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Method for Calculation of Average pH

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mdszj

mdszj

Civil/Environmental
Joined
Apr 10, 2003
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Hello

My question is regarding the proper method for calculating average pH. I just heard that the proper method for determining the average pH for a group of several pH measurements is:
1) Convert each pH measurement to its corresponding hydrogen ion concentration (i.e pH = -log [H+])
2) Calculate the average H+ concentration
3) Convert the average H+ concentration back to its corresponding pH value, which would be equal to the average pH value

Apparently this is required since pH is meaured on a log scale.

Thx for any info.
 
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I hope I understood your question.
Changes in concentrations of hydronium H3O+ aren't proportional to their dilutions. Take, for example, acetic acid in water at three different dilutions. The hydronium concentrations and the pH values would be:

1.0 M HOAc [→] 4.2 [×] 10-3 M pH = 2.4
0.1 M HOAc [→] 1.3 [×] 10-3 M pH = 2.9
0.01 M HOAc [→] 4.2 [×] 10-4 M pH = 3.4

Thus assuming no buffering action, no molecular rearrangements, no neutralizations, you'll need to know the dissociation constants of the various dissolved chemicals. For acetic acid as above, Ka at 25 Celsius: 1.8 [×] 10-5. One sees that the hydronium concentration decreases much more gradually than the acid concentration itself.

It appears a lab-bench check would be the answer.
 
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