Iodine Clock Reaction

Hazards: 

WARNING: NEVER LET SOLUTION B STAND IN OPEN BEAKER FOR OVER AN HOUR. TEST EXISTING STOCK SOLUTIONS. IF DEMONSTRATION DOES NOT WORK, DISCARD SOLUTION B.

Chemicals and Solutions: 

Iodine clock solution A

Iodine clock solution B

Solution preparation:

Solution A:  4.3 gms KIO3 per liter of solution (0.02M)

Solution B:

Make a paste of 4 grams of soluble starch in a small amount of distilled water. Slowly add 800ml of boiling distilled water. Boil for a few minutes until clear. COOL!

Add 0.2 grams Na2S2O5 (sodium metabisulfite). Add 5ml 1M H2SO4 Dilute to 1 liter.

Materials: 

Electronic stop clock

Ice bath in large refrigerator dish

2 - 100 mL cylinders

2 - 400 ml beakers

Magnetic stirrer and stirring bars for both beakers

Procedure: 

Only one cylinder and beaker are needed if this is done in conjunction with part I.

Into cylinders put 100 ml of solution A.

Into beakers put 100 ml of solution B.

Put one cylinder and beaker into ice bath at least 15 minutes before expected use.

Follow same procedure as above for different concentrations.

Electronic stop clock (optional)

3 - 100 mL cylinders

3 - 400 ml beakers

Iodine clock solution A

Iodine clock solution B

Magnetic stirrer and stirring bars for each beaker

Label cylinders and beakers in pairs; "1", "2", "3".

Into all cylinders put 100 ml of solution B.      

Into beaker #1, put 100 ml of solution A.

Into beaker #2, put 50 ml of sol. A + 50 ml dist. water.

Into beaker #3, put 25 ml of sol. A + 75 ml dist. water.

Make duplicates of items 3 and 5 and test before class  time. Make a note of time required to complete reaction.

Start stirring in beaker #1.

Add contents of cylinder.

Stop timer immediately when reaction occurs. Note time lapse.

Reset timer and follow same procedure for #2 and #3.

Reaction at different temperature.

 

Discussion: 

A simplified explanation of the reaction is as follows:

I- reacts with IO3- to form I2.

5HI (aq) + HIO3 (aq) ==> 3I2 + 3H2O

I2 is immediately consumed by reaction with HSO3-.

I2 + HSO3- + H2O ==> 2I- + SO42- (aq) + 3H+ (aq)

When HSO3- has been consumed, I2 accumulates.  I2 + starch forms a blue colored starch-I2 complex.

When you dilute solution A (0.02 M KIO3) in half, it takes twice as long to form the blue starch - I2 complex. When solution A is diluted to 1/4 the concentration, it takes four times as long to form the blue starch-I2 complex.  The reaction is much slower at colder temperatures.

Disposal:

The solutions can be poured down the drain. (The bisulfite and iodate are consumed and the iodine is complexed with starch so there is no oxidizer hazard).  Demo generates 800 mL of 0.2% starch iodine complex aq.

References:

Summerlin and Ealy, Chemical Demonstrations, pp. 75-76.

 

CLOCK II

Summary:  Iodine Oscillating clock

Hazards: 

30% hydrogen peroxide is very reactive.

Chemicals and Solutions: 

Solution #1 (Make this solution fresh):

Dilute 36 ml of 30% H2O2 to 100 ml

Solution #2  (To make 1 liter of solution):

Dissolve 43 g of KIO3 in approx. 800 mL of water.

Add 4.3 mL of conc. sulfuric acid.

Dilute the solution to 1 liter.

Solution #3 (To make 1 liter of solution):

Dissolve 15.6 g malonic acid and 3 g of MnSO4 in 970 mL of water.

Stir in 30 mL of a 1% starch solution

Materials: 

magnetic stirrer and bar

Three 100 mL graduated cylinders

400 mL beaker

Procedure: 

Measure out 100 mL of each solution into graduated cylinders.

With stirring, quickly add each solution to the 400 mL beaker.

(The solution will oscillate between colorless, amber and dark blue.)

Hint: 

Discussion: 

The oscillations are due to the shifting concentrations of I2 and I-. The amber color is due to the presence of I2.  When  I- is present, it reacts with I2 and starch to produce a dark blue complex.  This color fades as iodine is consumed.

A very simplified explanation of this reaction is:

2 HIO3 + 5H2O2 ==> I2 + 5O2 + 6H2O  GOLD

I2 + CH2(COOH)2 ==> ICH2(COOH)2 + H+ + I-

I2 + I- + starch ==> starch-iodine complex  DARK BLUE

I2 + 5H2O2 ==> 2HIO3 + 4H2O COLORLESS

Disposal:

The hydrogen peroxide and iodate are consumed and the iodine is complexed with starch so there is no oxidizer hazard.  Therefore the solution can be rinsed down the drain.  Demo generates  300 mL of 2% starch iodine complex aq.

 

 

CLOCK III

 

Summary:

Hazards: 

30% hydrogen peroxide is very reactive.  Use gloves!

Chemicals and Solutions: 

Recipe below makes 500 mL of each solution.  All solutions must be made fresh. 

Solution #1:  (Hydrogen peroxide solution)

Dilute 180 ml of 30% H2O2 to 500 ml

Solution #2:  (Potassium iodate solution)

Dissolve 21.5 g of KIO3 in approx. 400 mL of water.

Add 2.2 mL of conc. sulfuric acid.

Dilute to 500mL.

Solution #3:  (Starch solution)

Boil gently 1.5g starch in 75-100mL.  Solution should take on a clear, gel like quality.

Cool solution.

Dissolve 7.8g malonic acid and 1.5 g of MnSO4 in 350 mL of water.

Stir in the starch solution, adjust volume to 500mL.

Materials: 

Magnetic stirrer and bar (optional)

Three 250 mL graduated cylinders

1000 mL beaker

Three 100 mL graduated cylinders (for test run)

400mL beaker (for test run)

Procedure: 

For the test run (always test this demo in advance of class!)

Measure out 100 mL of each solution into graduated cylinders.

Add one solution to the 400mL beaker.

At the same time, add the remaining two solutions to the beaker, note color changes.

(The solution will oscillate between colorless, amber and dark blue.)

For the classroom demo, measure 250mL of each solution.  Place one solution in the 1000mL beaker, leave the remaining solutions for demonstrator to add simultaneously.

Clock will oscillate for about 5 minutes typically.

Hint: 

When no stir bar is used, regions of the solution will change first,

Discussion: 

The oscillations are due to the shifting concentrations of I2 and I-. The amber color is due to the presence of I2.  When  I- is present, it reacts with I2 and starch to produce a dark blue complex.  This color fades as iodine is consumed.

A very simplified explanation of this reaction is:

2HIO3 + 5H2O2 --->I2 + 5O2 + 6H2O

                             (GOLD)

I2 + CH2(COOH)2---> ICH2(COOH)2 + H+ + I-

I2 + I- + starch ---> starch-iodine complex

                               (DARK BLUE)

I2 + 5H2O2 --->2HIO3 + 4H2O

                        (COLORLESS)

Disposal:

The hydrogen peroxide and iodate are consumed and the iodine is complexed with starch so there is no oxidizer hazard.  Therefore the solution can be rinsed down the drain.  Demo generates  300 mL of 2% starch iodine complex aq.

To schedule a demonstration, please send an e-mail to the demonstration lab.

 

Contact:

Eric Camp

Lecture Demonstration Technician

Bagley Hall 171

(206) 543-1606

ericcamp@uw.edu

 

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