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Title: Buffering the pH: Exploring Citric Acid and Citrate Solutions
Introduction:
This experiment delves into the characteristics of a citric acid-sodium citrate buffer,
following a preliminary study on pH buffers across a range from 1 to 12. The focus is on
understanding how a buffer mitigates pH changes, demonstrated through the creation
of a buffer using citric acid and its conjugate base. Similar procedures are applied to
citrate solution and a 1:1 mixture of citric acid and citrate solution.
Experimental Procedure:
Materials like a pH probe, scintillation vials, HCl, NaOH, and Thymol Blue were
assembled. Calibration of the pH probe occurred, and citric acid solution underwent the
addition of NaOH, while citrate solution experienced HCl addition. A 1:1 citric acidcitrate solution was also tested for pH changes.
Data and Observations:
Citric Acid Solution, with Thymol Blue, displayed a pale orange color. The pH
progression with added NaOH is detailed in a table. Notably, a sudden pH change
occurred from 26 to 28 drops. Citrate Solution, with a pale green color, exhibited an
initial pH of 8.17. pH changes with added HCl are recorded, with a substantial shift
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around the 20 to 22 drop range. The 1:1 solution turned pale yellow with an initial pH of
4.20.
Data Analysis:
Fig. 1 illustrates the observed pH versus drops of NaOH/HCl. At the intersection, [A-]
equals [HA], indicating a pH around 4. The experimental Ka for citric acid is calculated
as 1x10^-4.
Results and Conclusions:
The lab aimed to observe buffer resistance to pH changes, utilizing the HendersonHasselbalch equation to find pH equals pKa. The results align well, with the expected
intersection around pH 4. Mixing procedures and probe cleaning were critical for
accuracy. The experiment contributes to a standard approach in examining buffers
quantitatively and qualitatively.
Discussion:
Citric acid's triprotic nature was explored, showing increased pKa with each
dissociation. The addition of sodium citrate shifted the equilibrium leftward. While
minor pH reading discrepancies were noted, they did not significantly impact the final
results.
Improvements:
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Enhanced mixing procedures could improve accuracy, addressing instances where pH
readings initially appeared odd but normalized after thorough mixing.
Conclusion:
This experiment underscores the Henderson-Hasselbalch formula's validity, illustrating
the pH-pKa relationship and buffer resilience until a specific point.