Experiment 15 Objectives

1
Experiment 15 - Objectives

• To determine the available chlorine in commercial
  bleaching agents.
• To prepare a standardized sodium thiosulfate
  solution.

2
Experiment 15 Introduction

• Natural fibers such as cotton, wool, and paper
  products tend to retain some of their natural
  color which is not white.
• Natural and synthetic dyes exist as a result of
  the presence of molecules with multiple bonds.
• The electrons of the multiple bonds absorb
  visible radiation, reflecting the colors that are
  not absorbed.

3
Experiment 15 Introduction

• The electrons of the double/triple bonds are
  removed by oxidizing agents.
• Because the oxidized form of the molecules
  causing the colors/stains no longer absorb
  visible radiation, the fabric or paper appears
  whiter.

4
Experiment 15 Introduction

• All the halogens and solutions of halogens are
  excellent oxidizing agents and are relatively
  safe to handle.
• Chlorine is an oxidant that is also an excellent
  bactericide used in swimming pools, drinking
  water, and sewage treatment plants.

5
Experiment 15 Introduction

• Most cheap commercial bleaching agents contain
  the hypochlorite ion, ClO-.
• The ClO- ion removes light-absorbing electrons
  from the multiple bonds of molecules. Therefore,
  the absorption of visible radiation does not
  occur all visible light is reflected/transmitted,
  giving the object a white appearance.
• The ClO- ion is generally in the form of a Na or
  Ca salt in the bleaching agent.

6
Experiment 15 Introduction

• Industry expresses the strength of bleaching
  agents as the mass of molecular chlorine, Cl2,
  per unit mass of bleach (solution or powder).
  This mass ratio, expressed as percent, is called
  the percent available chlorine in the bleach.

7
Experiment 15 Introduction

• Liquid laundry bleach is generally a 5.25 NaClO
  solution. Laundry bleach is manufactured by the
  electrolysis of a cold, concentrated NaCl
  solution. The chlorine gas and the OH- that are
  generated in the electrolysis cell combine to
  form the ClO- and, in the presence of Na, a NaClO
  solution.
• Cl2(aq) 2OH-(aq) ? ClO-(aq) Cl- H2O(l)

8
Experiment 15 Introduction

• The percent available chlorine in a 5.25 NaClO
  solution is calculated as
• gCl2 x 100 5.25g NaClO x mol NaClO
• gsoln 100 g soln 74.44g
• x 1mol Cl2 x 70.90g Cl2 x 100 5.00 Cl2
• 1mol NaClO mol Cl2

9
Experiment 15 Introduction

• Solid household bleaches often contain sodium
  perborate, NaBO2H2O2H2O, in which the bleaching
  agent is hydrogen peroxide, a compound that is
  also found in hair bleaches.

10
Experiment 15 Procedure

• Starch acts as an indicator by forming a strong
  bond to molecular iodine whereas molecular
  iodine has a yellow-brown color, the I2starch
  complex is a deep blue. Just before the
  brown-yellow color disappears during the
  titration, you will add starch and continue
  titrating with sodium thiosulfate until the blue
  color disappears. Repeat twice and average the
  Na2S2O3.

11
Experiment 15 - Procedure

• With the standardized Na2S2O3, you will titrate
  two different brands of bleach, two trials each.
  
• Calculate the percent available chlorine in each
  bleach.

12
Experiment 15 Procedure Notes Questions

• Procedure Notes
• Omit Parts A D
• Questions
• 3, 5