Ask A Scientist Chemistry Archive

(Created prior to 1993)

Question: Why are the bounds on pH the values 0-14?  I see no compelling 
reason that ion - clustering should spontaneously occur at 1M strong base or 
1M strong acid . . . after all, molarity is based on moles (which is based on 
grams, and therefore the density of water) and of course liters, which is 
based on the cubic meter.  Since neither of these have any inherent chemical 
properties, I do not see why a negative pH or one greater than 14 should be 
possible.
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Planck, like your namesake you have asked a very deep question 
for which I can give no simple answer.  However, for aqueous solutions at room 
temperature the pH of pure water is 7, right?  And the pOH is also 7.  This is 
because the dissociation constant of water at 25 degrees Celsius happens to be 
10^-14, right?  In fact, from this you can show that for any aqueous solution 
at room temperature, pH + pOH = 14.  So, the reason why the pH is restricted 
to the range (0-14) is because of these two facts.  Now, if you considered 
other solvents (like, say, ethanol) you might be able to get a completely 
different range.  Note also that changing the temperature has an effect on the 
dissociation constant.  I hope this is food for thought. An excellent 
question!
Topper
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