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Titration and pH Curves

Titration and pH Curves. Titration – the controlled addition of a solution of known concentration ( titrant ) in order to determine the concentration of a solution of unknown concentration

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Titration and pH Curves

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  1. Titration and pH Curves • Titration – the controlled addition of a solution of known concentration (titrant) in order to determine the concentration of a solution of unknown concentration • Equivalence Point – the point in a titration at which the reaction between titrant and unknown has just been completed • Indicator – a substance which undergoes a color change in the pH interval of the equivalence point

  2. Titration • Titration Curve (pH curve) – plotting of the pH of the solution as a function of the volume of titrant added Strong Acid/Base v. Weak Acid/Strong Base

  3. Strong Acid/Strong Base • Before the equivalence point • [H+] (and pH) can be calculated • At equivalence point • pH = 7 • After equivalence point • [OH-] can be calculated from the excess [OH-] and the volume of the solution • [H+] and pH can be calculated from [OH-] and Kw

  4. Weak Acid/Strong Base • Step 1 – Stoichiometry Problem • The reaction of OH- with the weak acid is assumed to run to completion • The concentration of the acid remaining and the conjugate base formed are determined • Step 2 – Equilibrium Problem • The position of the weak acid equilibrium is determined • pH is calculated • pH at the equivalence point of a weak acid/strong base is always greater than 7 • The anion of the weak acid is a base; the stronger the basic anion, the higher the pH of the equivalence point

  5. Weak Bases/Strong Acids • Identify the major species in solution before, at and after equivalence point • Study each reaction to determine if the reaction can run to completion • Do the stoichiometry • Do the equilibrium • At equivalence point, the pH is less than 7 • Conjugate acid of a weak base lowers the pH

  6. Acid-Base Indicators • Determination of Equivalence Point • Use a pH meter – find midpoint of vertical line in the titration curve • Use of indicators • How Indicators Work • Most are weak acids and shift from acid to conjugate base formsHIn(aq) H+(aq) + In-(aq) • Color change occurs as the proton is lost or acquired

  7. Using Indicators • Indicator color changes will be sharp, occurring with the addition of a single drop of titrant • Suitable indicators must be selected based on the equivalence point • Strong acid/strong base titrations may use indicators with end points as far apart as pH 5 and pH 9 • Titration of weak acids or weak bases requires more careful selection of an indicator with appropriate transition interval

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