# Chemical Forums

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

Title: How to calculate the percentage error in change in pH?
Post 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
Title: Re: How to calculate the percentage error in change in pH?
Post by: mjc123 on January 28, 2019, 07:14:22 AM
Assuming the error is random, not systematic, variances not errors are additive. Thus the variance of the sum (or difference) of two measurements is 0.22 + 0.22, 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.
Title: Re: How to calculate the percentage error in change in pH?
Post by: Enthalpy on January 28, 2019, 08:24:02 AM
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.
Title: Re: How to calculate the percentage error in change in pH?
Post by: Evann on February 17, 2019, 11:13:00 PM
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!