Science Buddies: "Ask an Expert"

Hello,

For the Phosphate buffer solution it says to use Deionized water. I was wondering if I could just used distilled water instead since it's easier and quicker for me to get.

Thanks!

Hi there,

For the water to fuel to water experiment I would guess that distilled water would work fine. There are likely different types of impurities in distilled vs. deionized water that may affect your overall outcome, but not significantly. Deionized water is also usually more acidic, so this may affect the relative amounts of KH₂PO₄ and Na₂HPO₄ it takes to reach the desired pH 7 for your buffer. If you have access to a pH meter or pH strips (you can purchase these on Amazon) you could test the pH to be sure.

Best,

Erin

Hi Erin,

Thanks for the confirmation. I do have a pH meter so I will double check. In the procedure it says to use 2.63 g monopotassium phosphate and 4.35 g sodium phosphate and then add 500 ml of water. If I need to tweak it in order to achieve the correct pH what's the ratio of the two element that I need to maintain?

Hi there,

In the initial setup with deionized water it looks like they used a bit more conjugate base than necessary for a neutral solution since the solution is already a bit more acidic and therefore would neutralize some of the conjugate base as it is being added.

If we assume that distilled water will be at pH 7 to start, and we know that we are aiming for a pH 7 solution of 0.1 M we can do the math to figure out how much of each component is appropriate.

The relevant equation here would be the Henderson-Hasselbach equation: pH=pKa+log[A-/HA]. Where your conjugate base (A-) is the Na₂HPO₄ and your acid (HA) is the KH₂PO₄. This gives you the ratio of conjugate base to acid that will give you a certain pH based on the pKa of your conjugate acid.

Once you've found the ratio of A- to HA you can then put this into absolute amounts using the molarity of solution that you are aiming to create.

I can post a photo of the math I did if you are interested, but the numbers I got were 2.83g KH₂PO₄ and 4.14 g Na₂HPO₄. Make sure to test your final pH with the meter anyway.

Hope this helps!

Erin

Awesome! Thank you so much Erin.

No problem! Best of luck with your experiment.