Determination of Copper Concentration by Redox Titration: An Analytical Chemistry Approach

Abstract

In this experiment, the amount of Cu2+ is determined by titrating thiosulfate solution to a copper sulfate solution where Cu2+ is reduced to Cu+ by a redox reaction with Potassium iodide. Starch and KSCN act as an indicator for the color change during the titration. Sulfuric acid is added to give the solution a slightly acidic solution to obtain the best result.

Introduction

Titration is the addition of a substance with a known concentration to a known volume of another solution until equilibrium is achieved.

Change of color is the indication that the titration has achieved equilibrium. In this lab, the objective is to determine the amount of copper in the sample. Copper (II) in Copper Sulfate is reduced to Copper (I) when iodine in Potassium Iodide is introduced. This result in the formation of I- and Cu+. Thiosulfate is used as a titrant to allow the solution to reach equilibrium.

Starch is introduced to form a dark blue solution with iodine.

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The purpose of this step is to allow us to observe the color change easier when the titration reach equilibrium. A color change from dark blue to whitish pink is observed when equilibrium is achieved. By measuring the volume of thiosulfate solution titrated and the adding of a known amount of Potassium Iodide, we are able to estimate the amount of Cu2+ in the sample solution.

Methods

Objectives: to determine the amount of copper (II) in a sample 0.2 g of potassium iodide is added to an unknown concentration of copper sulfate solution that have been diluted with deionized water.

5ml of sulfuric acid is also added to the solution prior to the addition of potassium iodide. 5 ml of thiosulfate solution is titrated to the mixed solution. Starch is added to give a dark blue solution. 1ml of thiosulfate solution and 0.2 g of potassium thiocyanate is added. thiosulfate solution is titrated until a change of color is observed. The solution turns whitish pink.

Results

Chemical Equation

2Cu2+ + 2I- 2Cu+ + I2

I2 + 2S2O32- 2I- + S4O62-

Experimental results

Average amount of KI: 0.215 g

Average amount of KSCN: 0.213 g

Average vol of Na2S2O3: 21.5 ml

Comments: 0.215 g of potassium iodide is added to 25 ml of diluted copper sulfate solution. This result in a redox reaction which reduced copper (II) to copper (I) while iodine ions is oxidized to iodine. Starch and 0.213 g of KSCN is added to give the solution a dark blue color.

The titration reached equilibrium when 21.5 ml of thiosulfate solution is added. A color change from dark blue to whitish pink is observer. From the volume titrated, the mol of S2O32- is calculated to be 0.00215 mol. By the chemical equation and the stoichiometric calculation, we can determine the mol of copper to be 0.00215 mol. Thus the amount of copper in the unknown sample is 0.137 g.

Discussion

The first titration is significantly different then the 2nd and 3rd titration. This could be due to not mixing the diluted copper sulphate solution sufficiently. The density of sulphate solution is slightly higher than of water thus most of the copper sulphate solution might settle down to the bottom of the flask which explains why there is higher volume of thiosulfate solution titrated in the 2nd and 3rd titration. This could be avoided if the solution is mixed well. During the experiment, KSCN that was collected was observed to have some form of moisture. The moisture from the KSCN could have an effect to the reaction.

Chemical Reactions and Stoichiometry

The redox reaction underpinning this analysis is represented by the equations:

2Cu2++2I−→2Cu++I2

I2+2S2O32−→2I−+S4O62−

These reactions elucidate the reduction of Cu2+ to Cu+ alongside the concurrent oxidation of iodide ions to iodine, which is subsequently titrated with thiosulfate. The stoichiometric relationship between thiosulfate and iodine, and by extension Cu2+, forms the basis for calculating the copper content in the analyzed sample.

Results and Stoichiometric Calculations

The experimental outcomes, summarized in the table below, reveal the average quantities of reagents employed and the resulting volume of thiosulfate solution titrated.

From the volume of thiosulfate used, the moles of S2O32- and consequently I2 and Cu2+ are determined, enabling the calculation of Cu2+ content in the sample:

Moles of S2O32−=0.00215 mol

Moles of I2=0.001075 mol

Moles of Cu2+=0.00215 mol

Amount of Cu2+=0.137 g

Conclusions

This experiment underscores the precision of redox titration in determining the concentration of metallic ions in a solution. Despite potential experimental variances, such as incomplete mixing or the presence of moisture in reagents, the method's efficacy in estimating Cu2+ concentration is evident. The experiment not only solidifies the understanding of redox reactions and titration principles but also demonstrates the practical application of these concepts in real-world analytical scenarios. Through careful execution and interpretation of results, the experiment successfully achieves its objective, highlighting the role of analytical chemistry in quantitative substance analysis.