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Acid Rain Experiments – Experiment 6 – Soil Buffering

Soil sometimes contains substances, like limestone, that buffer acids or bases. Some salts in soil may also act as buffers. In this experiment you will find out if soil from your lawn, garden, or school can buffer acids. You will observe the pH change of an acid mixture poured over soil in a filter. If the water collected from the filter is less acidic than the original mixture, then the soil is buffering some of the acid. If it does not change, then the soil may not be capable of buffering acids. Since the buffering capability of soils differs, you may want to do this experiment with several different soil types including those collected for the “Soil pH” experiment (experiment 5).

Materials

Instructions

  1. Pour 1 teaspoon of vinegar into 2 cups of distilled water, stir well, and check the pH with either pH paper or a garden soil pH testing kit. The pH of the vinegar/water mixture should be about 4. If it is below that, add a sprinkle of baking soda, stir well, and recheck the pH; but if it is above pH 4, add a drop or two of vinegar and again recheck the pH.
  2. Put 1 coffee filter into the funnel, and fill the filter with soil from one location. Do not pack the soil down.
  3. Hold the filter over a paper cup and slowly pour the vinegar/water mixture over the soil until some water collects in the paper cup (the filter may clog quickly, but you need only a small amount of water).
  4. Check the pH of the collected water using either pH paper or a garden soil pH testing kit and record the results.
  5. Repeat the experiment with other soil samples, using a new coffee filter for each sample.

Questions and Answers

Did the pH of the collected water stay the same as the original mixture, increase, or decrease?

If the pH stayed the same, the soil did not buffer the acid. Each pH value above 4 indicates that the soil buffered increasing amounts of the acid. Even soil capable of buffering acids can be overpowered if enough acid is added. As more acid is added to the soil, the buffering capability decreases, and the water from the filter becomes more acidic.

What can you add to the soil to increase its buffering capability?

Limestone can be added, but it takes weeks or even months for the limestone to work into the soil.


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