how to prepair a buffer of 9.8 p.h using Na3PO4 and Na2HPO4. Na:PO4 raio must be 2.8:1

To prepare a buffer solution with a pH of 9.8 using sodium phosphate compounds, specifically Na3PO4 (trisodium phosphate) and Na2HPO4 (disodium hydrogen phosphate), while maintaining a Na:PO4 ratio of 2.8:1, you'll need to follow a systematic approach. This involves understanding the Henderson-Hasselbalch equation, which relates the pH of a buffer solution to the concentration of its acidic and basic components.

The Henderson-Hasselbalch Equation

The equation is expressed as:

pH = pKa + log([A-]/[HA])

In this context, [A-] represents the concentration of the basic form (Na3PO4), and [HA] represents the concentration of the acidic form (Na2HPO4). For phosphate buffers, the relevant pKa values are approximately 7.2 (for H2PO4-) and 12.3 (for HPO4^2-). Since your target pH of 9.8 is between these two pKa values, we will use the pKa of 12.3 for our calculations.

Calculating the Ratio of Components

First, we need to determine the ratio of [A-] to [HA] that will yield a pH of 9.8:

• pH = 12.3 + log([Na3PO4]/[Na2HPO4])
• Rearranging gives: log([Na3PO4]/[Na2HPO4]) = pH - 12.3
• Substituting the pH value: log([Na3PO4]/[Na2HPO4]) = 9.8 - 12.3 = -2.5
• This implies: [Na3PO4]/[Na2HPO4] = 10^(-2.5) ≈ 0.00316

This means that for every part of Na2HPO4, you need about 0.00316 parts of Na3PO4. However, since you need a Na:PO4 ratio of 2.8:1, we can express this in terms of total moles of phosphate.

Setting Up the Molar Ratios

Let’s denote:

• [Na3PO4] = x moles
• [Na2HPO4] = y moles

From the Na:PO4 ratio of 2.8:1, we have:

• 3x + 2y = 2.8(x + y)

Solving this equation will give you the relationship between x and y. After some algebra, you can find the specific amounts of Na3PO4 and Na2HPO4 needed to achieve both the desired pH and the specified ratio.

Preparation Steps

1. **Calculate the required moles**: Based on your total desired volume of buffer, calculate the moles of Na3PO4 and Na2HPO4 needed using the ratios derived.

2. **Weigh the compounds**: Accurately weigh the calculated amounts of Na3PO4 and Na2HPO4.

3. **Dissolve in water**: Add the weighed compounds to a volumetric flask containing distilled water. Stir until fully dissolved.

4. **Adjust the volume**: Once dissolved, add more distilled water to reach the final desired volume.

5. **Check the pH**: Use a pH meter to verify that the pH is indeed 9.8. If necessary, make slight adjustments by adding small amounts of either Na3PO4 or Na2HPO4.

Final Thoughts

By following these steps, you can successfully prepare a buffer solution with a pH of 9.8 using Na3PO4 and Na2HPO4 while adhering to the specified Na:PO4 ratio. Remember, precision in measurement and pH adjustment is key to achieving the desired buffer characteristics.