Visible spectroscopy |Principle, Instrumentation & Applications
Visible spectroscopy is also called as colorimetry (color+metry= color+ measurement).
Colorimetry is the study of absorption of visible radiation by a colored substance within the wavelength range of 400nm to 800nm.
The range of radiation from 400 to 800 nm is called as visible radiation.
So this is a type of spectroscopy which relies on visible light for measurement.
The measurement of absorbance of this wavelength by a colored compound is called as visible spectroscopy.
A colored substance absorbs visible light of different wavelengths in different manner.
But the wavelength at which its absorbance is maximum is termed as λmax.
This λmax is specific and unique for every compound or substance.
So any unknown substance can be taken and its absorption curve checked. Based on the λmax one can easily identify it.
Thus colorimetry serves to identify the compound and serve the purpose of qualitative analysis.
Similarly by drawing a calibration curve of a standard compound, the quantity of test compound can be measured.
The absorption increases with rise in concentration but the wavelength of absorption λmax remains same.
This absorption is explained by two laws viz.
Beer’s law: The intensity of beam of monochromatic (single colored) light decreases exponentially with increase in the concentration of the absorbing substance arithmetically.
Lamberts law sates that the rate of decrease in intensity of monchromatic light with thickness of the medium is directly proportional to the intensity of incident light.
Further details can be seen in principle of spectroscopy.
Instrumentation of visible spectroscopy:
There are two types of instruments like simple colorimeters and sophisticated spectrophotometers. Their sensitivity and range differ but the basic components for both remain same.
1. Source of light: Any lamp which can produce visible light in the range of 400 to 800nm is enough for the purpose. But it should have adequate intensity and be free from fluctuations.
The common sources of light are tungsten lamp and carbon arc lamps. Of them tungsten lamp is widely used while carbon arc is meant for high intensity requirements.
2. Filters and monochromators: The visible white light has many wavelengths and colors. So filters are used to filter unwanted wavelengths. The filters are again as two types as
a) Absorption filters:
b) interference filters:
These filters are less expensive and find place in most of the colorimeters. The problem with them is less accuracy of band pass. For example we want a wavelength of 530nm and when we use absorption filters we get around ± 30nm difference i.e. either 500nm or 560nm range. While interference filters have band pass of ± 10nm only.
Monochromators are more sophisticated and accurate. They are of two types as
a) Prisms. (1. Refractive types 2. Reflective type)
b) gratings. (1. Diffraction 2. Transmission type)
They are expensive but very accurate. They have a band pass of ± 0.1nm and have a high range like 400 to 1000nm wavelength.
Sample cells: These are the one which hold the sample under test such that light falls and passes through it. The sample cell should be transparent and also tough. They are mostly either cylindrical or rectangular in shape. They are made of glass or polystyrene. The sample cells are mostly of 0.5ml to 5ml in volume.
Detectors: Once the monochromatic lights passes through the sample in the sample cell, it has to be measured. The unabsorbed light is allowed to fall onto a detector for measurement of light intensity. The detectors are of different types like
1. Photo voltaic cells
2. photo tubes
3. Photo multiplier tubes.
Of the above three, photomultiplier tubes are highly sensitive and accurate. So they are used in spectrophotometers while others are used in colorimeters.
The instruments are of two types as single beam and double beam types. The single beam colorimeter and single beam spectrophotometers have a single cell for sample alone. While the double beam photometers have two sample cells. One cell is meant for test sample and other is meant for reference solutions.
Applications of visible spectroscopy:
Spectroscopy has many applications in the fields like diagnosis, medicine manufacture, chemistry, research etc.
1. For quality control of samples: In industry many products are manufactured and there are chances of contamination. Colorimetry can be used to assess the quality and purity of the finished product. The presence of impurities gives additional peaks other than λmax of the substance.
2. Quantitative analysis: Th percentage concentration of substance in a given sample can be determined by absorbance characteristics
3. Structural elucidation can be done by using colorimetry. The absorbance spectrum of the substance under test can be compared with that of known ones. This helps to define most probable structure of the given test sample. but due to availability of modern methods like mass spectrometry, I.R and NMR methods this method is not used.
4. Determination of dissociation constants.
5. Determination of elements, functional groups etc.