Gas Exchange Practical Work


The J-tube

The J-tube can be used to calculate the percentages of oxygen and carbon dioxide in air. The procedure normally involves:

  • use the syringe to trap an air column between water columns within the capillary tube. This is done by filling the J-tube with water, using the syringe to introduce air, then sucking in more water to trap and seal the air bubble.
  • measure the length (A) of the air column trapped between the air columns.
  • most (not all) of the water below the air column is expelled and then potassium hydroxide is drawn into the tube. 
  • Shuttling the air column several times up and down the tube allows the air column to come in contact with the sides of the capillary tube wetted with potassium hydroxide - this then absorbs the carbon dioxide in the air column. 
  • The air column is then re-measured (B). The percentage of carbon dioxide in the air column is A-B/A x 100.
  • Most (not all) of the potassium hydroxide is now expelled and pyrogallol is drawn up into the tube. The potassium hydroxide and pyrogallol react to form potassium pyrogallate.
  • Again the air column is shuttled up an down the tube using the syringe. This time the air column comes in contact with capillary sides wetted with potassium pyrogallate, which absorbs the oxygen in the air sample. 
  • The air column is then re-measured (C). The percentage of oxygen is B-C/A x 100.

The J-tube is typically used to compare the composition of inhaled and exhaled air. Due to small volumes of gas involved, it is important that temperature fluctuations are avoided (e.g. through not holding J-tube at position of air column and by setting it up under water) and that a number of repeats are taken for reliability. Additionally, it is important that the tube is allowed to equilibriate, for a few minutes, at each stage before measurements are accurately taken.

The respirometer

Respirometers can be used to calculate CO2 and O2 consumption in living organisms.

Calculating oxygen uptake - carbon dioxide is produced…