[Environmentalist]

Hello -

Ultimately, my question is what kinds of phenomena or factors are making a buffer solution pH different from what is predicted by a Henderson-Hasselbach equation?

In my lab experiment, we used a pH meter to measure pH of a 50 mL acetic acid buffer. We pipetted 25 mL of 0.100 M acetic acid and 25 mL of 0.100 M sodium acetate solution into a 50 mL volumetric flask. Then we placed 20 mL of buffer solution into a beaker to be measured by the pH. My pH meter observed that the pH was 5.83, but Henderson-Hasselbach predicts otherwise.


Ka of acetic acid = 1.8*10^-5
pKa = 4.74

pH = pKa + log(base/acid)
pH = 4.74 + log(0.0025 mol base / 0.0025 mol acid)
pH = 4.74 of buffer solution

Henderson Hasselbach predicts pH = 4.74, while observed pH is found to be 5.83. How can this be?

[Borek]

pH of a buffer is never exact, plenty of reasons why - but when it is off by 1 unit it means some huge mistake in preparation or measurement.

How do you know concentrations were 0.100 M?

Was the probe freshly calibrated?

[Babcock_Hall]

Obviously I agree with Borek.  The only thing I would like to add is that the thermodynamic pK_a values are fine when the ionic strength is near zero, but as ionic strength increases, one must correct for this.  The effect for a buffer of moderate concentrations (such as yours) is too small to explain your observation.

[Borek]

the thermodynamic pK_a values are fine when the ionic strength is near zero, but as ionic strength increases, one must correct for this

That's within "plenty of reasons" 

Still, even for high ionic strengths pKa of acetic acid doesn't change by more than about 0.25 (it is listed as 4.50 for IS=0.5 and 5.0 for IS=3.0).

[Environmentalist]

Borek and Babcock_Hall, thank you both so much for your insights on this matter!

To answer your question, in our lab assignment, we were given labeled bottles of acetic acid and sodium acetate solution. Both bottles indicated 0.100 M concentrations.

That's really discouraging to hear, I was hoping that I had prepared the buffer solution properly but perhaps measurement error emerged in the fact that I transferred 20 mL of solution to a dry 150 mL beaker that I had left untouched for the span of a week and didn't rinse beforehand.

Yes, I followed the directions laid out in my laboratory notebook: remove the pH meter from the electrode storage solution, rinse the lower third of the meter with DI water, and dry with a KimWipe before placing into solution. I assume that the pH meter was calibrated before our laboratory class started.

Another potential explanation that I would like your feedback on is about the spectator Na+ ions in solution. My thinking is that they might have also been transferred into the 20mL portion of the buffer solution. And less H+ ions might be present in the 20mL portion than in the original 50mL buffer solution that might have led to a +1 pH increase in the pH reading. Comments or critiques on this idea of mine?

[Borek]

Another potential explanation that I would like your feedback on is about the spectator Na+ ions in solution.

Good thinking.

My thinking is that they might have also been transferred into the 20mL portion of the buffer solution.

They were definitely transferred, you can't transfer solution without all of its constituting components.

And less H+ ions might be present in the 20mL portion than in the original 50mL buffer solution that might have led to a +1 pH increase in the pH reading.

Bad thinking. While you are perfectly right that the amount of H⁺ present in 20 mL is lower than the amount of H⁺ in 50 mL, it doesn't matter, as you are measuring concentration - and concentration is an intensive property, identical for every sample of the given solution.

Comments or critiques on this idea of mine?

Back to good thinking - pH electrodes are sensitive not only to H⁺, but also to other cations, especially those small, with a single charge (Li⁺, Na⁺). These cation always make the electrode show pH that is a bit lower than in reality, this is called a sodium error (or alkaline error), compare http://www.ph-meter.info/pH-electrode-selectivity . For most commercial pH electrodes alkaline error shouldn't change the reading by more than few hundreds of a unit though, so it is still not enough to explain your result, but you may ask lab assistant (or whoever was responsible for the experiment preparation) for the electrode specification (or the electrode type and maker, every serious model should have the specification available on the web). Knowing selectivity coefficient estimating possible error should be trivial.

[Environmentalist]

Thanks Borek, I have learned so many new things about buffers in theory and creating buffers in a lab. I will be more careful about my buffer preparation techniques in the future! I wouldn't doubt that our final end-of-class lab practicum will involve buffer preparation.