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Instructions are Available Here
 

Measuring Oxygen Consumption:

Introduction

One of the best features of the Electro-Chem-Technic cells is that their design allows the consumption of oxygen to be measured. The rate at which oxygen is consumed can be compared with the electric current. The results can be used to:-

  • Notice that the oxygen consuption is proportional to current, and use this to reinforce an understanding of how fuel cells work.
  • Relate the volume of gas consumed to the charge, and using known formulas such as PV=nRT (the universal gas law), verify the chemistry of the cell, i.e. 4 electrons released per oxygen molecule.
  • Assume the chemistry of the cell given in the manual is correct, and use the same calculations of charge and gas volume to find the space taken up by one oxygen molecule at NTP.
The apparatus is as shown below:-

The assembly of bung, adaptor, flexible tube and graduated tube can by purchased. It is called the "oxygen use measurement kit". However, you could easily make it yourself. The graduated tube is made from a plastic 10ml pipette, obtainable from all laboratory equipment suppliers.

The basic procedure for an experiment is as follows:-

  • 1. Start up a mini fuel cell in the normal way.
  • 2. Connect it to a variable resistor load, in the range 0 - 20 ohms, in series with an ammeter.
  • 3. Set up the oxygen use measurement kit as in the diagram above. The graduated tube should be held steady with a clamp.
  • 4. Adjust the variable resistor till a suitable current is flowing, e.g. 40 mA. Anything in the range 20 - 100 mA is suitable.
  • 5. Observe that the water level very slowly rises up the graduated tube. When the water level inside the tube has risen to being just above the level outside start a timer.
  • 6. Time how long it takes for the level to change by 1.0 ml (1 cm3). While this is happening, keep adjusting the load resistor so that the current is constant. (With a current of 50mA, this time will be about 5 minutes.)
  • 7. Disconnect the load. Remove the graduated tube from the water so that it empties, and then replace.
  • 8. Repeat instructions 4 - 7 at different currents as necessary and as time allows.
The results can then be processed in a way suitable for the level of students performing the experiment. For some it will be enough to draw a graph of current vs. rate of oxygen use. More advanced students should be able to use values of R, Avagadro's number, the charge on one electron, the Universal Gas Constant and the temperature and pressure at the time of the experiment, together with PV = nRT to find the number of electrons released by each oxygen molecule. (See the "likely results" section below.)
Precautions

There are some precautions that need to be taken to get good results with this experiment:-

  • The liquid level inside the graduated tube must always be above that of the water outside - as in the diagram. In other words the pressure inside must be a little less than air pressure. If it is a little higher then air tends to diffuse through the porous air cathode, and the gas use readings are too high.
  • Don't be tempted to use a narrow tube for the water to go up. If you do then capillary action effects complicate things so much that you will get useless results.
    Likely Results

    Using the method descirbed above, a likely results is:-

    • Using a fuel cell, the load resistor was adjusted so that the current was 40 mA.
    • The water level slowly rose up the graduated tube till the level inside was the same as that outside. The timer was then started.
    • After seven minutes the level inside the tube had risen by 1.0 cm3.
    • Atmospheric Pressure was 100 kPa, and the temperature was 21C.
    These measurements can be used to find the number of electrons passing round the circuit for each molecule of oxygen, in order to better understand the chemistry of a fuel cell. This is done as follows:-
    • 1. Use PV = nRT to find the number of moles of oxygen gas used.
      • P = 100 kPa = 1.0 x 105Pa.
      • V = 1 cm3, being the amount the water level rose, = 1.0 x 10-6 m3.
      • T = 21 C = 294 K
      • R = 8.31.
    • So, n = PV/RT = (105 x 10-6)/(8.31 x 294) = 4.1 x 10-5 moles.
    • Now we find the number of molecules of oxygen by multiplying this by Avagado's number, giving 4.1 x 10-5 x 6.02 x 1023 = 2.5 x 1019.
    • Next we find the number of electrons that flowed round the circuit in the 7 minutes.
      • First we find the electrical charge in Coulombs.
      • Charge = Current x Time = 40mA x 7 x 60 = 16.8 Coulombs
      • The charge on one electron = 1.6 x 10-19 Coulombs.
      • So the number of electrons = 16.8 / 1.6 x 10-19 = 1.05 x 1020
    • We now find the number of electrons per molecule by dividing the number of electrons that flowed round the circuit by the number of oxygen molecules used.
    • The number of electrons per oxygen molecule = 1.05 x 1020/2.5 x 1019 = 4.2
    • The "accepted" answer is 4, as explained in Section 3 of the fuel cell instruction manual.

    This should be repeated for different currents and times, as time allows.