Measuring RQ with a Respirometer

A respirometer measures the rate of oxygen uptake by small organisms such as germinating seeds or woodlice. It can also be used to find the RQ.The organisms are placed in a tube on a gauze platform above soda lime (or potassium hydroxide solution), which absorbs carbon dioxide. A control tube contains glass beads of the same volume as the organisms. Both tubes are connected by a U-tube manometer containing coloured fluid, and are kept in a water bath at constant temperature, because gas volume changes with temperature and pressure. The control tube compensates for these changes.As the organisms respire, they take in oxygen and the carbon dioxide they release is absorbed. The volume of gas in the tube falls, so the manometer fluid moves towards the organisms. The distance moved in a set time gives the volume of oxygen used (volume = πr²h). A syringe resets the fluid level.Measuring RQ. Repeat the experiment with the soda lime replaced by water. Now the fluid movement shows the difference between oxygen used and carbon dioxide produced:CO₂ produced = O₂ used − change without soda limeWorked example: with soda lime, the fluid moves by a volume of 30 mm³ in 10 minutes, so 30 mm³ of O₂ is used. Without soda lime, the volume falls by 6 mm³. CO₂ produced = 30 − 6 = 24 mm³. RQ = 24 ÷ 30 = 0.8.If the volume does not change without soda lime, CO₂ produced equals O₂ used and the RQ is 1.0.