Method for Calculation of Average pH

[mdszj]

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.

 

[25362]

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, K_a 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.