I need to know the theoretical and physical explanation for the negative value of PH in a water sample and their relation with water sample sources........
pH ranges from 0-14 units, each unit an order of magnitude in H ion concentration. pH 7 is considered neutral pH. If you pH meter shows negative, the pH probe may need to be replaced, or something else may be wrong.
Dear Zinabu,
the solutions pH is defined by the dissociation of the water molecule,
If you consider that water molecules are amphoteric and the following equilibrium is present,
H2O + H2O H3O + + OH-
Keq = [H3O +] [OH-] / [H2O]
from the definition of pH, we have:
pH = -log [H3O +]
the following "extreme situations" can be observed:
Case 1. If the molar concentration of H3O + tends to zero, then the pH will be very close to 14 (basic disolution).
Case 2. If the molar concentration of H3O + tends to one, then the pH of the solution will be close to zero (disolution acid).
Case 3. If the concentration of hydronium ions is greater than 1 mol / L, then the pH of the solution would be negative (super acid´s case).
With the current pH scale these values can not be measured because the definition based on logarithms of base 10, but there are possible in extreme environmental situations.
Please refer always to your equipment manual to calibrate your pHmeter´s.
Hope it will help!
Regards,
Mario
Thank you for your answer the case is not because of the PH electrode. We have measured the water sample with different calibrated Ph meter and the result of all those sample values was not negative, some are within the acceptable range positive value and the rest samples are negative value.
In principle, the pH value will be vary from 0 to 14 and most peoples know this range. But, In reality, I don't believe this range is the best indication of all PH value because some strong acids has a PH value Negative.
For sure Zinabu, your observation is right. For "super acids" its posible to have [H+] greater than 1 mol/l, as consequence pH values will be negative in the "traditional scala". So if you want to measure this values, may be better to use the activities of Hydrogen ions, instead their molar concentration.
best regards,
Mario
Therefore, what you suggest for those water samples that have negative values
Hi Zinabu,
I can´t suggest you anything, because I dont now what you need to do. Iam sending you some papers that can be interesting for your work.
best regards,
Mario
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