TEMPERATURE, PH, AND CARBON DIOXIDE SATURATION: A LABORATORY EXERCISE

We designed a laboratory exercise to demonstrate: 1) the relationship between temperature and solubility of CO2 in water, 2) the effect of increasing CO2 on the pH of water at different temperatures, and 3) the equilibration of CO2 between gaseous and aqueous phases. We saturated water of three different temperatures (14, 23 and 71 degrees C) with CO2 by dropping dry ice into a large Erlenmeyer flask filled with distilled water. Temperature and pH were monitored continuously with Logger Pro 1.2. Saturation with CO2 was determined to have occurred when the solution reached constant pH. For each treatment, triplicate samples of about 230 mL were poured into home-made equilibrators. Each equilibrator was prepared using a 760 mL canning jar sealed with lids into which plumbing connectors capable of holding standard GC septa were installed. O-rings above and below the lid made the systems gas-tight. Samples were shaken to equilibrate the CO2 between the headspace and the water. Gases were withdrawn by gas-tight syringe from the headspace and analyzed for CO2 by both gas chromatography and infrared spectrometry. In the headspace, CO2 was highest in the low temperature samples and lowest in the high temperature samples corresponding to the inverse relationship between temperature and carbon dioxide saturation.