chem 407

The experiment involves mixing Fe⁺³ with varying concentrations of SCN- or thiocyanate ion. A strong red complex forms and this complex is measured spectrophotometrically.

• All runs employ the same starting concentration for Fe⁺³ ion.
• The concentration of SCN- is allowed to vary.
• Inert salts are added to maintain a constant ionic strength

A plot of Absorbance (Y-axis) vs. [SCN-] (X-axis) should start at zero absorbance but should level off in relatively concentrated SCN- solution.

• One may assume 100% of the Fe(III) is converted to complex in the most concentrated SCN- solutions.
• It is usually necessary to extrapolate to determine this reading
• From the (maximum) absorbance and concentration of complex you can compute the molar extinction coefficient for the complex.
• Other absorbance readings can be used to compute the actual concentration of complex.
  • Subtraction from the initial Fe(III) concentration gives the remaining free Fe(III) concentration.
  • You can now compute Kequil = [complex] / [Fe(II)] [SCN-]
• You get a different value of K_eqfor each solution,but they should be approximately the same value
  • In practice the solutions with the high and low [SCN-] are subject to more error. (why?) The most reliable results will come from the intermediate solution.
• There's a simplified way to compute K_eq
  • K = {Abs} / [{asymptote} -{abs}] / [SCN-]