In search of accurate pH measurement

When you hold a ruler against something you need to measure, you can be sure that the six-inch mark really is six inches from an end of the ruler. It isn’t so easy, however, to measure soil pH.pH testing tools

We discovered in the fall that different tools sometimes produce starkly different results with a single sample of soil. In one instance, we came up with an acidic 5.0 when we tested with pH paper and an alkaline 8.0 with a system that mixes soil with an indicator solution. So what to do? And do user errors or schlocky instruments account for the unsettling results?

Interestingly, we sent soil from the same batch to two different professional labs and again got different answers, though not nearly as different – 7.5 and 8.0. And with the help of Mary Cosgrove at the College of Saint Rose, we tested our tools, by testing buffer solutions that we knew with some certainty had pHs of 4.66, 7.0 and 10.0.

The exercise strengthened the case for a combination of user error and questionable instruments. But we’re still not really sure. Either way, working through this challenge would seem to provide a good lesson in science. And we definitely invite science teachers/scientists to help us get to the bottom of this.

—Bill Stoneman

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