If you’re a pharmacy or life sciences student, chances are you’ll need to prepare buffer solutions during lab work. I recently needed to make a phosphate buffer of pH 6.8, and during my exploration, I had some real doubts — like what volumes to mix, how pH relates to molarity, and whether there’s a smart shortcut to scale volumes.
Let’s walk through the whole thing — simply and clearly.
🔬 The Basic Chemistry Behind It
We use the Henderson–Hasselbalch equation to calculate the right ratio of acid to base needed:
pH = pKa + log([Base]/[Acid])
For a phosphate buffer, we typically use:
– Monobasic sodium phosphate (NaH₂PO₄) → Acid component
– Dibasic sodium phosphate (Na₂HPO₄) → Base component
And since the second dissociation constant (pKa₂) for phosphate is around 7.2, it’s perfect for making buffers around pH 6.8.
📊 Step-by-Step Calculation for 100 mL of Buffer
1. Apply the Henderson–Hasselbalch equation:
6.8 = 7.2 + log([Base]/[Acid]) → log([Base]/[Acid]) = -0.4
2. Convert the log to a ratio:
[Base]/[Acid] = 10^(-0.4) ≈ 0.398
3. Let acid volume = x mL, then base volume = 0.398x
Total volume = x + 0.398x = 1.398x
Solve:
1.398x = 100 → x ≈ 71.5 mL (acid)
→ Base = 28.5 mL
✅ Final Buffer Composition (100 mL, 0.2 M, pH 6.8)
| Component | Volume (mL) | Concentration |
| NaH₂PO₄ (Acid) | 71.5 | 0.2 M |
| Na₂HPO₄ (Base) | 28.5 | 0.2 M |
🤔 What If We Use 0.1 M Solutions?
This was my next doubt. Will the volumes change?
Answer: No. Because the ratio stays the same. The buffer capacity (i.e., its resistance to pH change) will be lower, but the volumes of acid and base remain the same as long as both components are of equal molarity.
🧠 What is Buffering Capacity?
Buffering capacity refers to a buffer solution’s ability to resist changes in pH when small amounts of acid or base are added. Higher concentration = stronger capacity to resist pH change.
🔁 The Shortcut Trick (Scaling Up!)
This was my “Eureka!” moment.
Since the ratio is fixed, you can simply scale up the acid and base volumes linearly:
| Total Volume | NaH₂PO₄ (mL) | Na₂HPO₄ (mL) |
| 100 mL | 71.5 | 28.5 |
| 200 mL | 143.0 | 57.0 |
| 300 mL | 214.5 | 85.5 |
| 400 mL | 286.0 | 114.0 |
| 500 mL | 357.5 | 142.5 |
🧠 Key Takeaways
– Use NaH₂PO₄ and Na₂HPO₄ in a 0.398:1 base-to-acid ratio to get pH 6.8.
– Use the Henderson–Hasselbalch equation if you’re unsure.
– You can scale the 100 mL calculation linearly for 200, 300, 500 mL, etc.
– If you change the concentration (e.g., from 0.2 M to 0.1 M), the volumes stay the same, but buffering capacity decreases.
– Buffering capacity is the solution’s ability to resist pH change when acid/base is added.
– Always check final pH with a pH meter and adjust slightly if required.