acid base titrations

use of acid-base titrations

• Used to calculate concentration of an unkown solution of an acid or an alkali

• During a titration a pH meter can be used and a pH curve plotted

• A pH curve is a graph showing how the pH of a solution changes as the acid (or base) is added

• The result is characteristically shaped graph which can yield useful information about how the particular acid and alkali react together with stoichiometric information

end point meaning

the point at which an indicator just changes colour

equivalence point

when an acid and a base have reacted in exact proportions of the stoichiometric equation

pH at equivalence point depends on the combinaiton of acid and base-salt

shape of titration curves

• All pH curves show an s-shape curve and the midpoint of the inflection is called the equivalence or stochiometric point

what are diprotic acids

acids that can donate 2 protone, H+.

eg H2SO4, H2CO3

This occurs in 2 stages:

1) H2SO4(aq) —> H+(aq) + HSO4-(aq)

2) HSO4- —→ (reversible) H+(aq) + SO42- (aq)

• the titration curve has 2 equivalence points, oen for each H+ in the acid

what are acid-base indicators?

acid-base indicators are often weak acids

HIn (colour 1-red) ——> H+ + In- (colour 2-yellow) (reversible)

• If H+ is added to the indicator, it shifts the equilibrium to the left and it appears red

• When (H+) is low (OH- added—> H2O (formed) the eqm shifts right and it appears yellow

• When (HIn) = (In-) there are equal amounts of the acid and its conjugate base, In-, the indicator is at its end point and appears orange

working pH range of indicators?

• Most indicators change colour over a range of 2pH units.

• The end point is usually in the middle of the range

• The equilibrium constant, K_a, for indicators, is known as K_In and is normally expressed in the logarithmic form, pK_In

• The working pH range of an indicator is a range of pH on either side of the pK_In value

how to choose indicator

• The vertical section of the graph must cross the colour change range of the indicator

• this will produce a sharp end point

when to use phenolphthalein

high pH range- better for strong acid -strong base and weak acid- strong base

when to use methyl orange?

• the pK_In of methyl orange is 3.7

• Below pH 3.7 the dominant colour is colour 1 (red for methyl orange) and above it is colour 2 (yellow for methyl orange)

• better for strogn acid- strong base and strong acid- weak base

how to measure pH of titration of weak acid- weak base?

• No suitable indicator as there isn’t vertical section

• Use a pH meter or change in temperature or changes in concentration