pHYSICAL cHEMISTRY - v

BSC6CH03

Experiment 2

Aim of the Experiment

Determination of copper in unknown solution using colorimeter based on BeerLambert’s law.

Materials required

Chemicals: Standard solution of Copper sulphate pentahydrate, Distilled water, ammonia solution

Apparatus: Colorimeter, burette , funnel

Principle

Principle of spectrophotometry is that every substance absorbs or transmits certain wavelengths of radiant energy but not other wavelengths. For example, chlorophyll always absorbs red and violet light, while it transmits yellow.green, and blue wavelengths.The transmitted and reflected wavelengths appear green. The light energy absorbed or transmitted must match exactly the energy required to cause an electronic transition in the substance under consideration. Only certain wavelength photons satisfy this energy condition. Thus, the absorption or transmission of specific wavelengths is characteristic for a substance, and a spectral analysis serves as a "fingerprint" of the Compound.

Light is a form of electromagnetic radiation. When it falls on a substance, three things can happen

1.The light can be reflected by the substance

2.It can be absorbed by the substance

3.Certain wavelengths can be absorbed and the reminder transmitted or reflected

Since reflection of light is of minimal interest in spectrophotometry. ignore it and turn to the absorbance and transmittance of light. The colour of sample solution is due to the selective absorption of certain wavelengths of visible light and transmittance of the remaining wavelengths If a sample absorbs all wavelengths in the visible region of the spectrum, it will appear black. If it absorbs none of them, it will appear white or

colourless. A beam of white light is passed, substance that absorbs light of particular

colour and transmits the light of complementary colour.

Solution colour Wavelength(nm)of absorbed light Observed filter colour (Complementary Colour)

Yellowish green 400-435 Violet

Yellow 435-480 Blue

Orange 480-490 Greenish blue

Red 490-500 Bluish green

Purple 500-560 Green

Violet 560-580 Yellowish Green

Blue 580-610 Yellow

Bluish green 610-750 Red

Beer's law is applicable only to dilute solutions. It states that when a monochromatic

light is passed through a homogeneous absorbing medium, the decrease in the intensity of light with the thickness of the medium is directly proportional to the intensity of incident light as well as concentration of the solution.

When a monochromatic light of intensity Io is incident on a transparent medium, a part Ia is absorbed, a part Ir is reflected and the remaining part It is transmitted.

Io=Ia+Ir+It

log It/Io=T called the transmittance, log I/T= Io/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 Io/ It=εct

Where ε is the molar extinction coefficient,it is a constant for a given substance at a given wavelength. If the path length is kept constant, then A α c. hence a plot of absorbance against concentration gives a straight line as shown in figure (calibration curve). The color is measured using a spectrophotometer for a fixed wavelength(λmax= 620).A series of standard solutions containing cupric ions is treated with ammonia to get deep blue cuprammonium complex and is diluted to a definite volume.

Cu2+ + 4(NH3) ---> Cu(NH3) 4 2+ (deep blue color)

The absorbance of each of these solutions is measured at 620 nm since the complex shows maximum absorbance at this wavelength. The concentration is plotted against absorbance to get a calibration curve. An unknown volume of the test solution is treated with strong ammonia and diluted to the same volume as above. The absorbance of this solution at 620 nm is measured and its concentration is determined from the calibration curve.

Procedure

Place the given copper sulfate solution in burette and transfer 5.0,10.0,15.0,20.0 and 25.0ml of the solution into separate, labeled 50ml standard flasks. Add 5ml of ammonia solution to each of them and make up to the mark using distilled water. Stopper the flasks and mix well. To the test solution taken in 50ml standard flak, add 5 ml ammonia solution and make up-to the mark. Prepare blank solution in a 50ml flask by adding 5ml ammonia and make up to the mark using distilled water and mix well.Adjust the Absorbance to zero by keeping blank solution at620nm. After 10 minutes, measure the absorbance of the solutions against blank at 620nm using spectrophotometer. Tabulate the readings as shown. Draw a calibration curve by plotting volume of copper sulfate solution against absorbance. Calculate the amount of copper present in test solution.