Acid-Base Titrations

Titration Lab technique commonly utilized to determine an UNKNOWN concentration of a chemical compound with a KNOWN concentration of another chemical compound. Chemical compounds combine with exact stoichiometric proportions Analyte— Chemical compound with unknown concentration Titrant— Chemical compound with known concentration Measured with volume and concentration Added to chemical compound with unknown concentration in titration

Acid-Base Titration Technique used to perform a neutralization reaction Acid/Base with KNOWN concentration used to determine acid/base with UNKNOWN concentration Neutralization Reaction: Acid + Base  Salt + H2O

Titration Terminology End Point: point in a titration where a color change is observed due to indicator. Equivalence Point: point during a titration when neutralization has occurred. Equal amount of acid and base Cannot usually be observed We can plot a titration on a graph—titration curve **Ideally, equivalence point = endpoint ** End point—color change with indicator Want to use indicators where end point and equivalence points are roughly the same. Why? So we can tell when neutralization has happened and stop adding base. Some indicators (phenolphtalein) do not change color right at equivalence point—so stop titration when a pink color persists for 10 seconds

2 Types of Acid-Base Titrations Strong Acid/Strong Base Titrations Weak Acid/Strong Base Titrations

1. Strong Acid/Strong Base Titrations Low initial pH value Sharp increase in pH before equivalence point Equivalence point is pH = 7 Rapid pH increase after equivalence point **Indicators with pH range 4-10 helpful for these titrations **Neutralization reactions

Strong Acid with Strong Base Titrant Strong Base with Strong Acid Titrant

Example 1: Strong Acid/Strong Base Titration Calculations 30 ml of 0.50M HCl is titrated with 0.50M NaOH. a) Find the pH of 0.50M HCl pH = 0.30

Example 1: Strong Acid/Strong Base Titration Calculations 30 ml of 0.50M HCl is titrated with 0.50M NaOH. b) Find the pH after 15 ml of NaOH added pH = 0.77

Example 1: Strong Acid/Strong Base Titration Calculations 30 ml of 0.50M HCl is titrated with 0.50M NaOH. c) Find the pH after 30 ml of NaOH added pH = 7, equivalence point for strong acid/base titrations

Example 1: Strong Acid/Strong Base Titration Calculations 30 ml of 0.50M HCl is titrated with 0.50M NaOH. d) Find the pH after 45 ml of titrant added pH = 13

2. Weak Acid/Strong Base Titrations High initial pH value pH = pKa at half-neutralization [weak acid] = [conjugate base] Ka = [H3O+] [A -] SO Ka = [A-]/[HA] is 1:1 [HA] Ka = [H3O+], SO pH = pKa

2. Weak Acid/Strong Base Titrations Equivalence point > 7 on pH scale **Indicators with pH range > 7 helpful as pH equivalence point is basic

Example 1: Weak Acid/Strong Base Titration Calculations 30 ml of 0.5M HC2H3O2 titrated with 0.50M NaOH. Ka for acetic acid is 1.8x10 -5. a) Find the initial pH of 0.5M acetic acid. HC2H3O2 + OH-  C2H3O2- + H2O (titration view) HC2H3O2  H+ + C2H3O2- (in detail) pH = 2.52

Example 1: Weak Acid/Strong Base Titration Calculations b) Find the pH after 15ml of NaOH were added pH = pKa + log [C2H3O2-] / [HC2H3O2] -used when dealing with buffer solutions -buffer—mixture of weak acid/conjugate base -more on this concept later pH = 4.74

Example 1: Weak Acid/Strong Base Titration Calculations c) Find the pH at the equivalence point. C2H3O2- + H2O  HC2H3O2 + OH- pH = 9.08

Example 1: Weak Acid/Strong Base Titration Calculations c) Find the pH after 40ml of NaOH are added to 30ml of 0.50M HC2H3O2. pH = 12.8

Homework Titration Worksheet