Question

What are the effective buffering ranges of MOPS (3-(N-morpholino)propanesulfonic acid) and SHS (sodium hydrogen succinate)? The Nitrogen atom of MOPS can be protonated (pKa=7.2). The carboxyl group of SHS can be ionized (pKa=5.5).Calculate the ratio of basic to acidic species for each buffer at pH 6.5.

Answer 1

1)Effective buffering is about 1 pH unit on either side of the pKa of the conjugate acid.

1) For MOPS [3-(N-Morpholino)propanesulfonic acid) pKa = 7.2

Hence effective buffer range for MOPS is 6.2-8.2.

2) For SHS (Sodium Hydrogen Succinate) pKa = 5.5

Hence effective buffer range is 4.5-6.5.

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2) Henderson equation for acidic buffer is,

pH = pKa + log{[conjugate base]/[Acid]}

A) For MOPS buffer,

pKa = 7.2, then at pH = 6.5 with above equation we get,

6.5 = 7.2 + log{[conjugate base]/[Acid]}

log{[conjugate base]/[Acid]} = 6.5-7.2

log{[conjugate base]/[Acid]} = - 0.7

[conjugate base]/[Acid] = 10^-0.7

[conjugate base]/[Acid] = 0.1995

B) For SHS buffer,

pKa = 5.5, pH = 6.5

Hence,

pH = pKa + log{[conjugate base]/[Acid]}

6.5 = 5.5 + log{[conjugate base]/[Acid]}

log{[conjugate base]/[Acid]} = 6.5 – 5.5

log{[conjugate base]/[Acid]} = 1

[conjugate base]/[Acid] = 10¹

[conjugate base]/[Acid] = 10

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