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I work in a soil science lab and not very much a chemist.

Recently I took the $\mathrm{pH}$ of a large number of soil samples using a glass electrode on a mixture of $\pu{10 g}$ soil and $\pu{20 mL}$ Millipore water. Partway through the assay, I discovered that the water I was using had a $\mathrm{pH}$ of $\pu{5.2}$.

Is there a way to correct my recorded $\mathrm{pH}$'s to values I would have measured had the water had a $\mathrm{pH} = 7$?

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  • $\begingroup$ I also know the water concentration of each soil sample if that helps $\endgroup$ Aug 16, 2023 at 19:58
  • $\begingroup$ There should be a standard procedure for this analysis. The procedure would include how to handle the distilled water. An educated guess is that $\ce{CO2}$ from the atmosphere is causing the low pH. (I remember boiling water and then putting a drying tube filled with $\ce{NaOH}$ on the container's headspace for some experiment that I did.) Also the procedure should detail how to handle the soil-water mixture. If the soil has a pH above 5 or so, then the soil-water mixture too will eventually react with $\ce{CO2}$ from the atmosphere. $\endgroup$
    – MaxW
    Aug 18, 2023 at 3:35
  • $\begingroup$ The pH is not as important as the quantity of acid. When I have done similar tests , i assumed the several grams of soil had much more material to affect the pH than the water. $\endgroup$ Aug 28, 2023 at 15:24

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Regrettably, one cannot simply add or subtract pH to correct for that error. Correction would depend on knowing how much and what type of each contaminant was in the water.

For example, a very dilute, 0.00001 molar solution, of $\ce{HCl}$ would have pH about 5. It would take considerably more dissolved $\ce{CO2}$ to lower the pH to 5. The first would likely have less effect on your measurements, because the actual amount of $\ce{HCl}$ would have been small, but in the second case, far more $\ce{CO2}$ would be present to react with the soil sample.

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  • $\begingroup$ Ah well, back to pH-measuring then. Thanks for answering! $\endgroup$ Aug 16, 2023 at 20:19
  • $\begingroup$ We often acid-wash our containers in HCl in our lab. If the contaminant were enough HCl to lower the water pH to 5.2, how would I correct my pHs? $\endgroup$ Aug 16, 2023 at 20:24
  • $\begingroup$ @Tillandsia123, How much water is left in the containers? If a 10 liter carboy were rinsed with HCl and shaken out, and if a 1 ml pipette were rinsed with HCl and shaken out, is that the same amount of remnant HCl? Answer: rinse with pure H2O, shake out, rinse, shake out, reiterate until the pH approaches that of the "pure" water. In other words, establish how much rinsing is needed, measuring pH each time, and establish a routine so that you need not repeat those tests. Even then, if the detergent or water source changes, you'd need you'd check again. $\endgroup$ Aug 16, 2023 at 22:07
  • $\begingroup$ I'll make sure to do that. Thank you! $\endgroup$ Aug 17, 2023 at 0:33

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