All water-based solutions exhibit a pH, which indicates the concentration of hydronium ions --- the species responsible for acidic behavior --- in the solution. If the solution exhibits a pH of 7, then chemists refer to it as neutral. If the solution exhibits a pH less than 7 it is acidic, and if the solution exhibits a pH greater than 7 it is basic. Science projects involving pH can focus on one of two different aspects: the chemicals and equipment used to measure pH or the actual pH values of various substances.
Measuring pH
Chemists typically use one of three methods for measuring the pH of solutions: paper test strips, wet chemical test kits or an electronic pH meter. Test paper and chemical test kits do not provide the level of precision that a pH meter offers. A student, however, can prepare his own test strips and kits in his kitchen from cabbage, whereas the construction of a pH meter probably lies outside the skill set of most high school students. Nonetheless, if the student finds the additional precision of a pH meter indispensable, scientific supply retailers offer small, hand-held units at modest cost. Likewise, such retailers also offer test strips and chemical test kits, such as those used for swimming pools.
pH of Substances
Once the student has determined how she will measure pH, she should next find something to test. Students should try to develop a thematic approach, rather than simply testing solutions at random. Possibilities include testing the pH of beverages or household cleaners. A more environmentally themed approach might include testing the pH of local ponds, streams and lakes, as well as rainwater. Soil also exhibits pH, although it requires special testing procedures.
pH Monitoring
A more prolonged investigation could include environmental monitoring of surface waters, the discharge from a water treatment facility or even a compost pile. This would involve collecting water samples and measuring the pH at, for example, one-week intervals for a period of a few months. This becomes as much an exercise in record keeping as pH measurement because the dates and locations of each sample must be carefully tracked. After the student has collected the necessary data, he should construct plots that detail the pH as a function of time to see if any patterns emerge. Then the student should attempt to correlate any significant deviations in pH to events that might explain their occurrence, such as a heavy rainfall or a prolonged drought.
Advanced Topics
More advanced students may wish to delve into advanced topics involving pH, such as using pH as a detection system for various other analyses. Lakes, for example, actually contain carbonates that form a buffer system. Buffer solutions resist changes in pH values, and water scientists carefully monitor a parameter known as buffer capacity --- the molarity of sodium hydroxide necessary to change the pH of the buffer solution by one pH unit. Other possibilities include determining the pH levels at which microbes can no longer survive in a nutrient solution. Such projects require some specialized equipment and glassware, but most high schools should include this equipment in their inventory.
Tags: chemical test, chemical test kits, solution exhibits, test kits, test strips
