Experiment 22: Colorimetric determination of an equilibrium constant

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Experiment 22: Colorimetric determination of an equilibrium constant
PURPOSE To determine the value of the equilibrium constant for the equilibrium system involving Fe3+(aq), SCN–(aq) and FeSCN2+(aq) using colorimetric analysis.

THEORY In aqueous solution, Fe3+ ions react with SCN– ions to form the blood red coloured FeSCN2+ ion: Fe3+(aq) (pale yellow) + SCN–(aq) (colourless)  FeSCN2+(aq) (red) The colour of the solution is directly proportional to the concentration of FeSCN2+ ions present. The concentration of these ions can be determined by measuring the absorbance of the solution and comparing it with the absorbance of a solution of known concentration. If the initial concentrations of Fe3+ and SCN– are known, a value for the equilibrium constant, K, for the reaction can be calculated. Colorimeters measure the amount of light that is transmitted or absorbed by a solution. A description of how they work can be found in your text book

SAFETY PROCEDURES Follow all instructions for using the equipment in this activity. 2. Wear safety glasses and a laboratory coat for this experiment. 3. Potassium thiocyanate is irritating to the skin and eyes. Avoid contact. 4. The iron(III) nitrate solution used in this experiment contains nitric acid. The solution is irritating to skin and body tissues.

Part 1. Preparation of Equilibrium Solutions
Label six ml flasks 1-6 Pipet ml of .2M Fe(NO3)3 into each flask Then pipet 1.00, 2.00, 3.00, 4.00, and 5.00 ml of .002M (NaSCN)- into flasks 2 – 6 respectively Add .1M nitric acid to each flask to make a total of ml per flask and stopper each flask

Part 1. Preparation of Equilibrium Solutions
Calculate the final Fe(NO3)3 concentration and report them in part A Calculate the final Fe(NCS)+2 concentration in each flask NaSCN ml soln 1 2 3 4 5 6 Initial [SCN-] M 4.0x 10-5M Equil FeNCS+2M Percent T 100 78.5 Absorbacnce 0.105

Part 1. Preparation of Equilibrium Solutions
Obtain 2 cuvettes, rinse one cuvette with soln 1, and discard in waste jar Fill the rinsed cuvette with soln 1, insert cuvette into spectrophotometer and adjust 10 0 absorbance and 100 transmittance. Rinse the second cuvette with soln 2 and discard in waste jar and measure the absorbance and transmittance of soln 2 at 447nm Repeat procedures for soln 3 -6

calibration curve Prepare calibration curve by plotting absorbance vs concentration

Part B: Determination of the calibration curve
Label six clean dry test tubes 1- 6 Pipet ml of .002M Fe(NO3)3 into each test tube Then pipet 1.00, 2.00, 3.00, 4.00, and 5.00 ml of .002M (NaSCN)- into flasks 2 – 6 respectively Add .1M nitric acid to each flask to make a total of ml per flask and stopper each flask

Part B: Determination of the calibration curve
Measure and record the absorbance and transmittance of these solns at 447nm From your calibration curve determine the equilibrium concentration of Fe(NCS)+2

Part B: Determination of the calibration curve
NaSCN ml soln 1 2 3 4 5 6 Volume of (FeNO3)3 ml 5.00 ml 5.00 m Volume of NaSCN ml 1.00 2.00 Initial Fe +3 M 1.0 x 10-3M Initial SCN -1M 2.0 x 10-4M Absorbance .164

Part B: Determination of the calibration curve
NaSCN ml soln 1 2 3 4 5 6 Equil [FeNCS+2] M 3.28 x 10-5 Equil [Fe+3] M 1.0 x 10-3M 9.67 x 10-4 Equil [SCN-] M 1.6 x 10-4M Keq - 211