When preparing and handling solutions of potassium permanganate, it is advisable to wear rubber gloves to avoid contact of the solution with your skin; it will cause unsightly stains on your hands for several days. If this oxidant comes in contact with the skin. wash the affected area thoroughly with soap and warm water.

Residues of manganese dioxide on glassware can be difficult to remove. Rinsing the glassware with 10% sodium bisulfite solution will sometimes work. In more difficult cases, 6 M HCI should be placed in the glassware and heated in a ventilation hood.

1. In your 1 L beaker, add 31.5 gm. (0.2 mol) potassium permanganate in 250 mL water,

2. Place a glass thermometer into the beaker and measure the temperature of the mixture. ( It

3. Place your hot plate into your hood and heat the mixture until the temperature of

the mixture reaches 45^oC. IMMEDIATELY, remove the beaker from the hot plate and place

4. If the temperature of the mixture drops, return it to the hot plate. If the temperature

of the mixture continues to increase, allow it to reach 55^oC, then ice your beaker until

5. When the temperature no longer rises above 50^oC upon removal of the beaker from the

ice bath, allow the mixture to stand for an additional 5 - 10 minutes; its temperature

6. Heat the mixture to boiling CAREFULLY, WITH STIRRING on your hot plate (set at 3 - 4)

for 5 - 10 minutes, to complete the oxidation. The mixture has been known to bump out of

7. While your mixture is heating, attach your 500 mL filter flask assembly to the vacuum

source, wet your filter paper with water and turn on the vacuum/water aspirator.

8. Once the mixture in your beaker has begun to boil, test the solution for unchanged

permanganate by placing a glass stirring rod into the mixture and removing it. At least

one drop of the mixture will hang onto the rod. Place that drop on a piece of filter

permanganate has been reacted. If a purple ring appears around the brown spot then

you have to continue boiling the mixture and test it until no purple color appears on the

filter paper. If the spot on the filter paper is completely purple when you first test the

9. Destroy any excess permanganate by adding small amounts of solid sodium bisulfite to

the mixture until the spot test is negative. Do not add a large excess of bisulfite.

10. Filter the mixture by vacuum and rinse the reaction flask and filter cake with two 10-mL

11. Rinse out your 1 L beaker and pour the contents of your filter flask back into it.

Concentrate the filtrate to about 65 mL by heating on a hot plate, setting 3 - 4. Use a

stiorring rod rather than boiling chips in your beaker to prevent bumping while you are

12. If the concentrate is colored, add a spatula of decolorizing carbon and re-heat the

mixture to boiling for about 2 minutes, then vacuum filter, using your small filter flask

13. Pour the clear, colorless filtrate into a 100 mL beaker and allow it to cool to room

14. Acidify the filtrate with 50 mL 6M hydrochloric acid and test the solution with

blue litmus paper. If the paper turns red, the solution is acid and no further additions

of acid are necessary. If the blue litmus remains blue after being dipped into your

15. Cool this mixture in an ice-water bath until the solution is less than 10^oC, and isolate the

16. Set up your 500 mL filter flask and large buchner funnel for vacuum filtration, using

17. Recrystallize your crude product by placing it in a clean 100 mL beaker and adding 20

18. Bring your solvent product mixture to a boil on your hot plate at your bench. If your

solid does not dissolve completely once the mixture starts to boil, add an additional

5 mL of room temperature solvent and again bring the mixture to a boil. Continue this

procedure until a final volume of 30 mL is attained. If at this point you still have

20. Remove the boiling mixture from the hot plate and allow it to cool to body temperature

on your desk top.

21. At this point, place the beaker containing your product into an ice bath and allow your

product to crystallize out of solution.

22. Isolate your product by vacuum filtration, using Whatman #1 filter paper but NO filter

aid. Wash the solid in the funnel with 5 mL of the 50% ethanol-water solvent.

23. Discard your filtrate down the sink.

Day 2

Determine the yield, melting point and molecular weight of the pure Adipic acid obtained, and record them in the Results section of your lab report, complete with appropriate calculations.

Molecular Weight Determination

1. Write a balanced equation for the titration

2. Weigh approximately 0.2 gm of your dried product into a 2500 mL Erlenmeyer Flask.

Do your weighing to three decimal places and keep the weight between 0.990 gm and

0.110 gm. Dissolve your sample in 25 mL 95% Ethanol. Titrate

the solution with 0.100M NaOH to a faint pink end point, using 3 drops of

phenolphthalein as your indicator. Calculate your equivalent weight using the following

equation.

3. Calculate your molecular weight

2 x grams of acid

molecular weight = -----------------------------------------------------------------

( volume of base consumed in liters ) x M

4. Record this calculation in the Results section of your lab report.

5. Be prepared to derive the mathematical equation from the balanced chemical equation.

6. Why are the grams of acid multiplied by 2 ? See Your balanced equation and be able

to derive the above from the balanced equation and the titration data. (Hint: It doesn't

come from the "2" fairy).

Return the remainder of your product, if any, to the jar on the East Bench, next to hood #11.

Theory Behind the Experiment

A. Equations: The net equation is the sum of two equations. The first

one is a redox equation. The cyclohexanone is oxidized to

1,6-hexanedioic acid (adipic acid) while the permanganate ion is

reduced to MnO2. (Which is the oxidizing agent & which is the

reducing agent?)

C₆H₁₀O + 2 KMnO₄ -----> 2 MnO₂(s) + C₆H₈O₄K₂ + H₂O + heat.

The second equation describes the reaction which converts

dipotassium adipate to adipic acid. Adipic acid is insoluble in water

but the dipotassium salt is almost completely soluble in water;

therefore the salt is converted to the acid.

C₆H₈O₄K₂ + 2 HCl ----> C₆H₁₀O₄ + 2 KCl.

If you add up these two equations, you will obtain the net equation at

the beginning of the experiment.

B. Procedure:

Step 1 OH is needed to start the reaction. It acts as a catalyst.

Steps 2-3 The reaction is exothermic but the activation energy is not reached until 45^oC. Above 60^oC the reaction tends to run out of control and gas forms so rapidly, that the reaction mixture is violently splattered all over the room. That is why you keep the reaction temperature between 45^o + 55^oC.

Steps 8-9 If permanganate is not destroyed, a pink solid crystallizes out with the hexanedioic acid and contaminates it. Adding an excess of bisulfite can cause some of the manganese dioxide to reduce to manganous ion (Mn⁺²) which is soluble in water. When the hexanedioic acid crystallizes out, some of it may crystallize out as the manganous salt, rather than the free acid; thus your melting points and melting ranges will indicate very impure substances.

Step 10 The brown MnO₂ has to be removed from the solution by filtration. The MnO₂

particles are very small and can clog the pores of the filter paper.

Step 11 The solution may be unsaturated at this stage, so you want to reduce the volume to where the amount exceeds the solubility limit at 0^oC. It was experimentally found that 30 ml would be a convenient volume from which to crystallize the adipic acid.

Step 12 If you need charcoal to decolorize the solution, you do Steps 12, 13, and 14.

Step 14 You are converting the dipotassium salt of adipic acid (dipotassium adipate) which is soluble in water, into adipic acid or hexanedioic acid which is insoluble in water.

Step 15 You are reducing the solubility of adipic acid.

Step 17 21 ml of 95% (v/v) ethanol and 9.0 ml of water gives 1:1 or a 50% (v/v) solution of ethanol and water. 21 ml of 95% ethanol gives 19.95 ml ethanol and 1.05 ml of water. The 1.05 ml of water + 19 ml of additional water gives 20.05 ml, which is close enough to 50:50.

Step 18 You are dissolving the crystals, thus the molecules of adipic acid become separated from each other enough so that the impurities will float away in the solvent.

Step 20 The solvent cools, thus allowing the molecules of adipic acid to come close together while the impurities remain far apart. The molecules begin to attach to each other into crystal formation. The molecules attach to each other to form large enough crystals to be caught by the filter paper but not large enough to trap impurities.