Determination of Copper by Iodimetric Titration

 

Introduction

Copper in its elemental form is a metal commonly used for electrical conductors and coins as well as a variety of alloys including bronze and brass. As with many chemical species, copper is essential to health and is considered crucial to avoid slipped discs, hemorrhoids, emphysema, aneurysms, and poor immunity. Shellfish and liver are considered the best food sources. Copper can also be toxic, primarily by interfering with zinc, although the amounts needed for acute toxicity are large. In this experiment copper is determined by a method called iodimetric titration. This is an indirect iodine redox titration method and has widespread use in analytical chemistry. Initially, the copper ion oxidizes I- to I3-.

2Cu2+ + 5I- à 2CuI + I3- Equation 1

I3- is a pale yellow color but forms a deep blue complex in the presence of starch. At the endpoint, thiosufate reduces all of the I3- and the blue color will disappear.

2S2O32- + I3- à S4O62- + 3I- Equation 2

Iodate can be used to produce triiodide for the standardization of thiosulfate according to the equation:

IO3- + 8I- + 6H+ à 3I3- + 3H2O Equation 3

 

Procedure

  1. Pipet 5.00mL of 0.01M standard potassium iodate solution into a conical flask and add approximately 20mL of deionized water(treat each sample individually)
  2. Add 2g of potassium iodide with swirling
  3. Add 2ml of dilute hydrochloric acid and immediately titrate with sodium thiosulfate solution until the solution becomes pale yellow
  4. Add 5 ml of starch indicator and titrate, with swirling, to the disappearance of the blue color
  5. Repeat to achieve 3 close results
  6. Add 10mL of dilute nitric acid to 0.1g of the unknown (copper oxide mixture)
  7. In a fume hood, heat until dissolved
  8. Add approximately 25mL of deionized water
  9. Add 10mL of 5%(w/v) urea and boil briefly
  10. Cool and add concentrated ammonia dropwise with mixing (in a fume hood) until an intense blue solution is obtained
  11. Add 3M sulfuric acid dropwise until the color just disappears and then add 2.0mL of 85% phosphoric acid
  12. Treating all samples individually, add 4.0g of potassium iodide and titrate immediately with standardized sodium thiosulfate until a pale yellow color is obtained
  13. Add 5mL of starch indicator and titrate until the blue color becomes faint
  14. Add 2.0g of KSCN and swirl vigorously for 30s
  15. Complete the titration to the disappearance of the blue color
  16. Repeat for 3 more samples

Calculations

Calculate the mean concentration of copper in your unknown sample. Quote the 95% confidence interval and relative standard deviation.

Questions

  1. Write the balance chemical equation for the reaction that takes place in part 10 of the procedure.
  2. Why is it important to conduct the titrations independently?
  3. Gold (III) can also oxidize iodide to form triiodide and the AuI species. Write a balance chemical equation for this reaction.
  4. Many books show I2 as the reactive species in this method rather than I3-. Why is this not possible?
  5. 0.23g of copper metal was dissolved in acid and used to standardize a solution of sodium thiosulfate. If 33.02mL of the solution was required to reach the endpoint, what is the concentration of the solution?

Reference:

Skoog, West and Holler, Fundamentals of Analytical Chemistry, 6th Edition, Saunders College Publishing