Please help with question!!

[Nimret]

How would you prepare a 250 ml solution, 0.5 M in sodium phosphate buffer at pH of 7.0. You have in the laboratory monobasic sodium phosphate (NaH2PO4) and diabasic sodium phosphate (Na2HPO4). pKa = 7.2

[chrispaul3]

I know what you mean.I'm having the same problem.Waiting for the help...

[helan]

Hi

You just refer to below link. so u can get little bit ideas. i am not 100% Sure. but at this link you can got ans for your question..

[Kunal Pandya]

I get this method from internet and this for preparation of 1M buffer so use directly this procedure and then dilute accordingly for your use otherwise calculate directly for 0.5M solution


Phosphate Buffer Materials

* monosodium phosphate
* disodium phosphate
* water
* phosphoric acid to make the pH more acidic or sodium hydroxide to make the pH more alkaline
* pH meter
* glassware

Prepare the Phosphate Buffer

1. Decide on the concentration of the buffer. Most buffers are used at a concentration between 0.1 M and 10 M. If you make up a concentrated buffer solution, you can dilute it as needed.

2. Decide on the pH for your buffer. This pH should be within one pH unit from the pKa of the acid/conjugate base. So, you can prepare a buffer at pH 2 or pH 7, for example, but pH 9 would be pushing it.

3. Use the Henderson-Hasselbach equation to calculate how much acid and base you need. You can simplify the calculation if you make 1 liter of buffer. Select the pKa value that is closest to the pH of your buffer. For example, if you want the pH of your buffer to be 7, then use the pKa of 6.9:

pH = pKa + log ([Base]/[Acid])

ratio of [Base]/[Acid] = 1.096

The molarity of the buffer is the sum of the molarities of the acid and conjugate base or the sum of [Acid] + [Base]. For a 1 M buffer (selected to make the calculation easy), [Acid] + [Base] = 1

[Base] = 1 - [Acid]

substitute this into the ratio and solve:

[Base] = 0.523 moles/L

Now solve for [Acid]. [Base] = 1 - [Acid] so [Acid] = 0.477 moles/L

4. Prepare the solution by mixing 0.477 moles of monosodium phosphate and 0.523 moles of disodium phosphate in a little less than a liter of water.

5. Check the pH using a pH meter and adjust the pH as necessary using phosphoric acid or sodium hydroxide.

6. Once you have reached the desired pH, add water to bring the total volume of phosphoric acid buffer to 1 L.

7. If you prepared this buffer as a stock solution, you can dilute it to make up buffers at other concentrations, such as 0.5 M or 0.1 M.

Regards
Kunal pandya