Materials Included In Kit

Additional Materials Required

Prelab Preparation

To make the 10% SrCl₂ solution, 500 mL: Weigh out 50 g of strontium chloride in a weigh dish. Fill a 600 mL beaker with about 400 mL of distilled or deionized water. Add the 50 g of strontium chloride and stir to dissolve. Once the solid has completely dissolved, fill the beaker up to the 500 mL line. Label the beaker “10% SrCl₂.” 

Safety Precautions

Sodium hydroxide solutions are corrosive to all body tissue, especially to the eyes. Hydrochloric acid is toxic by ingestion or inhalation and corrosive to all body tissue. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Work in a fume hood. Remind students to wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. The excess hydrochloric acid may be disposed of by neutralizing with a base, such as the leftover sodium hydroxide solution, and then disposed of down the drain with excess water according to Flinn Suggested Disposal Method #24b. The excess sodium hydroxide solution may be disposed of by neutralizing with an acid, such as the leftover hydrochloric acid, and then disposed of down the drain with excess water according to Flinn Suggested Disposal Method #10. The excess strontium chloride solution can be disposed of down the drain with excess water according to Flinn Suggested Disposal Method #26b. The wine solutions may be disposed of down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• To quantitatively analyze the sulfite concentration, have the students filter and weigh the white precipitate that forms upon the addition of Sr²⁺.
• Make this lab open inquiry by depriving students of the leading questions. Simply present them with white wine and the necessary chemicals, inform them of the goal of the lab and challenge them to design their own procedure.

Answers to Prelab Questions

1. Define the following terms.
   1. Chemical equilibrium

      Equilibrium occurs when the forward rate of reaction is equal to the reverse rate of reaction for a reversible reaction.

   2. Le Chatelier’s Principle

      When stress is applied to a system at equilibrium, a shift will occur to compensate for the change and allow the system to return to equilibrium.

2. Given the following reaction, what is the correct K_c expression?
   {14115_PreLab_Reaction_1}
   K_c = [H₃O⁺][OH^–]
3. If H₂ is removed from the following reaction, which way will the reaction shift to return the system to equilibrium?
   {14115_PreLab_Reaction_2}
   The reaction will shift towards the left (reactants).
4. If NH₃ is added from the following reaction, what will happen to the concentration of AgCl?
   {14115_PreLab_Reaction_3}
   The concentration of AgCl will decrease. NH₃ will react with the Ag⁺ ions to form Ag(NH₃)₂⁺, reducing the amount of Ag⁺ in the solution. To return to equilibrium, the system will shift toward the products and increase the concentration of AgCl.

Answers to Questions

1. How might you adjust the experiment to more quantitatively determine the sulfite concentration in a sample of white wine? Add the 10% SrCl₂ solution until precipitation ceases. Filter, dry and weigh the solid SrSO₃. Convert grams of SrSO₃ to grams of SO₃^– and divide by the grams of wine analyzed. Assume the density is equal to approximately 1.0 g/mL owing to the fact that the predominant component of wine is water.

References

Chiaverini, N. & Mortier, T. J. Chem. Educ. 2015, 92, 877–880.

Introduction

Sulfites are commonly found in wines, specifically white wines, to prevent oxidation and microbial spoilage. However, a small percentage of the population is allergic to sulfites, so many methods have been developed to analyze the concentration of sulfites in wine. The sulfites in wine can be described by a system of chemical equilibrium equations. How can you use those equations to qualitatively determine the sulfite concentration?

Concepts

• Precipitation reactions
• Acid–base reactions
• Analytical chemistry
• Qualitative analysis
• Chemical equilibrium
• Le Chatelier’s Principle

Background

If you read the back of most white wine labels you will see the phrase “contains sulfites.” In fact, most white wines have pH values in the range 3–4, where HSO₃^– is the predominant species. HSO₃^– exists in equilibrium with sulfurous acid, H₂SO₃, and sulfite ions, SO₃^–, according to the following equations:

The positions of these equilibria can be shifted according to Le Chatelier’s principle. If we add strong base, such as NaOH, to the equilibrium system described by Equation 2, the equilibrium shifts to the right and results in the generation of increased levels of sulfite ions in white wine. Also, the solution turns a dark brown color.

Given this information, you will be challenged to design a qualitative method, with the given chemicals and materials, for determining the sulfite ion concentration in white wine. Also, you will need to determine how to bring the wine back to its original pH and color.

Experiment Overview

The purpose of this activity is to qualitatively determine the concentration of sulfites in white wine by applying Le Chatelier’s Principle to a system of equilibrium equations. Each group of students will be responsible for developing a hypothesis and designing a procedure to determine the amount of sulfite ions in white wine.

Materials

Prelab Questions

1. Define the following terms.
   1. Chemical equilibrium
   2. Le Chatelier’s Principle
2. Given the following reaction, what is the correct K_c expression?
   {14115_PreLab_Reaction_1}
3. If H₂ is removed from the reaction below, which way will the reaction shift to return the system to equilibrium?
   {14115_PreLab_Reaction_2}
4. If NH₃ is added from the following reaction, what will happen to the concentration of AgCl?
   {14115_PreLab_Reaction_3}

Safety Precautions

Sodium hydroxide solutions are corrosive to all body tissue, especially to the eyes. Hydrochloric acid is toxic by ingestion or inhalation and corrosive to all body tissue. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Work under a fume hood. Please follow all laboratory safety guidelines.

Procedure

Form a working group of 3–4 students to design a qualitative method for determining the sulfite concentration in white wine. Use the following prompts and guiding questions to aid in the development of your qualitative method.

1. Obtain a sample of white wine from your teacher.
2. Recall that the equilibrium reactions that describe the occurrence of sulfite and its associated species in white wine are:
   {14115_Procedure_Equation_1}
   {14115_Procedure_Equation_2}
3. Predict which way the equilibrium will shift for Equation 2 if NaOH is added to the white wine. Explain.
4. Add 5 mL of NaOH to 100 mL of white wine and record any observations. Note: When NaOH is added to the wine, the equilibrium of the system (Equation 2) will shift toward the right and produce more sulfite ions. The color of the wine will darken. Adding Sr²⁺ to the wine will result in a precipitation reaction and produce a white precipitate. To bring the wine back to the original color and pH, HCl can be added because this shifts the equilibrium of the system (Equation 2) to the left.
5. What cations are insoluble when combined with sulfite?
6. Predict what will happen when you add Sr²⁺ to your sample of white wine.
7. Add the Sr²⁺ to the white wine and record any observations.
8. How can you use these observations to more precisely determine the sulfite content in wine?
9. Describe how to restore the white wine back to its original state, where the HSO₃^– species predominates at pH 3–4. Would you add acid or base?
10. Test your hypothesis described in step 9.
11. Consult your instructor for proper disposal of your solutions.

Post-Lab Question

1. How might you adjust the experiment to more quantitatively determine the sulfite concentration in a sample of white wine?