272_Exp4BC

As a result the overall titration curve will look

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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...
View Full Document

Ask a homework question - tutors are online