Double beam filter photometer:
It consists of a source of light (tungsten filament lamp), lens to make the light beam parallel, filter of wavelength selection, cuvette with sample holder for keeping the solution to be analyzed, mirror for incident the part of light beam onto reference photocell, two photocells (one as reference and other as measuring), potentiometers for zero and span adjustments and a recording device (galvanometer).
In double beam photometer, the light rays from the source are made parallel and passed through a filter. It is divided into two parts; one part passes through the sample solution cuvette and falls on the measuring photocell and the other part passes directly onto the reference photocell. The galvanometer receives opposing currents from the two photocells.
Steps in experiment:
1) With the lamp off, the galvanometer is adjusted to zero mechanically.
2) The potentiometer R1 is adjusted for T=1 or A=0.
3) Then with lamp on blank solution is placed in the light path of measuring photocell and potentiometer R2 is adjusted until the galvanometer reads zero.
4) The solution to be analyzed is then placed in the light path of measuring photocell and R1 is adjusted until the galvanometer reads zero, keeping R2 unchanged. The absorbance or transmittance can be read directly on the scale of potentiometer R1. Since the potentiometer R1 is calibrated in terms of transmittance and absorbance.
In double beam filter photometer errors due to fluctuations of the lamp intensity are minimized also the scale of potentiometer R1 can be made much larger in size than the scale of meter in single beam filter photometer.
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