The fundamental principle of a respirometer is the reduction in air pressure within a sealed container as oxygen is removed from the air by the respiring organism.
To ensure that the change in pressure is solely due to oxygen uptake, a carbon dioxide absorbent (such as potassium hydroxide or soda lime) must be present to remove all produced by the organism.
As the volume of gas decreases, the internal pressure drops below atmospheric pressure, causing a manometer fluid or a colored liquid drop to move toward the experimental chamber.
The distance moved by the liquid is proportional to the volume of oxygen consumed, allowing for the calculation of the metabolic rate using the formula for the volume of a cylinder: .
Setup: Place the living specimen (e.g., germinating seeds or small invertebrates) on a gauze platform above the absorbent to prevent direct contact with the chemical.
Equilibration: Allow the apparatus to sit in a temperature-controlled water bath for several minutes before sealing to ensure the air inside reaches the same temperature as the surroundings.
Measurement: Record the starting position of the colored liquid and use a timer to measure the distance moved over a specific interval (e.g., 10 minutes).
Resetting: Use a syringe attached to a three-way tap to push air back into the system, returning the liquid drop to its original position for repeat trials.
Units and Calculations: Always check if the question asks for the 'rate' (distance/time) or 'volume' (). Ensure the radius of the capillary tube is used, not the diameter.
Temperature Control: Emphasize the use of a water bath in your answers; even small temperature changes significantly alter gas volume (), leading to inaccurate results.
The Role of the Absorbent: If a question asks what happens if the absorbent is removed, the answer is that the liquid will likely not move (or move very little) because the produced replaces the consumed.
Ignoring Respiration in Plants: Students often forget that plants respire 24/7. When investigating plant gas exchange in the dark, they only perform respiration, but in the light, photosynthesis may mask respiration.
Confusing Pressure and Volume: Remember that the liquid moves because the volume of gas decreases, which creates a lower pressure inside the tube compared to the outside.
Leaky Apparatus: A common source of experimental error is an imperfect seal. Any air entering the system will prevent the pressure drop and stop the liquid from moving.
