Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: [1]   Go Down

Author Topic: How to calculate the percentage error in change in pH?  (Read 303 times)

0 Members and 1 Guest are viewing this topic.

marimelida

  • Very New Member
  • *
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 1
How to calculate the percentage error in change in pH?
« on: January 26, 2019, 02:53:27 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
Logged

mjc123

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +212/-8
  • Offline Offline
  • Posts: 1502
Re: How to calculate the percentage error in change in pH?
« Reply #1 on: January 28, 2019, 01:14:22 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.
Logged

Enthalpy

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +240/-54
  • Offline Offline
  • Posts: 2796
Re: How to calculate the percentage error in change in pH?
« Reply #2 on: January 28, 2019, 02:24:02 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.
Logged
Pages: [1]   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.058 seconds with 23 queries.