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As a result the overall titration curve will look

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Unformatted text preview: As each proton in a polyprotic acid is titrated, each successive pair of species in the sequence- - from fully protonated to fully deprotonated- - is linked by a separate equilibrium with water, and defined by a unique equilibrium constant: − H 3 PO4 ( aq ) ⇔ H 2 PO4 ( aq ) + H(+aq ) K a = 7.5 × 10−3 − − H 2 PO4 ( aq ) ⇔ HPO4 2( aq ) + H(+aq ) K a = 6.2 × 10−8 − − HPO4 2( aq ) ⇔ PO4 3( aq ) + H(+aq ) K a = 4.8 × 10−13 The titration curve for phosphoric acid will be determined by directly measuring the pH of a solution that results when a known volume of a known concentration of base is added to the acid. However, it is also possible to calculate the pH expected from the addition of a specific amount of base before any measurement € is made. Such calculations are made easier when we can assume that the deprotonations occur sequentially. This is possible when the equilibrium constants for the successive deprotonations differ by 3- 4 orders of magnitude. When this is true, at any pH one acidic form and its conjugate base are present in significantly higher concentration than any others. As a result, the overall titration curve will look like a series of single proton titrations where the region beyond the equivalence point of one proton merges with the buffering region of the next. Let’s look at a specific example: Oxalic acid (C2H2O4) is used in waste water treatment, where it removes calcium from water, it’s used in cleaning and sterilizing home brewing equipment, and is useful as a reducing agent for photography and ink removal. It is also a diprotic acid (with Ka1 = 6.5 x 10- 2, Ka2 = 6.1 x 10- 5). If we were to titrate this acid with a 0.1000 M standardized NaOH solution, we can use ou...
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