Titrations

Standard Solution Sample Solution Burette A titration is a volumetric analysis technique used to find the [unknown] of a sample solution by adding a standard solution of [known] and volume

A sufficient amount of standard solution is added until equal moles of each have reacted or the equivalence point has been reached.

A common titration involves an acid-base neutralisation reaction. One of the reactants must be either a strong acid or base or both (must go to completion) as follows: ACID + BASE  WATER + SALT Titrations do not always yield an equivalence point with a pH of 7. Why? –The salt can alter the final pH of the solution.

Depends on the strength of the reacting acids/bases: An indicator must be chosen to signal the equivalence point of a titration. It does this by a change in colour at the equivalence point.

Choosing Indicators For Titrations will depend on the overall pH of the salt produced yellowblue

Strong Acids and Strong Base Titrations Example: HCl (strong acid) and NaOH (strong base) Formula Equation:__________________________________ Complete Ionic Equation: ____________________________ Net Ionic Equation:__________________________________ pH at equivalence point: _____________________________ Possible Indicators:__________________________________ HCl + NaOH  H 2 O + NaCl H + + Cl - + Na + + OH -  H 2 O + Na + + Cl - H + + OH -  H 2 O pH = 7 (neutral salt) Bromothymol blue

Titration Curve for Strong Acid & Strong Base pH of sample solution is very low  SA Base is being added; pH increases

Example: CH 3 COOH (weak acid) and NaOH (strong base) pH at equivalence point: _____________________________ Possible Indicators:__________________________________ Weak Acids and Strong Base Titrations

Weak Acid & Strong Base pH starts higher as CH 3 COOH is a weaker acid E.P > 7

Weak Base and Strong Acid Titrations Example: NH 3 (weak base) and HCl (strong acid) pH at equivalence point: _____________________________ Possible Indicators:__________________________________

Weak Base & Strong Acid Standard Acid is added Sample of Strong Base E.P < 7

Example: A 25.00mL sample of HCl was titrated with a standard NaOH solution of concentration molL -1. If 32.45mL of base was needed to reach the EP, what was the concentration of the HCl?