Make your own free website on Tripod.com

BLEACH ANALYSIS

 

 

NAME:____________________________________________________ PERIOD:_______

 

Prelab

 

1.         A student followed the procedure of this experiment to determine the percent NaOCl in a commercial bleaching solution that was found in the basement of an abandoned house.  The student diluted 50.00 mL of bleach to 250.0 mL in a volumetric flask and titrated a 20.00mL sample of the diluted bleaching solution.  The titration required 35.46 mL of 0.1052 M Na2S2O3 solution. A faded price label on the gallon bottle read $0.79.  The density of the bleaching solution was 1.10 g/mL. Note: 1.00gal = 3.78L

 

a)         Calculate the number of moles of S2O32- ion required for the titration.

 

 

 

 

b)         Calculate the number of moles of I2 produced in the titration mixture.

 

 

 

 

c)         Calculate the number of moles of OCl- ion present in the diluted bleaching solution

            titrated.

 

 

 

 

d)                  Calculate the mass of NaOCl present in the diluted bleaching solution titrated (the 20.00mL sample).

 

 

 

 

e)                  Calculate the mass of NaOCl present in the original 50.00mL sample of undiluted bleach.

 

 

 

 

f)          Calculate the mass of the original 50.00mL sample of undiluted bleach.

 

 

 

 

g)         Determine the mass percent NaOCl in original 50.00mL sample of undiluted bleach.

 

 

 

 

 

h)         Calculate the mass of undiluted bleaching solution required to give 100.0 g of NaOCl.

 

 

 

 

 

 

 

i)          Calculate the volume of undiluted bleaching solution required to give 100.0 g of

            NaOCl.

 

 

 

 

 

 

 

j)          Determine the cost of the amount of undiluted bleaching solution required to give

           100.0 g of NaOCl.

 

 

 

 


BLEACH ANALYSIS

 

Objectives:

 

·        To carry out a redox reaction between acidified iodide ions and hypochlorite ions.

·        To titrate the triiodide ion (I3-) formed by the iodine combining with the excess iodide ion to determine the concentration of commercial bleach solution.

·        To use a starch complex indicator to determine the end point in the titration of bleach with acidified iodide potassium iodide solution.

 

Theory:

 

Many commercial products are effective because they contain oxidizing agents. Some products, which contain oxidizing agents, are bleaches, hair coloring agents, scouring powders, and toilet bowl cleaners. The most common oxidizing agent in bleach is sodium hypochlorite, NaOCl. Commercial bleaches are produced by bubbling chlorine gas into a sodium hydroxide solution. Some of the chlorine is oxidized to the hypochlorite ion, OCl- and some is reduced to the chloride ion, Cl-.  The solution remains strongly basic. The chemical equation for the process is:

 

 Cl2 (g)  +  2 OH- (aq)  ®   ClO- (aq)   +  Cl- (aq)   + H2O (l)

 

The amount of hypochlorite ion present in a solution can be determined by an oxidation-reduction titration. One of the best methods is the iodine-thiosulfate titration procedure. Iodide ion, I-, is easily oxidized by many oxidizing agents.  In acid solution, hypochlorite ions oxidize iodide ions to form iodine, I2.  The iodine that forms is then titrated with a standard (known molarity) solution of sodium thiosulfate.

 

The analysis takes place in a series of steps:

 

(1)        Acidified iodide ion is added to the hypochlorite ion solution and the iodide is oxidized

            to iodine.

2 H+ (aq)   +   OCl- (aq)    +    2 I- (aq)     ®     Cl- (aq)    +   I2 (aq)     +    H2O (l)        (reaction 1)

 

(2)        Iodine is only slightly soluble in water.  It dissolves very well in an aqueous solution of

            iodide ion, in which it forms a complex ion called the triiodide ion, I3-. Triiodide ion is a

            combination of a neutral I2 molecule and an I- ion.  The triiodide ion is yellow in dilute

            solution, and dark red-brown  when concentrated.

I2 (aq)   +     I- (aq)      ®     I3- (aq)    (reaction 2)

 


(3)        The triiodide ion is titrated with a standard solution of thiosulfate ions, which reduces the

            iodine back to iodide ions.

I3- (aq)    +   2 S2O3-2 (aq)     ®     3 I- (aq)    +    S4O6-2 (aq)     (reaction 3)

 

During the last reaction the red-brown color of the solution of the triiodide ion fades to yellow and then to the colorless solution of the iodide ion. It is possible to use the disappearance of the color of the I3- ion as a method of determining the end point in the titration, but this is not a very sensitive procedure. Addition of starch to a solution that contains iodine or triiodide ion forms a reversible deep blue complex. The disappearance of this blue colored complex is a much more sensitive method of determining the end point. If the starch is added to a solution, which contains a great deal of iodine, the starch-iodine complex, which forms, may not be reversible.  Therefore, the starch is not added until shortly before the end point is reached. The moles of thiosulfate used in the titration and the stoichiometry in reactions 1-3 are used to determine the amount of hypochlorite initially present in the sample.

 

Procedure:

 

1. Rinse a 50.00mL buret with a small amount of the standardized thiosulfate solution. Fill the buret with the thiosulfate solution. Be sure the tip of the buret is filled.

 

2. Record the initial buret reading and the molarity of the thiosulfate solution.

 

3. Use a 5.00mL pipet and bulb to transfer a 5.00mL portion of commercial bleach into a 100.00 mL volumetric flask.  Dilute to the line with distilled water, stopper and mix well.

 

4. Mass 2.00 g of solid KI in to plastic weighing boat. The KI is in excess so a centigram balance can be used and the mass of KI does not need to be known accurately.

 

5. Pipet 25.00mL of the dilute bleach into a 125mL Erlenmeyer flask.

 

6. Add the KI and approximately 25mL of distilled water, swirl to dissolve the KI.

 

7. Add approximately 2.0mL of 3 M HCl to the solution. It will be dark yellow to red-brown from the presence of the triiodide complex ion.

 

8. Titrate the iodine with the standardized sodium thiosulfate solution until the iodine coloris light yellow. Add 1-2 droppers of starch solution and observe the blue starch-iodine complex. 

 

9. Slowly continue titration dropwise with good mixing until the blue color disappears.

 

10. Record the final buret reading and calculate the volume of thiosulfate solution used.

 

11. Rinse the flask with hot water being careful not to lose the magnetic stir bar,

 

12. Repeat the titration twice more.

BLEACH ANALYSIS

 

NAME:__________________________________________ COURSE:___________

 

LAB PARTNER:___________________________________ PERIOD:___________

 

DATA TABLE

 

Trial number

1

2

3

Initial buret reading (mL)

 

 

 

Final buret reading (mL)

 

 

 

Volume of thiosulfate used (mL)

 

 

 

Molarity of thiosulfate (M)

 

 

 

Moles S2O3-2

 

 

 

Moles I2

 

 

 

Moles OCl-1 in the 25.00mL of dilute bleach

 

 

 

Moles OCl-1 in the 5.00mL of undiluted bleach

 

 

 

Density of the undiluted bleach

1.08 g/mL

 

 

Mass of 5.00mL of undiluted bleach (g)

 

 

 

Molar mass of NaOCl

 

 

 

Mass NaOCl in the 5.00mL of undiluted bleach

 

 

 

Mass % NaOCl in the undiluted bleach

 

 

 

Relative % Error*

 

 

 

*Use 5.25% as the actual value or the percent listed on the label of the bleach bottle.

Show a sample calculation for one trial below:

 

 

 

 

 

 

 

 

Discussion: Comment on precision, errors and difficulties, and whether or not the commercial bleach meets, exceeds, or falls short of the advertised NaOCl content (5.25%).


BLEACH ANALYSIS

Setup Sheet

 

Equipment:

 

10        Burets labeled: Na2S2O3

10        Buret reading cards

10        25.0 mL graduated cylinders

10        250 mL Erlenmeyer flasks

10        10 mL graduated cylinders

10        Magnetic stirrers

10        Medium magnetic stir bars

3          Stir bar retrievers

4          Wide mouth screw top bottles labeled: KI

3          250ml Plastic reagent bottles labeled Clorox

3          Dropper bottles of starch indicator solution

10        Wash bottles with distilled water

10        Boxes Kimwipes

3          1 Liter plastic reagent bottles labeled Na2S2O3

10        Medium plastic weigh boats

10        Scoopulas labeled KI

10        100mL volumetric flasks with stoppers

10        Pipet racks

10        Rubber pipet bulbs

10        5.00mL volumetric pipets labeled undiluted

10        25.00mL volumetric pipets labeled undiluted

 

10 liters of 0.100 M Na2S2O3 (24.8 g /l.or 248g /10 l) Standardize the solution.

3 liters of Chlorox (100 mL..1 liter x 3)

 

Solution Preparation:

2% starch solution:   prepare 200 mL of starch solution by first dissolving 4 grams of soluble starch into ~10 mL of distilled water. Pour into 80 mL of boiling distilled water while stirring. Dilute to 200mL.

 

3 M HCl:  500 mL of HCl solution by adding 125 mL of 12 M HCl to 200 mL of distilled water and diluting to a final volume of 500 mL.

 

0.1000M Na2S2O3:   Dissolve 24.8 g of Na2S2O3 · 5 H2O in 150 mL of distilled water in a one liter volumetric flask and dilute with distilled water to a final volume. Mix thoroughly.

 

Alternate.Starch prep.(Shakasheri).

1% starch:1 liter of solution (heat 500 mL of distilled water to boiling in a 1-liter beaker. Prepare a slurry in a 50mL beaker of 10 g starch in 20mL of distilled water. Pour the slurry into the boiling water and boil for 5 minutes. Put 450 mL of cold DI water in a 2 liter beaker and pour the hot starch solution into it. Dilute to 1 liter after the solution reaches room temperature. Rinse the storage bottle with boiling DI water before storing the solution to inhibit bacterial growth.


BLEACH ANALYSIS

 

Instructor’s Notes

 

1. Put a white surface under the titration flask to see the loss of the iodine color and titrate to a very pale yellow, If the solution does not turn blue when the starch is added, the sample has been over-titrated (the endpoint overrun) and the sample needs to be repeated. Adding the starch too early results in too much iodine adhering to the starch and an endpoint that is not sharp.

 

2. Chlorine bleach is very basic and should be handled with care. Safety glasses must be worn. The students should be reminded to wear old clothes during this lab. The bleach will affect many dyed fabrics.

 

3. Answers to the prelab questions.

a. Calculate the number of moles of S2O32- ion required for the titration.

Mole S2O3-2 = (3.546x10-2L)(0.1052M) = 3.730x10-3mole S2O3-2

 

b. Calculate the number of moles of I2 produced in the titration mixture.

3.730x10-3mole S2O3-2 (1 mol I2/ 2 mol S2O3-2) = 1.865x10-3 mol I2

 

c. Calculate the number of moles of OCl- ion present in the diluted bleaching solution titrated.

1.865x10-3 mol I2 (1 mol OCl-1/ 1 mol I2) = 1.865x10-3 mol OCl-1

 

d. Calculate the mass of NaOCl present in the diluted bleaching solution titrated (the 20.00mL sample).

1.865x10-3 mol NaOCl (74.45g/mol) = 0.1388g NaOCl

 

e. Calculate the mass of NaOCl present in the original 50.00mL sample of undiluted bleach.

0.1388g NaOCl (250mL total sample/ 20mL sample titrated) = 1.736g

 

f. Calculate the mass of the original 50.00mL sample of undiluted bleach.

(50.00mL)(1.10g.mL) = 55.0g bleach

 

g. Determine the mass percent NaOCl in original 50.00mL sample of undiluted bleach.

(1.736g NaOCl)/(55.0g bleach) x 100 = 3.16%

 

h. Calculate the mass of undiluted bleaching solution required to give 100.0 g of NaOCl.

100.0g NaOCl = (0.0316)(X g bleach) X g bleach = 3.16x103g

 

i. Calculate the volume of undiluted bleaching solution required to give 100.0 g of NaOCl.

V=mass/volume            (3.16x103g)/ (1.10g/mL) = 2.88x103mL           2.88L

 

j. Determine the cost of the amount of undiluted bleaching solution required to give 100.0 g of NaOCl.

2.88L(1.00gal/L)(0.79/gal) = 0.60        $0.60

 

 

 

4. Sample Data

 

Trial number

1

2

3

Initial buret reading (mL)

0.00

 

 

Final buret reading (mL)

19.30

 

 

Volume of thiosulfate used (mL)

19.30

 

 

Molarity of thiosulfate (M)

0.1000

 

 

Moles S2O3-2

1.930x10-3

 

 

Moles I2

9.650x10-4

 

 

Moles OCl-1 in the 25.00mL of dilute bleach

9.650x10-4

 

 

Moles OCl-1 in the 5.00mL of undiluted bleach

3.860x10-3

 

 

Density of the undiluted bleach

1.08 g/mL

 

 

Mass of 5.00mL of undiluted bleach (g)

5.40

 

 

Molar mass of NaOCl (g/mol)

74.45

 

 

Mass NaOCl in the 5.00mL of undiluted bleach (g)

0.2874

 

 

Mass % NaOCl in the undiluted bleach (%)

5.32

 

 

Relative % Error* (%)

1