Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: marimelida on January 26, 2019, 08:53:27 AM

In my experiment, I added HCl(aq) into a buffer solution until there was a pH change of 1 pH unit. The pH meter has an uncertainty of +/ 0.2. I was wondering how I would calculate the percentage uncertainty of the solution facing a pH change of 1 unit.
I did it by calculating Change in pH = 7.2 +/ 0.2  6.2 +/ 0.2 = 1 +/ 0.4
but the percentage uncertainty is then 40%, is this correct? It's just such a large amount

Assuming the error is random, not systematic, variances not errors are additive. Thus the variance of the sum (or difference) of two measurements is 0.2^{2} + 0.2^{2}, and the error of the sum is the square root of that.
Rule of thumb: error of the sum or difference between 2 measurements with the same error is sqrt(2)*error of one measurement.

Beware pH are logarithms! Percentages of logarithms are most often meaningless.
That is, an absolute change in a logarithm, for instance a change of 0.2 in the pH, is already a relative (percentage) change in the H^{+} concentration.
Consistently, you could also have 0.2 uncertainty on a pH of zero.

I'm also doing an experiment that uses a pH probe.
What I’m wondering is whether it’s possible to apply other uncertainties like uncertainty from the measuring equipment to the data.
Thanks!