Acid-Base Indicators

behaviour depends on Bronsted-Lowry concept and the equilibrium concept

an indicator is a conjugate weak acid-base pair formed by dissolving it in water

e.g.

using Le Chatelier’s Principle, an increase in the [H3O+] shifts the equilibrium to the left
in acidic solutions, the primary form of the indicator is its un-ionized (HIn(aq)) form, eg. when litmus is added to an acidic solution
in basic solutions, the OH- ions remove the H3O+ ions and the equilibrium shifts to the right, than the base colour predominates (In-(aq))
different indicators have different acid strengths so the acidity or pH can vary (see p.609, Table 7)

in acid-base titrations, the pH changes sharply near the equivalence point

this change occurs over a small range of pH but in a titration the change occurs very quickly so that all we see is a very quick colour change at the indicator’s transition point
when selecting an indicator, the pH at the equivalence point must be known

the equilibrium constant equation for the indicator equilibrium is:

since KIn , [H3O+(aq)] , or [H+(aq)] are very small we can convert them by taking the negative logarithm of each value

an indicator works best when its pKIn equals the pH at the equivalence point of that particular titration

Titrations of Polyprotic Acids and Bases

polyprotic acids include: H2SO4(aq), H3PO4(aq), H2CO3(aq)

the titration of H3PO4 with a strong base follows the following multiple ionizing steps

1.H3PO4(aq) + NaOH(aq)  H2PO4-(aq) + H2O(l) + Na+(aq)

2.H2PO4-(aq) + NaOH(aq)  HPO42-(aq) + H2O(l) + Na+(aq)

3.HPO42-(aq) + NaOH(aq)  PO43-(aq) + H2O(l) + Na+(aq)

each ionizing step occurs in succession
the titration curve of the polyprotic acid, H3PO4, with a strong base also shows multiple endpoints as follows (p. 613, Figure 9)
the curves show two endpoints requiring equal volumes of base
the third endpoint does not register as the third ionizing step because HPO42- is an extremely weak acid (Ka = 4.2 x 10-13) and apparently does not quantitatively lose its proton – only quantitative reactions produce detectable endpoints in an acid-base titration

bases can also be considered polyprotic, such as: CO32-, PO43-, S2-

consider a titration of NaCO3 solution: the salt will dissociate into an anion (conjugate base), CO32-,of a weak acid - as a result it can hydrolyze first to HCO3- and then to H2CO3
in a titration with a strong acid the carbonate and bicarbonate anions will act as a base and accept a proton from the acid in two steps:

1.CO32-(aq) + HCl(aq)  HCO3-(aq) + Cl-(aq)

2.HCO3-(aq) + HCl(aq)  H2CO3(aq) + Cl-(aq)

The titration curve will appear with two endpoints representing both reactions (p. 611, Figure 8)

the volume to reach each endpoint will be the same

BUFFERS

Buffer Solution

pH curves involving a weak acid or weak base have at least one region where the pH changes very little, despite the addition of an appreciable amount of acid or base (buffering action)

the pH curve in this region is most nearly horizontal (flat) at a volume of titrant that is one-half the volume of the equivalence point

mixture in buffering region contains approximately equal amounts of unreacted weak acid and its conjugate base
prepared by mixing weak acid with a soluble salt of its conjugate base (may also use weak base with a soluble salt of its conjugate acid)

examples include blood

buffering action works with addition of a small amount of a strong acid or strong base

Capacity of a Buffer

buffers have a certain capacity,i.e. addition of a strong acid or strong base beyond a certain point will result in a dramatic change in pH