Adam C.

asked • 08/27/18

How to make a buffer?

So I have to make a 0.02 M Sodium Phosphate buffer @ pH of 7, easy enough I use monosodium and disodium phosphate as the acid and base, I used the henderson hasselbach equation. But now I have to make a 0.02 M Sodium phosphate buffer with 1 M NaCl?? I'm not sure what this means. When I asked the professor, she said that we have to have these two different solutions in one beaker and have the pH at 7. I need .75 ml of this solution. How can I accomplish this?

1 Expert Answer

By:

J.R. S. answered • 08/27/18

Ph.D. in Biochemistry--University Professor--Chemistry Tutor

Adam C.

Thank you!! The format of the question is weird but it says pH 7 then a period ending the sentence, the next character is the letter i not a 1, sorry about that! But of course I was overthinking this, I thought the NaCl was affecting the molarity of the buffer or that it was messing with the pH. Thank you! I also read somewhere that you can create a stock solution of monosodium phosphate and 1 M NaCl and then make a solution with Disodium phosphate and 1 M NaCl and add them together to the desired pH. Since I will actually have to make this solution in class, is there a benefit to doing it this way? Or should I just make 2 times the amount of 0.02 M sodium phosphate and use that as you have outlined above?
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08/27/18

J.R. S.

tutor
The NaCl will affect the [Na+] of the buffer, but not the acid/conjugate base concentrations.  What you read about making the stock solutions of mono and di sodium phosphates containing 1 M NaCl would be another way to do it, but I see no obvious advantage.  Which ever way is easiest for you to do.  Whenever I've made this (and I've made it many, many times), I've done it the way I outlined.  I  would have the stock phosphate buffer on hand, and would just use it to dissolve whatever solute I needed.  
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08/27/18

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