This isn't homework. It's for my job. I'm supposed to be making a 50mM phosphate buffer to pH 7.2, 0.5 NaOH. I have access to salts, K(adsbygoogle = window.adsbygoogle || []).push({}); _{2}HPO_{4}(dibasic), KH_{2}PO_{4}(monobasic), and NaCl. I've determined that I need two liters of buffer solution. I did the calculations but when I measured the pH after making the solution, it was more acidic than it should be. I adjusted the pH with strong base, so it's alright now. I'm just wondering if someone can verify my calculations.

FW K_{2}HPO_{4}= 174.18 g/mol

FW KH_{2}PO_{4}= 136.09 g/mol

FW NaCl = 58.44 g/mol

pH = pKa + log ([K_{2}HPO_{4}]/[KH_{2}PO_{4}])

Since pKa = 7.2 and pH =7.2, the HH equation reduces to [KH_{2}PO_{4}]=[K_{2}HPO_{4}]

For two liters, I need 0.1 mol phosphate buffer

0.1 mol= [KH_{2}PO_{4}]+[K_{2}HPO_{4}]

then

0.05 mol = [KH_{2}PO_{4}]

0.05 mol = [K_{2}HPO_{4}]

then I weigh out 136.09 g/mol * 0.05 mol = 6.8045 g [KH_{2}PO_{4}]

and 174.18 g/mol * 0.05 mol = 8.709 g [K_{2}HPO_{4}]

and 58.44 g NaCl

and fill the volume to 95% and adjust pH if necessary then fill to mark. Did I do something wrong? Some buffer calculators are showing something like 3 times the amount of base as acid and I don't know why. For example, http://www.biomol.net/en/tools/buffercalculator.htm

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# Phosphate buffer calculation help

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