Tabulate the readings as shown. Draw a calibration curve by plotting absorbance against volume of copper sulphate solution. Using the calibration curve, find out the volume of copper sulphate solution given i.e., the volume of test solution and calculate the amount of copper in the given solution.