B EER ’ S L AW P0P0

U SES OF B EER ’ S L AW - Relates concentration to the optical measurement of ‘absorbance’ - combined with spectrophotometry can be used to distinguish and compare different molecules in solution

T HE RELATIONSHIP BETWEEN ABSORBANCE AND TRANSMITTANCE IS ILLUSTRATED IN THE FOLLOWING DIAGRAM : Transmittance Absorbance 0% 100% 2.0 0

T HE AMOUNT OF RADIATION ABSORBED MAY BE MEASURED IN A NUMBER OF WAYS : Transmittance, T = P / P 0 % Transmittance, %T = 100 T Absorbance, A = log 10 P 0 / P A = log 10 1 / T A = log / %T A = 2 - log 10 %T

Q UESTION : W HY DO WE PREFER TO EXPRESS THE B EER -L AMBERT LAW USING ABSORBANCE AS A MEASURE OF THE ABSORPTION RATHER THAN %T ? Compare the two equations that we use: A=abc %T = 100 P/P 0 = e -abc

C OMPARE HOW EACH EQUATION G RAPHS

B EER ’ S L AW E QUATION A=abc Where, a= molar absorptivity (is a measure of the amount of light absorbed per unit concentration; this value is a constant for a given solution) b = cell path length (usually 1cm) (Cuvette) c = concentration (M)

B EER ’ S L AW A = abc Where ab = constant, then A = constant times c So when we plot this y=mx+b

B EER ’ S L AW In order to use A = abc, we need to define values for a and b. b = path length – “blank” – distilled water in a cuvette a = colorimeter(select a preset wavelength) spectrophotometer (choose wavelength where maximum absorption of photons occurs)