Today I was using my pH meter on wastewater samples but the meter was reading values near 14 and sometimes greater, I changed the probe with the spare one and it read well (sure I calibrated before reading) My question is if the probe must be damaged or is it only need cleaning?

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    $\begingroup$ Try testing it with standard buffer solutions, e.g., pH values of 4, 7, and 10. $\endgroup$
    – Ed V
    Commented Feb 22, 2021 at 19:15

2 Answers 2


You did not mention what kind of waste water are you talking about. Is it from a textile industry, paper mill, leather factory? Waste water is not a "compound", it can be any junk and its pH can vary over several orders of magnitude!

As I have stated before, there is nothing fundamental about pH. It is a scale of convenience. It can be negative and it can be greater than 14 depending upon the activity of the hydrogen or hydroxyl ion in water respectively.

In case the wastewater is in fact highly alkaline:

The point to ponder is whether a glass electrode is suitable or not for measuring really alkaline solutions. The answer is no. Glass electrode have something called as alkaline error at high pH. So you may have to dilute your samples quantitatively and quote the pH of dilute samples. If the samples are too alkaline, simply express the titre with 0.1 M HCl since pH measurements will not be reliable.


Just to add on, the pH can be greater than 14 if the temperature of the solution is low, in which case the water dissociation constant will decrease. If a given solution had a really high pH that is close to 14 at room temperature then it would definitely increase to a value beyond 14 if the temperature of the solution were to decrease.


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