Question

The purpose of this experiment was to determine the equilibrium constant of the reaction between iron (III) chloride (FeCl3) and potassium thiocyanate (KSCN) through the use of spectrophotometry.

Part A: Producing the calibration curve

Part B: FInding the equilibrium concentraions

Why were FeCl3 and HCl added to the blank solutions? Why not just use distilled water for the blanks? Why would you use two different blank solutions for parts A and B?

Answer 1

1) Here we are calibrating it to the buffer. Before any titrant is added, the buffer is absorbing some of the light. So to indicate that the metal is not having any effect on the solution, the amount of light absorbed by the metal itself must be measured. If water were used as the buffer, instead of FeCl₃-HCl, the measurements would also include the light absorbed by the buffer, resulting error in the experiment.

2) When we are plotting the calibration curve ( Absorbance vs [FeSCN²⁺] ), before measurement of transmittance of different concentration solution, the transmittance of a blank solution (before adding any KSCN to the cuvette) of FeCl₃-HCl measured and transmittance is set to 100%- which is actually the calibration part. (Part-A)

Now before calculation of equilibrium constant, we need to plot the I.C.E table for the equilibrium condition, and for that reason, we need a blank set of FeCl₃ and water to determine the initial concentration of FeCl₃.(Part -B)