Which one is the best procedure to prepare 100 mL of buffer solution?

Question

BlackTom AI · Accepted Answer

Buffers are best made by dissolving the buffer salts in less than the target volume, adjusting the pH, then bringing the solution to the final volume (so the pH adjustment is not later altered by dilution). This practice is the central reason why option C is correct.

1) Relevant knowledge points and concepts
- A buffer’s pH is set by the ratio of conjugate acid/base; adding strong acid or base changes both pH and volume.  
- If you adjust pH in the full final volume, subsequent volume changes will alter concentrations and pH.  
- If you adjust pH in too small a volume, additions of titrant produce large, hard-to-control pH jumps.  
- Standard practice: dissolve salts in ~70–90% of final volume, adjust pH, then transfer to a calibrated volumetric flask and bring to final volume.

2) Step-by-step reasoning
- Dissolve the calculated amount of buffer salts in a convenient partial volume (commonly ~80 mL for a 100 mL target). This ensures salts dissolve and gives room to titrate.  
- Measure pH and add small aliquots of acid or base to reach the target pH while solution is near its final ionic environment.  
- After the pH is correct and thoroughly mixed, add deionised water to reach exactly 100 mL in a volumetric flask (this fixes concentration precisely).

3) Intermediate-step illustration (conceptual)
- If you start in 30 mL and add 0.5 mL titrant, that titrant changes total volume by ~1.6%; in 80 mL the same 0.5 mL changes volume by ~0.6% — smaller relative effect and easier control. Also pH jumps are less extreme when you titrate in a larger pre-volume.

4) Why the correct answer is correct
- Option C follows best laboratory practice: partial-volume dissolution, pH adjustment at near-final composition, then final-volume adjustment for accurate concentration and pH stability.

5) Why the other options are incorrect
- A: No pH testing — unacceptable because buffer must be adjusted to target pH.  
- B: Dissolving in full 100 mL then adjusting pH will change composition when you add titrant (volume and concentration change), making final concentration inaccurate.  
- D: Dissolving in only 30 mL gives too little headroom; pH adjustment will be difficult and prone to overshoot and large dilution effects.

6) Conclusion
Following these principles confirms that option C is the best procedure; the final answer is C.

Which one is the best procedure to prepare 100 mL of buffer solution?单项选择题

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