ENVE 201 Environmental Engineering Chemistry 1 COLORIMETRY Dr

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 I Io Cell length l • Intensity decreases exponentially with length. • This law is valid for homogeneous solutions

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

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

Spectrophotometer

Colorimeter - Spectrophotometer