Our sodium phosphate buffer calculator aids in determining the precise quantities of monobasic and dibasic sodium phosphate needed to create a buffer solution with a specific pH and molarity.
When you need a 0.1 M sodium phosphate buffer at pH 7.4, the calculator would provide the exact amounts of Na₂HPO₄ (dibasic) and NaH₂PO₄ (monobasic) to mix.
For a 0.2 M buffer at pH 6.8:
Ratio = 10^(6.8 – 7.2) = 0.398
[Na₂HPO₄] = 0.0597 M,
[NaH₂PO₄] = 0.1403 M 3
Dissolve 8 .47 g Na₂HPO₄ and 16 .83 g NaH₂PO₄ in water, adjusting to 1 L.
To make a 0 .5 M buffer at pH 7 .6:
Ratio = 10^(7 .6 – 7 .2) = 2 .51 2
[Na₂HPO₄] = 0 .3571 M
[NaH₂PO₄] = 0 .1429 M 3.
Dissolve 50 .69 g Na₂HPO₄ and 17 .14 g NaH₂PO₄ in water , adjusting to 1 L.
Sodium Phosphate Buffer Calculator
| Desired pH | Molarity (M) | Na₂HPO₄ (g/L) | NaH₂PO₄ (g/L) |
|---|---|---|---|
| 6.0 | 0.05 | 0.82 | 4.68 |
| 6.5 | 0.05 | 1.63 | 4.38 |
| 7.0 | 0.1 | 5.49 | 7.35 |
| 7.2 | 0.1 | 7.10 | 6.00 |
| 7.4 | 0.2 | 19.33 | 12.96 |
| 7.6 | 0.1 | 9.18 | 2.82 |
| 8.0 | 0.15 | 18.37 | 2.46 |
| 8.5 | 0.1 | 14.79 | 1.21 |
| 7.2 | 0.3 | 21.29 | 17.99 |
| 7.8 | 0.2 | 13.98 | 4.02 |
| 6.8 | 0.1 | 3.10 | 6.90 |
| 5.8 | 0.05 | 0.25 | 3.75 |
| 9.0 | 0.1 | 24.70 | 1.00 |
| 8.2 | 0.15 | 16.80 | 2.20 |
| 5.5 | 0.05 | 0.10 | 3.90 |
Sodium Phosphate Buffer Calculation Formula
The foundation of sodium phosphate buffer calculations lies in the Henderson-Hasselbalch equation:
pH = pKa + log([A⁻] / [HA])Where:
- pH is the desired pH of the buffer
- pKa is the acid dissociation constant (7.2 for phosphate buffer)
- [A⁻] is the concentration of the conjugate base (dibasic phosphate)
- [HA] is the concentration of the weak acid (monobasic phosphate)
To calculate the ratio of dibasic to monobasic phosphate, we can rearrange this equation:
[A⁻] / [HA] = 10^(pH - pKa)Let’s create a 20 mM sodium phosphate buffer at pH 7.4
Using the Henderson-Hasselbalch equation:
Ratio = 10^(pH - pKa) = 10^(7.4 - 7.2) = 1.58Calculate the concentrations of each component Total concentration = 20 mM = 0.02 M
[Na2HPO4] (dibasic) = (0.02 * 1.58) / (1 + 1.58) = 0.01224 M [NaH2PO4] (monobasic) = 0.02 – 0.01224 = 0.00776 M
Calculate the mass of each component needed for 1 liter of solution Mass of Na2HPO4 = 0.01224 M 141.96 g/mol = 1.74 g Mass of NaH2PO4 = 0.00776 M 119.98 g/mol = 0.93 g
Prepare the buffer
- In a clean beaker, dissolve 1.74 g of Na2HPO4 (dibasic sodium phosphate) in about 800 mL of deionized water.
- In the same beaker, add 0.93 g of NaH2PO4 (monobasic sodium phosphate).
- Stir the solution until both salts are completely dissolved.
- Transfer the solution to a 1 L volumetric flask.
- Rinse the beaker with small amounts of deionized water and add the rinsings to the volumetric flask.
- Add deionized water to the volumetric flask until the volume reaches exactly 1 L.
- Mix the solution thoroughly by inverting the flask several times.
Check and adjust the pH
Calibrate a pH meter using standard buffer solutions.
Measure the pH of your prepared buffer.
If necessary, adjust the pH:
- To increase pH: add small amounts of 1 M NaOH
- To decrease pH: add small amounts of 1 M HCl
Sodium Phosphate Buffer Table
| pH | Component | Concentration (M) | Volume of Mono Basic Sodium Phosphate (NaH₂PO₄) | Volume of Di Basic Sodium Phosphate (Na₂HPO₄) | Total Volume (L) |
|---|---|---|---|---|---|
| 5.0 | NaH₂PO₄ | 0.1 | 0.15 L | 0.0 L | 1.0 L |
| 5.5 | NaH₂PO₄ | 0.1 | 0.1 L | 0.05 L | 1.0 L |
| 6.0 | NaH₂PO₄ | 0.1 | 0.1 L | 0.0 L | 1.0 L |
| 6.5 | NaH₂PO₄ | 0.1 | 0.075 L | 0.025 L | 1.0 L |
| 7.0 | NaH₂PO₄ | 0.1 | 0.05 L | 0.05 L | 1.0 L |
| 7.2 | Na₂HPO₄ | 0.1 | 0.025 L | 0.075 L | 1.0 L |
| 7.4 | Na₂HPO₄ | 0.1 | 0.01 L | 0.09 L | 1.0 L |
| 7.5 | Na₂HPO₄ | 0.1 | 0.005 L | 0.095 L | 1.0 L |
| 7.6 | Na₂HPO₄ | 0.1 | 0.0025 L | 0.0975 L | 1.0 L |
| 8.0 | Na₂HPO₄ | 0.1 | 0.0 L | 0.1 L | 1.0 L |
| 8.5 | Na₂HPO₄ | 0.1 | – | – | – |
How do you calculate sodium phosphate buffer?
To calculate a sodium phosphate buffer, follow these steps:
- Determine the desired pH and molarity of your buffer.
- Calculate the ratio of dibasic to monobasic phosphate using the Henderson-Hasselbalch equation.
- Compute the individual concentrations of each component based on the total molarity.
- Convert concentrations to masses using molecular weights.
Let’s calculate a 0.1 M sodium phosphate buffer at pH 7.0:
- pH = 7.0, Molarity = 0.1 M
- Ratio = 10^(7.0 – 7.2) = 0.631
- [Na₂HPO₄] = (0.1 * 0.631) / (1 + 0.631) = 0.0387 M
[NaH₂PO₄] = 0.1 – 0.0387 = 0.0613 M - Mass Na₂HPO₄ = 0.0387 141.96 g/mol = 5.49 g/L
Mass NaH₂PO₄ = 0.0613 119.98 g/mol = 7.35 g/L
How do you make sodium phosphate buffer pH 8?
To prepare a sodium phosphate buffer at pH 8, you’ll need a higher proportion of dibasic phosphate.
Here’s an example for a 0.1 M buffer:
- Calculate ratio: 10^(8.0 – 7.2) = 6.31
- [Na₂HPO₄] = (0.1 6.31) / (1 + 6.31) = 0.0863 M
[NaH₂PO₄] = 0.1 – 0.0863 = *0.0137 M - Dissolve 12.25 g Na₂HPO₄ and 1.64 g NaH₂PO₄ in water, adjusting to a final volume of 1 L.
How do you make sodium phosphate buffer pH 7?
For a pH 7 buffer, the ratio of dibasic to monobasic phosphate is close to 1:1.
Ratio = 10^(7.0 – 7.2) = 0.631
[Na₂HPO₄] = 0.0387 M, [NaH₂PO₄] = 0.0613 M
Dissolve 5.49 g Na₂HPO₄ and 7.35 g NaH₂PO₄ in water, adjusting to 1 L.
How do you make a 50 mM sodium phosphate buffer?
To prepare a 50 mM buffer at pH 7.4:
- Ratio = 10^(7.4 – 7.2) = 1.58
- [Na₂HPO₄] = 0.0307 M, [NaH₂PO₄] = 0.0193 M
- Dissolve 4.36 g Na₂HPO₄ and 2.32 g NaH₂PO₄ in water, adjusting to 1 L.
References
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