Unraveling the Role of Sulfur Compounds in Acid Rain Formation: Experiments on a Wetted Glass pH Electrode

Vast quantities, on the order of megatons, of pollutants are emitted monthly to the atmosphere both by natural and anthropogenic sources. The evaluation of rainwater composition has great importance in understanding the atmospheric chemical composition, as water drops scavenge particles and soluble atmospheric pollutants. Most students are aware that SO2 is one of the pollutants related to the acidification of the atmosphere owing to its oxidation in the gas or liquid phase that produces sulfate and H+, but the situation is more complex in the presence of other common pollutants such as ammonia or formaldehyde. To give insight into the chemistry occurring in the gas and liquid phases, a demonstration is described where the pH of a "raindrop" is monitored during absorption of SO2 followed by exposure to H2O2, with or without previous contact with gaseous CH2O. The effect of the presence of alkalinizing substances such as NH3 in the atmosphere is evaluated as well. The raindrop is simulated by the wet bulb of a combination glass electrode exposed to the gas phase. Measurements can be made with any pH meter, although data acquisition with a computer is advantageous to illustrate the kinetics of the processes, as shown in the article.