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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?

uhoh
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    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 03:24
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    @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 03:28
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    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 05:38
  • @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 06:14
  • 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
  • @DSVA check carefully what I've said. It's related to low ionic concentrations. "solutions of low ionic strength." There are other ions besides H${}^+$. – uhoh May 28 '17 at 13:39
  • @uhoh again: can you link any source on this. Afaik low ion concentration is no problem, high ion solution can be a problem (that's why you will get problems above pH=12 and below 0). You are claiming the exact opposite. – DSVA May 28 '17 at 14:29
  • @DSVA a half-dozen "problem measuring pH of solutions of low ionic strength" 1, 2, 3, 4, 5, 6. – uhoh May 28 '17 at 14:35
  • @DSVA There are all kinds of different explanations there. I am not qualified to search through these or another half dozen after this. I believe those more knowledgeable than me probably know which of all the possible things are likely to be the most likely, most common, and which are - while theoretically possible - not really the main sources of trouble. – uhoh May 28 '17 at 14:37
  • @DSVA 7, 8, 9, 10, 11, 12 – uhoh May 28 '17 at 14:47
  • @uhoh I've looked into several of those and all pretty much describe the same problems and have similar explanations. The main Problem is that you will only get stable and correct readings once your system is in an equilibrated state. Such pH-electrodes are calibrated in high ionic strength solutions and the reference solution is of high ionic strength too. In addition the glass and especially membranes have a high ion concentration. If you now use low ionic strength analytes the whole system (especially at membranes and at the liquid junktion) won't be at equilibrium – DSVA May 28 '17 at 15:08
  • @uhoh you'll get similar problems when doing measurements in D2O with electrodes calibrated in H2O – DSVA May 28 '17 at 15:08
  • @DSVA would you post that as a proper answer, which can then be up/down voted? I don't need a "pretty much"-type comment, I need a quality stackexchange answer. I've looked at these and others and I disagree that the reality of the problem is as simple as you make it sound. If you post it as an answer rather than a comment, we can see how well the rest of the community agrees with your statements. – uhoh May 28 '17 at 15:14
  • @uhoh well there's no point in repeating the content of your reference 3 and 6 (which explain the problems pretty well without any mathematics) as an answer if you seem to not accept these sources anyways. – DSVA May 28 '17 at 15:28
  • @DSVA the point would be to select from the large variety of explanations which ones are the best and most likely. This would be for my benefit and the benefit of future readers who would like to learn about the problems of measuring solutions of low ionic concentration. Indeed there is certainly a point to it. This is what stackexchange is for, this is how it works. It's not a repository of knowledgeable-sounding comments, it's a place for questions and answers. – uhoh May 28 '17 at 15:35

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