How to Calculate pH in a Titration of Citric Acid with NaOH?

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Discussion Overview

The discussion revolves around calculating the pH during the titration of citric acid, a weak triprotic acid, with sodium hydroxide, a strong base. Participants explore the implications of the acid's triprotic nature and the concentrations involved in the titration process.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant notes that there are equal moles of citric acid and sodium hydroxide, suggesting the pH should be just above 7 due to the weak acid-strong base interaction.
  • Another participant points out that citric acid is a triprotic acid, implying that trisodium citrate will form during the titration.
  • A question is raised about whether the triprotic nature of citric acid will affect the pH at the given concentrations.
  • One participant suggests starting with millimole amounts to find the first equivalence point and emphasizes the importance of knowing the concentration of the base.
  • It is clarified that the concentration of NaOH is 0.1000 M, which adds to the confusion regarding the use of the Henderson-Hasselbalch equation.
  • Another participant mentions that only one Ka value was provided, leading to the assumption that the first ionization is being considered, and suggests recalculating concentrations after each addition of NaOH.
  • A participant agrees to consider citric acid as monoprotic for the purpose of this discussion.

Areas of Agreement / Disagreement

Participants express uncertainty about the applicability of the Henderson-Hasselbalch equation and whether the triprotic nature of citric acid significantly impacts the pH calculation at the specified concentrations. No consensus is reached on the best approach to calculate the pH.

Contextual Notes

Limitations include the lack of additional Ka values for the other stages of deprotonization of citric acid and the potential confusion arising from the concentrations of acid and base being equal.

nautica
50.00 mL of 0.1000 M citric acid (Ka = 7.41 x 10^-4).

titrated 50.00 mL NaOH added.

Calculate pH.

I have equal moles of Acid and equal moles of Base. But, I am titrating a strong base into a weak acid. So, instead of my pH being 7, I assume it should be just above 7. But, I am not sure how to go about this.

I would not think that Henderson-Hass would work b/c we will be dealing with a log of undefined, which is undefined. The zero concentrations of each have me confused.

Thanks
Nautica
 
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Citric acid is a triprotic acid as you know, and as a result trisodium citrate will be formed. This may be a cue.
 
But, will that effect the pH at these concentrations?
 
Well, start with millimole amounts to find the first equivalence point; you have 50*.1=5 mmol of acid and 5 mmol of base is needed. How many mililiters will you use?

Also, you should know the starting concentration of the base, without knowing this property, it would be useless to consider anything.
 
Last edited:
Sorry, I see the confusion now. I left out that the NaOH concentration is 0.1000 M. This is what is confusing me. At these concentrations they cancel out so Henderson Hass can not be used.

I have also not worked with triprotics, but I wouldn't think at these concentrations it would matter. I was planning on using the above Ka.

Thanks
Nautica
 
Well, since only one Ka value was given, we can assume that the first ionization is under consideration. If you want to study the other stages of deprotonization, you'll have to give us the other Ka values along with this one.

When you start the titration, you add, say, 1 mL of NaOH onto 50 mL of HA (I will regard this acid as monoprotic here). Here, the total volume will be 51 mL, and you can recalculate the concentration after each addition step, and find the relevant concentrations (base concentration will be practically zero, since it will immediately reacted by the virtual pool of acid).
 
Yes, only one Ka was given, so I am considering it a monoprotic

Nautica
 

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