Evaporating dish, glass wool, disposable pipets, 2 mm diameter glass rod approximately 10-15 cm long, hotplate, small spatula, small funnel and short length of rubber tubing (~2 cm). If this is to be viewed in a large classroom, a video camera is almost essential.
Concentrated sulfuric acid, saturated potassium dichromate solution, 20% sodium silicate solution, ethanol
- Add 5 mL of 20% sodium silicate solution to an evaporating dish.These amounts will provide enough treated silica gel to make 6 trials.
- Add saturated potassium dichromate solution dropwise until you get an intense yellow color.
- Add concentrated sulfuric acid dropwise until the solution gels and turns orange.
- Gently warm the evaporating dish to dry out the silica gel.
- Stir and scrape the gel until it is fairly dry and about the consistency of coarse sand.
- Add concentrated sulfuric acid dropwise to return a little moisture to the silica gel. Note: Too dry and the reaction will not occur, too wet and you cannot fill the pipet.
- Push a loose plug of glass wool into the constricted end of a disposable pipet.
- Attach a small funnel to the large end of the pipet with a short length of rubber tubing.
- Add the silica gel to the pipet, tap lightly to loosely pack the gel. Note: You want a loose packing without large channels or voids, but not so tight that the gas won't easily pass through the pipet.
- Place a loose glass wool plug on top of the gel to hold it in place.
- Seal the ends of the pipet until ready for use. The pipet may be stored indefinitely so long as the pipet is kept sealed so that no moisture is transferred and no contamination prematurely reduces the dichromate.
- Clamp the pipet horizontally to a ringstand and remove the seals from the ends of the pipet.
- Take a clean one hole rubber stopper size 6-8 and place a balloon lip over the large end of the stopper.
- Add approximately 2-5 drops of ethanol to the balloon through the stopper hole. The more drops that are added, the faster and more complete the reaction will be.
- Inflate the balloon, pinch the neck of the balloon and carefully insert the large end of the pipet into the hole of the stopper just far enough so that the stopper will remain connected to the pipet.
- Release the balloon neck, so that the air and ethanol vapor can pass through the pipet.
- If you don't want to perform these steps in class, you can perform steps 2-4 before class and clamp the neck of the balloon shut. Release the clamp when you are ready to perform the demonstration.
Sulfuric acid: This substance is a severe eye irritant and could cause permanent damage to your eyes and blindness. This substance is corrosive. Contact with the skin could cause permanent injury (including scarring) to the affected area. Ingestion may cause severe irritation or ulceration of the digestive tract which may result in nausea, vomiting, diarrhea, and in severe cases, collapse, shock and death. This substance is extremely irritating if inhaled. Inhalation of the aerosol or mist of this material may cause serious adverse effects including acute lung damage and death.
Potassium dichromate: Causes eye burns. May cause chemical conjunctivitis and corneal damage. May be fatal if absorbed through the skin. Causes skin burns. May cause skin sensitization, an allergic reaction, which becomes evident upon re-exposure to this material. Chrome ulcers, penetrating lesions of the skin, occur chiefly on the hand and forearm where there has been a break in the epidermis. May be fatal if swallowed. May cause severe and permanent damage to the digestive tract. Causes gastrointestinal tract burns. May cause asthmatic attacks due to allergic sensitization of the respiratory tract. May cause ulceration and perforation of the nasal septum if inhaled in excessive quantities. Causes chemical burns to the respiratory tract. Aspiration may lead to pulmonary edema.
Sodium silicate: May result in irritation to eyes, skin, lungs and gastrointestinal tract.
Ethanol: Vapors may be irritating to the eyes, nose and throat. Inhalation may cause headache, nausea, vomiting , dizziness, drowsiness, irritation to the respiratory tract and loss od consciousness. The vapors are highly flammable, keep away from open flames.
This demonstration illustrates the oxidation of ethanol by dichromate (orange) under acidic conditions to produce acetaldehyde, see first reaction below. The dichromate then oxidizes the acetaldehyde under acidic conditions to acetic acid, see second reaction below. As the dichromate is oxidizing the various reactants, it is in turn being reduced to Cr 3+ which is green. The animation shows a pipet filled with a bright orange mixture of silica gel impregnated with potassium dichromate and sulfuric acid. As the reaction proceeds, the color changes to green caused by the presence of Cr 3+ in the form of the sparingly soluble chromium (III) sulfate salt. This particular animation utilized 2 drops of ethanol added to the balloon and the reaction takes place over a period of 5 minutes. Adding more drops would have caused the changes to occur in a much shorter period of time. If a person is close enough to this demonstration, the odors of acetaldehyde and acetic acid may be detected exiting the pipet. Also the tube gets slightly warm. This is probably a combination of the exothermicity of the chemical reactions and the produced water diluting the concentrated sulfuric acid held in the silica gel.
This reaction will work on any primary alcohol. The reactions would be the same with appropriate substitutions of the initial alcohol and the resulting aldehyde and carboxylic acid. This reaction will convert a secondary alcohol to a ketone and the reaction will not proceed as the dichromate is incabable of oxidizing the resulting ketone. No reaction will occur for a tertiary alcohol. These reactions or lack of reaction are used in organic chemistry to identify the particular type of alcohol as a preliminary step in its identification.
This type of alcohol detection was once commonly used by law enforecement agencies. It has been replaced with other detection methods utilizing infrared spectroscopy, chromatography and amperometric detection. There are several reasons that this method has been phased out over the years; one being the toxicity of chromium compounds and another being the better accuracy and precision of the newer methods.
3CH3CH2OH (aq) + K2Cr2O7 (aq) + 4H2SO4 (aq) 3CH3CHO(aq) + Cr2(SO4)3 (s) + K2SO4 (aq) + 7H2O (l)
3CH3CHO(aq) + K2Cr2O7 (aq) + 4H2SO4 (aq) 3CH3CO2H (aq) + Cr2(SO4)3 (s) + K2SO4 (aq) + 4H2O (l)
The initial idea for this demonstration was found in an internet article 1, several modifications were made to achieve more reproducible results. The chemistry mentioned here was taken from an organic laboratory manual 2, and was expanded upon.