Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: [1]   Go Down

Author Topic: Measured pH of a buffer different from calculated pH??  (Read 541 times)

0 Members and 1 Guest are viewing this topic.

MR

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 8
Measured pH of a buffer different from calculated pH??
« on: March 23, 2017, 10:21:38 AM »

Hi all,

I made a buffer with the pH 6 from a weak acid/conjugated base with a pka of 4.76.
I calculated the amount of the acid and base needed to make a buffer solution with pH 6.

After I made the buffer solution the measured pH was different from 6.

How is this possible?  ??? ??? ???
Logged

scientific

  • Regular Member
  • ***
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 15
Re: Measured pH of a buffer different from calculated pH??
« Reply #1 on: March 23, 2017, 10:28:24 AM »

Can you show us your calculations? Maybe there is a mistake there.


Otherwise, it could be an issue with the buffers you used or the measuring equipment.
Logged

MR

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 8
Re: Measured pH of a buffer different from calculated pH??
« Reply #2 on: March 23, 2017, 11:11:46 AM »

The buffer is sorbic acid/potassium sorbate.
My calculations are:

6=4.76+log (potassiumsorbate/sorbic acid)
1.24=log (potassiumsorbate/sorbic acid)
17.378=potassiumsorbate/sorbic acid

potassiumsorbate= 17.378/ (1+17.378)= 0.9456

sorbic acid = 1/(1+17.378)=0.0544

Then i multiplied with the desired molarity and molar masse to get gram.
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1550/-389
  • Offline Offline
  • Gender: Male
  • Posts: 23551
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Measured pH of a buffer different from calculated pH??
« Reply #3 on: March 23, 2017, 11:31:49 AM »

I made a buffer with the pH 6 from a weak acid/conjugated base with a pka of 4.76.

Using buffer outside of the pKa┬▒1 range is asking for troubles.
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

MR

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 8
Re: Measured pH of a buffer different from calculated pH??
« Reply #4 on: March 23, 2017, 11:46:28 AM »

What kind of troubles?
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1550/-389
  • Offline Offline
  • Gender: Male
  • Posts: 23551
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Measured pH of a buffer different from calculated pH??
« Reply #5 on: March 23, 2017, 01:41:29 PM »

Pretty small errors in preparation meaning large differences in pH (not to mention the main problem - low buffering capacity, but that's not what we are talking about ATM).
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +183/-12
  • Offline Offline
  • Posts: 2894
Re: Measured pH of a buffer different from calculated pH??
« Reply #6 on: March 23, 2017, 03:21:48 PM »

One other thing to consider.  The thermodynamic pKa of a substance pertains to an ionic strength of zero.  Yet the actual ionic strength of a typical buffer is not zero.  This effect for your buffer is not likely to be large unless your buffer was fairly concentrated.  Some tables provide a practical pKa value, which attempts to correct for this effect.
Logged

MR

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 8
Re: Measured pH of a buffer different from calculated pH??
« Reply #7 on: March 23, 2017, 09:18:58 PM »

So what i have to make sure of is my preparation and not to make a fairly concwntrated buffer. But how concwntrated are we takling about and where can i find the practical pka values?   :)
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1550/-389
  • Offline Offline
  • Gender: Male
  • Posts: 23551
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Measured pH of a buffer different from calculated pH??
« Reply #8 on: March 23, 2017, 10:07:40 PM »

http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients

Any serious source should contain the thermodynamic values for different IS values.

Besides, if you need a precise pH control you should always adjust your buffer against a calibrated pH meter, not rely on the calculated recipe.
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

MR

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 8
Re: Measured pH of a buffer different from calculated pH??
« Reply #9 on: March 23, 2017, 10:58:46 PM »

Another question i have is, is it normal for this kind of buffer to have a LOWER buffer capacity for a high concentration compared to the buffer capacity of the same low conc buffer?
Logged

Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +183/-12
  • Offline Offline
  • Posts: 2894
Re: Measured pH of a buffer different from calculated pH??
« Reply #10 on: March 24, 2017, 04:10:59 AM »

So what i have to make sure of is my preparation and not to make a fairly concwntrated buffer. But how concwntrated are we takling about and where can i find the practical pka values?   :)
Obviously I agree with Borek that one should in general make the buffer using a calibrated pH meter.  when I calculate how much acid or base to add, I usually think of that calculation as an estimate, anyway.  Others may have different points of view.

The ability of the buffer to resist changes in pH is greatest at a pH that is identical its the pKa.  Assuming that I have the freedom to choose any buffer that I want, I typically look for buffers with pKa values close to the pH of my experiment.  However, when one is trying to duplicate someone else's experiment, this may not be an option.

I may have brought about unnecessary confusion when I mentioned ionic strength.  One usually choose the concentration of the buffer according to the needs of the experiment and then accepts the ionic strength for what it is.  For sorbate buffers the effect will probably be small anyway.
Logged
Pages: [1]   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.056 seconds with 24 queries.