Changes in Acid-Base Reaction Systems Acid / Base Titrations Changes in Acid-Base Reaction Systems

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

pH curves A pH curve is a graph showing the continuous change in pH during a titration (an acid-base reaction) Titration of NaOH and Na2CO3 with HCl 14 pH 7 vol. of titrant (HCl) added (mL)

Interpreting pH curves Titration of NaOH with HCl 14 buffer region titrant pH endpoint equivalence point 7 X equivalence point volume buffer region 0 10 20 30 40 volume of titrant added (mL)

Which is the titrant? pH pH Take 2 titrations involving NaOH and HCl vol. of titrant vol. of titrant HCl is titrant NaOH is titrant (pH high to low) (pH low to high)

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

Multiple Endpoints Titration of Na2CO3 with HCl 14 endpoint 1 8 X pH endpoint 2 equivalence point volume 1 4 X equivalence point volume 2 0 10 20 30 40 volume of titrant added (mL)

Choosing Indicators for Multiple Endpoints Titration of Na2CO3 with HCl 14 endpoint 1 - metacresol purple (7.4-9.0) 8 X pH endpoint 2 – methyl orange (3.2-4.4) 4 X 0 10 20 30 40 volume of titrant added (mL)

Polyprotic Acids and Bases 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 E.g. H2SO4 : can donate 2 protons E.g. H3PO4 : 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. CO3 2- : can accept 2 protons E.g. PO4 3- : can accept 3 protons

Explaining Polyprotic Endpoints Titration of Na2CO3 with HCl 14 8 X endpoint 1 pH 4 X endpoint 2 0 10 20 30 40 volume of titrant added (mL)

Explaining Polyprotic Endpoints Titration of Na2CO3 with HCl 14 8 X H3O+ + CO3 2- H2O + HCO3 - pH 4 X H3O+ + HCO3 - H2O + H2CO3 0 10 20 30 40 volume of titrant added (mL)

Other Polyprotic Systems Consider the titration of phosphoric acid, H3PO4, with sodium hydroxide, NaOH: 11 X endpoint 2 pH X endpoint 1 Vol. of NaOH (mL)

Other Polyprotic Systems Consider the titration of phosphoric acid, H3PO4, with sodium hydroxide, NaOH: 11 HPO42- + OH - H2O + PO43- (not observed) H2PO4- + OH- H2O + HPO42- X pH X H3PO4 + OH - H2O + H2PO4 - Vol. of NaOH (mL)