pH Titration Curves & Indicators

Objectives 1)Review AS understanding of titration calculations for: Reacting volumes & Reactant concentrations 1)Predict, draw and explain titration curves 2)Explain what an indicator is Describe the characteristics of indicators Use pH curves to select an appropriate indicator

25.0cm 3 of 0.14M NaOH are exactly neutralised by a solution of 0.2M HCl. What was the volume of HCl? cm 3

25.0cm 3 of ethanedioic acid, H 2 C 2 O 4, is neutralised by 28.6 cm 3 of a 0.28 moldm -3 solution of sodium hydroxide, NaOH. Calculate the concentration of ethanedioic acid solution moldm -3

The change in pH during acid-base titrations can be used to determine the equivalence point. A graph of pH of the solution being titrated plotted against volume of solution added gives a pH curve. “the point at which stoichiometrically equivalent amounts of acid and base have been reacted”

25 cm 3 of moldm -3 HCl titrated against moldm -3 NaOH

starting pH ending pH x equivalence point 25 cm 3 of moldm -3 HCl titrated against moldm -3 NaOH

Strong acid (HCl) : Strong base (NaOH) equivalence point = 7 Strong acids react with strong bases to form neutral solutions Major ions present are Na+ and Cl- ions which have no effect pH

(a)(b) (c) Identify each of the following as: 1)weak acid – weak base 2)weak acid – strong base 3)strong acid – weak base

Strong acid (HCl) : Weak base (NH 3 ) starting pH ending pH x equivalence point

Strong acid (HCl) : Weak base (NH 3 ) equivalence point < 7 NH 4 + ions are present which act as Bronsted-Lowry acid Write the equation for the reaction

Weak acid (CH 3 COOH) : Strong base (NaOH) starting pH ending pH x equivalence point

Weak acid (CH 3 COOH) : Strong base (NaOH) equivalence point > 7 CH 3 COO - ions are present which act as Bronsted-Lowry base Write the equation for the reaction

Weak acid (CH 3 COOH) : Weak base (NH 3 ) starting pH ending pH x equivalence point

equivalence point ~ 7 Weak acid (CH 3 COOH) : Weak base (NH 3 ) Difficult to detect as pH changes gradually and there are no sudden changes

Can you explain what titration this pH curve is representing?

Shapes of pH curves – main features System pH at start pH at equivalence pH at twice equivalence strong acid + strong base weak acid + strong base strong base + strong acid weak base + strong acid weak base + weak acid

Shapes of pH curves – main features System pH at start pH at equivalence pH at twice equivalence strong acid + strong base weak acid + strong base strong base + strong acid weak base + strong acid weak base + weak acid

Shapes of pH curves – main features System pH at start pH at equivalence pH at twice equivalence strong acid + strong base weak acid + strong base > 1.0> strong base + strong acid weak base + strong acid weak base + weak acid

Shapes of pH curves – main features System pH at start pH at equivalence pH at twice equivalence strong acid + strong base weak acid + strong base strong base + strong acid weak base + strong acid weak base + weak acid

Shapes of pH curves – main features System pH at start pH at equivalence pH at twice equivalence strong acid + strong base weak acid + strong base strong base + strong acid weak base + strong acid < 13.0< weak base + weak acid

Shapes of pH curves – main features System pH at start pH at equivalence pH at twice equivalence strong acid + strong base weak acid + strong base strong base + strong acid weak base + strong acid weak base + weak acid < 13.0~ 7

Acid-Base Indicator “A weak water-soluble organic acid whose acid form (HIn) and co-base (In - ) are different colours.” HIn H + + In - colour 1 colour 2

Common IndicatorsCommon Indicators IndicatorColour 1 acid (HIn) pH rangeColour 2 base (In - ) thymol bluered yellow methyl orangered3.2 – 4.4yellow methyl redred4.8 – 6.0yellow litmusred5.0 – 8.0blue bromothymol blueyellow6.0 – 7.6blue phenol redyellow6.6 – 8.0red phenolphthaleincolourless8.2 – 10.0pink alizarin yellowyellow10.1 – 12.0red

The end-point This is when a sufficient volume of titrant has been added to give a [H + ] where [HIn] = [In - ]. The equivalence volume of titrant can be found precisely when the indicator has an end point = equivalence point of the titration. The end-point (refers to the indicator) should NOT be called the equivalence point (refers to the titration)

Choosing Indicators The indicator is chosen by considering the specific pH curve for the titration. An indicator is appropriate if the rapid change of pH at equivalence overlaps the activity range of the indicator.