Water, pH and Macromolecules
Polyprotic acids such as H3PO4, can act as acid-base buffers

if their concentration is kept low
at pH values around any of their pKa's
at pH values around neutrality
only in combination with polyprotic bases

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Water, pH and Macromolecules
The main difference between an acid and a base is that

acids donate hydrogen ions in water while bases accept hydrogen ions
acids are polar molecules and bases are not
bases are polar molecules and acids are not
bases donate hydrogen ions in water while acids accept hydrogen ions

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