Acid Base Titrations Changes in AcidBase Reaction Systems

Acid / Base Titrations Changes in Acid-Base Reaction Systems

Titration Terminology n Titration – the progressive transfer of a solution from a buret (called the titrant) into a measured volume of another solution (called the sample). n Equivalence point – the volume of titrant required to n Endpoint – the p. H at the equivalence point of a titration. n Indicator – a chemical which is added to the sample that n Buffering region – a horizontal region of the p. H curve neutralize the sample (# mol acid = # mol base). changes colour at the equivalence point of a titration. where p. H is not changing significantly.

p. H curves n A p. H curve is a graph showing the continuous change in p. H during a titration (an acid-base reaction) n Titration of Na. OH and Na 2 CO 3 with HCl 14 p. H 7 0 vol. of titrant (HCl) added (m. L)

Interpreting p. H curves Titration of Na. OH with HCl n 14 p. H buffer region endpoint 7 0 X titrant equivalence point volume buffer region 0 10 20 30 volume of titrant added (m. L) 40

Which is the titrant? n Take 2 titrations involving Na. OH and HCl p. H vol. of titrant n n HCl is titrant (p. H high to low) vol. of titrant Na. OH is titrant (p. H low to high)

Choosing an Indicator p. H curves are used to determine which indicator(s) are suitable for a particular titration n The goal is to get an indicator that changes colour in the same region as the endpoint of your titration n n 14 p. H 7 Titration of Na. OH with HCl unsuitable: alizarin yellow (too early) good indicators: bromothymol blue, litmus and phenol red unsuitable: orange IV (too late) 0 vol. of titrant (HCl) added (m. L)

Multiple Endpoints Titration of Na 2 CO 3 with HCl n 14 endpoint 1 p. H 8 X endpoint 2 4 0 equivalence point volume 1 X equivalence point volume 2 0 10 20 30 volume of titrant added (m. L) 40

Choosing Indicators for Multiple Endpoints n Titration of Na 2 CO 3 with HCl 14 endpoint 1 - metacresol purple (7. 4 -9. 0) p. H 8 X endpoint 2 – methyl orange (3. 2 -4. 4) 4 0 X 0 10 20 30 volume of titrant added (m. L) 40

Polyprotic Acids and Bases n Polyprotic acids are those which can donate more than one proton; in most cases, it is equal to the number of hydrogens in the chemical formula – – n E. g. H 2 SO 4 : can donate 2 protons E. g. H 3 PO 4 : can donate 3 protons Polyprotic bases are those which can accept more than one proton; in most cases it is equal to the magnitude of the charge on the anion – E. g. CO 3 2 - : can accept 2 protons – E. g. PO 4 3 - : can accept 3 protons

Explaining Polyprotic Endpoints Titration of Na 2 CO 3 with HCl n 14 p. H 8 X 4 0 endpoint 1 X 0 endpoint 2 10 20 30 volume of titrant added (m. L) 40

Explaining Polyprotic Endpoints Titration of Na 2 CO 3 with HCl n 14 n 8 p. H 0 4 0 X H 3 O+ + CO 3 2 - X H 2 O + HCO 3 - H 3 O+ + HCO 3 - 10 20 30 volume of titrant added (m. L) 40 H 2 O + H 2 CO 3

Other Polyprotic Systems n Consider the titration of phosphoric acid, H 3 PO 4, with sodium hydroxide, Na. OH: 11 X p. H X endpoint 1 0 Vol. of Na. OH (m. L) endpoint 2

Other Polyprotic Systems n Consider the titration of phosphoric acid, H 3 PO 4, with sodium hydroxide, Na. OH: 11 HPO 42 - + OH H 2 PO 4 - + OH- H 2 O + PO 43 - (not observed) H 2 O + HPO 42 - X p. H X H 3 PO 4 + OH - 0 Vol. of Na. OH (m. L) H 2 O + H 2 PO 4 -