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Unformatted text preview: adjust the ratios and create a new pH for the buffer…
adjust Buffers do not prevent pH changes but
they do resist changes
• Determine the pH of a buffer made from 0.25 M Tris
hydrochloride and 0.35 M Tris. (Tris is a weak base,
abbreviate as B; Tris hydrochloride is the conjugate
acid BH+; the pKa of the acid is 8.072) Set up H-H:
• pH = 8.072 + log [.35/.25] = ….. 8.218
• Now if the buffer is “attacked” by adding enough HCl
to make it 0.010 M, what will be the new pH? (Note
that if 0.010 M HCl is just found in water the pH would
be ~ 2.00.)
• BH+ would now be a bit more, B would now be less. Some acids produce more than one H+;
some bases accept more than one H—
these are known as POLYPROTICS
When the amino acid proline is fully protonated it will
have two acid H+ (Proline is one of the main
components of our collagen).
Ka1 = 1.1 x10–2;
Ka2 = 2.3 x10–11;
• What is the pH of 0.010 M solution?
• Consider two reactions reaching equilibrium
• H2Pro+ H+ + HPro Ka1 = [H+][HPro]/[H2Pro+]
• HPro H+ + Pro– 2 Ka2 = [H+][Pro–]/[HPro]] • Take advantage of a simplifying assumption in Ka2. Polyprotic Acids: Twice as Nice!
• H2CO3 and others. Each H+ has its own Ka
• Several components are present at same time:
H2CO3, HCO3–; CO32–; H+
There is a fraction of species for each!
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