Cu2+ + 4NH3 → Cu(NH3)42+ (deep blue) ? % White KNO3 ? % Blue CuSO4.5H2O Cu2+ + 4NH3 → Cu(NH3)42+ (deep blue) EX8-1 (of 12)

A more saturated color means a higher concentration of the colored component The “saturation” can be determined by measuring the amount of light absorbed by the solution, called its ABSORBANCE EX8-2 (of 12)

A light bulb emits white light A prism separates the colors of light SPECTROMETER – A device that measures the amount of light absorbed by a sample A light bulb emits white light A prism separates the colors of light Light passes through the sample Light passes through a slit to form a narrow beam Rotating the prism allows just one color to pass through another slit A detector measures the final amount of light EX8-3 (of 12)

Concentration of CuSO4.5H2O (%) If you choose a specific wavelength of light, and measure the absorbances of CuSO4.5H2O solutions of known concentrations, and plot absorbance vs. concentration , the relation is linear Absorbance C: 0.000% 0.025% 0.050% 0.075% 0.100% A: 0.000 0.241 0.478 0.722 0.961 Concentration of CuSO4.5H2O (%) EX8-4 (of 12)

If an unknown solution has an absorbance of 0 If an unknown solution has an absorbance of 0.351 using a wavelength of light of 608.0 nm, find its concentration of CuSO4.5H2O A@608.0 = mx + b m (slope): 1.9 0.351 = (1.9 %-1)C + 0.001 0.350 = (1.9 %-1)C 0.350 = C _________ 1.9 %-1 b (Y-intercept): 0.001 A@608 = mC + b = 0.18 % m = Δy ____ Δx = Δ Abs. __________ Δ Conc. = no units ___________ % = %-1 EX8-5 (of 12)

USING THE SPECTROMETER Place a colorless solution (called the BLANK) in the spectrometer and set it to zero absorbance EX8-6 (of 12)

USING THE SPECTROMETER Place a colored standard solution in the spectrometer ABSORBANCE SPECTRUM – A graph of the absorbance of a solution at different wavelengths If a solution appears blue, it is primarily absorbing its complimentary color, orange EX8-7 (of 12)

USING THE SPECTROMETER LAMBDA MAX (λmax) – The wavelength of maximum absorbance Peak of the absorbance graph When measuring the absorbance of solutions, it is most accurate to measure the absorbance at λmax You will record the λmax in your Data Table λmax EX8-8 (of 12)

USING THE SPECTROMETER Determine the absorbance of each standard solution at λmax m (slope): 0.9612 b (Y-intercept): 0.001 A@620.0 = mx + b Correlation: 0.994 C: 0.000% 0.025% 0.050% 0.075% 0.100% A: 0.000 0.241 0.478 0.722 0.961 A@620 = mx + b A@620 = (0.9612 %-1)C + 0.001 This is called a CALIBRATION LINE EX8-9 (of 12)

USING THE SPECTROMETER Determine the percentage of CuSO4.5H2O in the sample if its absorbance is 0.246 m (slope): 0.9612 b (Y-intercept): 0.001 A@620.0 = mx + b Correlation: 0.994 y = mx + b A@620.0 = (0.9612 %-1)C + 0.001 0.246 = (0.9612 %-1)C + 0.001 0.245 = (0.9612 %-1)C 0.245 = C _____________ 0.9612 %-1 0.255 % = C = % CuSO4.5H2O 99.745 % = % KNO3 EX8-10 (of 12)

ɛ = extinction coefficient 1852 AUGUST BEER Proposed a mathematical explanation for the linear relationship between concentration and absorbance A = mC + b BEER’S LAW : A = ɛ l C + 0 A = absorbance ɛ = extinction coefficient (a constant for a given solute at a given λ) l = width of the cuvet holding the sample (for our cuvets it is 1.00 cm) C = concentration (in our lab it’s in “% CuSO4.5H2O”) l = 1.00 cm EX8-11 (of 12)

Calculate the extinction coefficient for absorbance at a wavelength of 620 nm and using a 1.00 cm cuvet given the calibration line: A@620 = (0.9612 %-1)C – 0.001 A@620 = mC + b A@620 = ɛ l C + b slope = ɛ l m = ɛ ___ l = 0.9612 %-1 _____________ 1.00 cm = 0.9612 %-1cm-1 EX8-12 (of 12)