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Topic: phosphate buffer  (Read 6752 times)

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versyss

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phosphate buffer
« on: October 26, 2005, 12:08:02 AM »
I am doing a lab to determine the concentration of H3PO4 and NaH2PO4.  I have already done the lab, and calculated Ka1 and Ka2 from my results, but ran into a problem.  I need help in how I would prepare a phosphate buffer, pH 7.00, about .05M using activity coefficients.  We haven't really go over it in class yet and the book is a confusing me a bit.  A nudge in the right direction would help.

Offline AWK

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Re:phosphate buffer
« Reply #1 on: October 26, 2005, 02:18:35 AM »
Start from phosphate buffer using concentration.
Your problem is of type:
x/y=a
x+y=b
then calculate activities of ions and check change of pH (probably concentration methot will be sufficient)
AWK

Offline Borek

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Re:phosphate buffer
« Reply #2 on: October 26, 2005, 03:57:59 AM »
I am doing a lab to determine the concentration of H3PO4 and NaH2PO4.  I have already done the lab, and calculated Ka1 and Ka2 from my results, but ran into a problem.  I need help in how I would prepare a phosphate buffer, pH 7.00, about .05M using activity coefficients.  We haven't really go over it in class yet and the book is a confusing me a bit.  A nudge in the right direction would help.

AWK already outlined possible general approach (he wakes up earlier then me ;) ). Buffer calculations and activity calculations are covered on my site.

That's - at least partially - trial and error. You may call it an iterative process if you pefer (sounds much better). To speed up calculation try BATE, as it calculates ionic strength and activity coefficients on the fly (in fact I have already did the question in less then a minute). The problem is, ionic strength of the solution is above 0.1 so it is outside of the Debye-Huckel theory usablity range.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

versyss

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Re:phosphate buffer
« Reply #3 on: October 26, 2005, 07:49:20 PM »
Thanks!  I am going to give it a try!

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