# Why is it difficult to measure the pH of environmental water samples accurately (near 7)?

I've read in several places (edit: 1, 2, 3, 4, 5, 6, 7, 8. 9, 10, 11, 12) that there are many practical factors that make accurate pH measurements of roughly neutral environmental water samples more challenging than significantly basic or acidic samples.

Since the glass probes are sensitive fairly specifically to hydrogen or hydronium activity, why would concentrations around $\ce10^{-7}$ be more challenging than a much higher or especially much lower concentration, say $\ce10^{-10}$?

Discussions of the problem seem to center around issues related "solutions of low ionic strength", so a basic solution near pH 10 and a buffered solution near ph 7 would both have less practical measurement issues than a non-ionic solution near pH 7.

Is there a way to explain the practical issues clearly, but in fairly simple, somewhat physical rather than purely mathematical terms?

• If you carry the pH electrode to the water source it would be easy to measure the pH. The problem would come with trying to transport the water back to the lab. The water could get CO2 from the atmosphere, and the water would have active biological organisms which could cause the pH to change. // Remember too that at a pH 7 water itself has little buffer capacity. So a very small amount of acid or base can change the pH a lot.
– MaxW
May 21 '17 at 3:24
• @MaxW variation in the dissolved CO2 is a very important consideration, since the sample would be non-buffering, but those pH measurements would not be incorrect. They would reflect the pH of the solution, which is no longer a valid sample. I'm really asking about effects that would lead to an incorrect measurement - the number on the meter not reflecting the hydrogen activity of the solution currently under test.
– uhoh
May 21 '17 at 3:28
• My major professor (chemistry) at the Univ of Ga was on the committee for a PhD student in the agriculture dept. The kid was sudying pH effect on some crop. In his PhD defense my professor threw the kid what he thought was a softball question. "How did you calibrate your pH meter?" The kid responded "Calibrate?!?"
– MaxW
May 21 '17 at 5:38
• @MaxW This line of enquiry started when helping a friend work with a third party who's anecdotal response was "Keep it wet?!?" There are quite a lot of interesting things going on with the measurement or even definition of pH, much more than I'd ever realized. I thought I'd carved out a tidy little bit of science to learn, but wow!
– uhoh
May 21 '17 at 6:14
• Can you link any of those sources? Since pH-electrodes measure the concentration of "H+" it doesn't make sense that they wouldn't work well at pH 7 but good at higher AND lower concentrations.
– DSVA
May 28 '17 at 13:07