ENVE 201 Environmental Engineering Chemistry 1 COLORIMETRY Dr. Aslıhan Kerç

Colorimetry Colorimetric methods are applicable to dilute solutions. For a colorimetric method to be quantitative, it must form a coumpound with definite color characteristics. Color amount must be directly proportional to the concentration. Colored compound must obey Beer’s Law and Lambert’s Law.

Lambert’s Law Relates the absorption of light to the depth or thickness of the colored liquid. Each layer of equal thickness absorns an equal fraction of the light which traverses it. Monochromatic light?

Lambert’s Law Intensity decreases exponentially with length. This law is valid for homogeneous solutions IoIo I Cell length l

Lambert’s Law l: length of absorbing layer T: transmittance of solution A: Absorbance or optical density k’ : constant for particular solution IoIo I Cell length l

Beer’s Law Intensity of a ray of monochromatic light decreases expponentially as the concentration of the absorbing medium increases. k’’ : constant for particular solution C: concentration

Beer’s Law If light is absorbed exponentially with concentration, the colored material is said to confirm Beer’s Law. Prepare a series of samples in desired concentration range, measure light transmission. If it forms a straight line on a semi-log paper  obeys Beer’s law

Lambert – Beer Law Two absorbtion laws are combined:

Spectrophotometer

Colorimeter - Spectrophotometer