TitrationChemistry: The Molecular Nature of Matter, 6th edition

By Jesperson, Brady, & Hyslop

acid–base, 198–199, 811–819redox reactions, 239–240

Titration

Titrations are a type of quantitative analysis that enables a researcher to measure the amount, not just identify, a substance in a sample. Titrations use a chemical reaction to capture all of the desired chemical species with a known reaction that produces a known product. From the amount of known product obtained the amount of the desired chemical species in the original sample can be determined.

Types of titration reactions:• Acid/Base• Redox

Components of a typical experiment: 1. A solution of the reactant with an unknown concentration is isolated in a

beaker along with an indicator (that will change color when the reaction is complete)

2. The other reactant (the titrant) will be added slowly drop-wise.3. The other reacted is added until the indicator changes color, meaning the

reaction is complete, or has reached the endpoint.4. The amount of product can be determined and then the amount of each

reactant can be extrapolated.

Titration: Equipment

• A burette will deliver a drop of a reactant at a time. The stopcock controls the deliver of the reactant (drop or steady stream). When reading the bottom of the meniscus, you will be reading the Volume delivered, not the volume remaining in the burette.

• An Erlenmeyer flask is placed below the burrette and will contain of of the reactants and the indicator. The solution will need to be constantly stirred.

Titration: Example

Titration: Acid Base

AH + B A- + BH+

Acid + Base Conjugate Base + Conjugate AcidHCl + NaOH NaCl + H2O

H+ (acid) + OH- (base) H2O (neutral water)Initial: Erlenmeyer flask: HCl + H2O which we can think of as H+ (aq) + Cl- (aq)Burette: NaOH + H2O which we can think of as OH- (aq) + Na+ (aq)*The pH in the Erlenmeyer flask will be low because HCl is a strong acid and there is a high concentration of H+ in solution.

Midway: Erlenmeyer flask: HCl + 0.5 NaOH H2O + 0.5 NaCl + 0.5 HCl*Half of the neutralization reaction has happened, the pH will higher because the concentration of H+ has decreased by half.

Close to Final: Erlenmeyer flask: HCl + NaOH NaCl + H2O

The neutralization reaction has been completed and the pH = 7.

Final: Erlenmeyer flask: HCl + (a bit more) NaOH NaCl + H2O + small amount of NaOH*Once ~ pH = 8 (slightly basic) has been reached the indicator will be pink and the endpoint reached.

Titration: Acid Base

Phenolphthalein is a common acid/base indicator that is magenta colored between pH 8.2 - 12, and colorless from 0 – 8.2.

Initial Midway Final

Colorless Pink

Titration: Acid Base

Calculations: Know: the concentration of NaOH, the Volume of NaOH added.

1. VNaOH (L) x MNaOH (moles/L) = moles NaOH added to the acid

2. Mole : Mole ratio of HCl : NaOH is 1:1, therefore moles NaOH added = moles of HCl reacted

3. Determine the Molarity of the HCl solution initially by dividing the # moles that reacted with the initial volume of the solution (L).

Titration: Redox

• Often rely on color changes of the reactants rather then an indicator.

• KMnO4 is a common redox reactant in titration experiments:

MnO4- + 5e- Mn2+

• Powerful oxidant• Intense purple color• Reduction product is Mn2+ which is

colorless• Reaction is complete when the purple

color stays in the flask.