4 minutes reading time (701 words)

    The "Breathalyser" Reaction

    Description

    A U-tube is packed with orange crystals of potassium dichromate moistened with dilute sulfuric acid. Air saturated with ethanol vapour is blown or sucked over these crystals which turn from brown to green as the alcohol reduces the chromium (VI) to chromium(III).

    Apparatus

    • One U-tube (length of arm about 10cm). Each arm should have a one-hole bung fitted with a short length of glass tube.
    • Two 250 mL conical flasks each with a two-holed bung fitted with one long and one short length of glass tube.
    • One rubber bung with a single hole fitted with a short length of glass tube.
    • One plastic sandwich bag.
    • Rubber tubing to make connections.
    • Access to a filter pump.
    • Cable tie or length of thread.

    Chemicals

    • Ethanol, 100mL
    • Potassium dichromate (potassium dichromate (VI), K2Cr2O7) crystals, 30g
    • Sulfuric acid, 2M, 3mL
    • Bottles of ethanol (acetaldehyde) and of an approximately 2M solution of ethanoic acid (acetic acid) (optional).

    Method
    Before the demonstration

    Wearing plastic gloves, weigh out into a beaker sufficient potassium dichromate crystals to half fill the U-tube (about 30g). Add 2M sulfuric acid to the crystals, in the ratio 1mL of acid to 10g of dichromate and mix thoroughly. This should produce moistened crystals of potassium dichromate. Put these into the U-Tube, tapping the tube gently to pack them down.

    Place ethanol in the conical flask to such a depth that the longer of the glass tubes is below the surface of the ethanol and the shorter one is not.
    Attach a plastic sandwich bag to a one-hole rubber bung by gathering the neck of the bag around the bung and attaching it with a cable tie or by winding thread around it.

    The demonstration

    Blow air by mouth through the ethanol in the flask so that the air in the flask is saturated with ethanol vapour. Make sure the plastic bag is deflated. Connect the conical flask, U-tube and plastic bag as shown above. Blow into the ethanol-containing flask so that the breath bubbles through the ethanol, passes over the acidified potassium dichromate and into the plastic bag. The dichromate crystals in the arm of the U-tube nearest the ethanol will turn brown. This is caused by a mixture of unreacted orange crystals and green chromium (III), a product of the reaction. If desired, continue the reaction by removing the plastic bag and connecting the apparatus to a filter pump to draw more ethanol vapour over the crystals. The brown colour will spread and eventually turn green although this will take several minutes. Remove the stoppers from the U- tube and pass it around the class for the students to smell the products of the reaction. Let them compare the smell with the smells of ethanol and ethanoic acid solution - possible products of the reaction.

    Visual tips

    A white background will make the colour changes easier to see and a bottle of unreacted potassium dichromate is useful for comparison.

    Teaching tips

    The teacher could also demonstrate the reaction of ethanol with acidified potassium dichromate solution in a test tube in the usual way if the students are not familiar with it.
    This could be a suitable opportunity to discuss the dangers of drinking and driving. Balancing the equations can be set as homework for post-16 students.

    Theory

    The reactions are:

    Cr2O72-(s) + 3C2H5OH(g) + 8H+(aq) → 3CH3CHO(g) + 2Cr3+(aq) + 7H2O(l)

    producing ethanol, followed by:

    Cr2O72-(s) + 3CH3CHO(g) + 8H+(aq) → 3CH3CO2H(l) + 2Cr3+(aq) + 4H2O(l)

    producing ethanoic acid. Or, overall:

    2Cr2O72-(s) + 3C2H5OH(g) + 16H+(aq) → 3CH3CO2H(l) + 4Cr3+(aq) + 11H2O(l)

    Further details

    Chemical breathalysers are no longer in use; roadside screening is done using a tester based on a fuel cell while testing at the police station for use in evidence is done by measuring infrared absorption. Further confirmation, if requested by the suspect, may be done by GLC. Details can be found in C.B. Faust, Modern chemical techniques London: RSC, 1992.

    Safety

    Wear eye protection. Some teachers may wish to include an extra conical flask in the apparatus to act as a trap to guard against the possibility of a suck back while blowing into the ethanol.
    It is the responsibility of teachers doing this demonstration to carry out an appropriate risk assessment.

    Allotropes of Sulfur
    Movement of Ions During Electrolysis
     

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    Thursday, 18 October 2018

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