Spectrophotometric Determination of an Equilibrium Constant





The magnitude of an equilibrium constant, Keq, expresses the equilibrium position for a chemical system. The larger the equilibrium constant the more the equilibrium "lies to the right". The value of Keq is constant for a chemical system at a given temperature. The chemical system studied in this experiment is:

Fe(H2O)63+ + SCN1- <====>Fe(H2O)5SCN2+ + H2O

Since the concentration of water is constant in dilute aqueous solutions the above reaction is usually simplified to:

Fe3+ + SCN1- <======> FeSCN2+

The equilibrium expression of this chemical system is:



Five equilibrium systems will be prepared for the above reaction by mixing known concentrations of Fe3+ and SCN1- . Since the product FeSCN2+ is a deep, blood-red complex ion with an absorption maximum of 447 nM, its concentration can be determined spectrophotometrically. By knowing the initial concentrations of each of the reactants and the measured concentration of the product the equilibrium concentration of Fe3+ and SCN- can be calculated. The equilibrium constant can then be calculated by substituting into the equilibrium expression. The five calculated equilibrium constants will then be averaged to obtain the "best" value.

Before the spectrophotometer can be used to measure the concentration of the FeSCN2+ ion it must be calibrated with a set of standard FeSCN2+ solutions. These solutions will be prepared in part A by mixing a dilute solution of SCN1- with a relatively concentrated solution of Fe3+. The large excess of Fe3+ will cause the equilibrium to shift nearly 100% to the right thus enabling you to assume that virtually all of the SCN1- reacted. The calibration curve will then be constructed by plotting the Absorbance of the solution at 447 nM. Vs. the concentration of the FeSCN2+ ion.


Procedure: Part A

1. Pipet 1, 2, 4, 6, and 8 mL of 0.00200 M KSCN into separate 100 mL volumetric flasks. Add to each flask 25 mL of 0.2 M Fe(NO3)3 and dilute to the 100 mL mark. Measure the absorbance of each solution at 447 nM using 0.2 M Fe(NO3)3 in 0.25 M HNO3 as a blank.

2. Calculate the equilibrium concentration of FeSCN2+ and construct a calibration curve by plotting Absorbance Vs. [FeSCN2+].


Procedure: Part B

1. Prepare the following test solutions and determine the concentration of the FeSCN2+ ion by measuring the Absorbance of each solution at 447 nM and consulting the calibration plot you constructed above. The blank for this part should be a solution consisting of 50% 0.0002 M KSCN and 50% 0.25 M HNO3.

Measure the absorbance of each of these solutions and determine the concentration of the FeSCN2+ complex ion from your calibration chart. Calculate the equilibrium concentrations of both Fe3+ and SCN1- ions and then determine the equilibrium constant for each trial. Average these five equilibrium constants to obtain a best value to report.



1. What affect does a dirty cuvette have on the absorbance reading for a FeSCN2+ solution?


2. How does the error in question 1 affect the reported equilibrium constant?


3. In our calculations the solutions's thickness and the probability of light absorption by the absorbing species (a in the equation A=abc) are not considered. Explain.


4. Absorbance is defined as the negative log of the transmittance (fraction of light transmitted throught the solution). What is the transmittance for a solution that has an absorbance reading of 2?


5. How can the procedure be modified to obtain a more accurate reading for solutions having an absorbance of 2 or greater?


6. Over a period of time the transmittance of the blank solution may drift from its initial calibration. If the percent transmittance of the blank drifts to lower values how does this affect the

a) absorbance readings.

b) the calculated [Fe3+]

c) the calculated FeSCN2+

d) the calculated [SCN-]

e) the calculated Keq


7. Why is 0.0002 M KSCN in 0.25 M HNO3 used as the blank in part B rather than distilled water or 0.25 M HNO3?


8. From the class values listed on the chalkboard for the Keq being studied determine which ones would be considered outliers at the 96% confidence level and rejected. After rejecting these values calculate the class average for the and the standard deviation.

Questions? Comments??