Volumetric Analysis:

TitrationAn introduction

The Big Picture

Use a solution of known concentration (the

standard solution) to determine the

concentration of an unknown solution

What we need

• The chemical equation

• A way to measure amounts of solution added – Fixed and variable

• A way to determine that the titration is complete

• Reaction should be fast

The Chemical Reaction• Must know the equation and correct

stoichiometric ratios

NaOH(aq) + H2SO4(aq) Na2SO4(aq) + H2O(l)

Example:

Write the equation for the titration of sulfuric acid with sodium hydroxide

2 2

Therefore the base to acid ratio is 2:1Not balanced

Volume Measurements

Fixed (known) volume should be measured with high precision: Use pipette (p.17)

Variable volume should be as high precision as possible: Use burette

• Titrate a variable amount of solution into a known volume of solution until titration is complete

How to determine titration is complete

• Usually an indicator is used

Terms

End point: when indicator changes color

Equivalence point: when a stoichio-metrically equivalent amount of titrant has been added to solution

More on indicators

Phenolphthalein is a very common acid-base indicator

Colorless in acid

Pink in base

Example

25.00 mL of 0.460 M sulfuric acid is titrated with NaOH, requiring 28.45 mL to reach the endpoint. What is the NaOH concentration?

We place 25.00 mL of acid and a couple drops of the indicator into the flask and titrate with base until the solution turns slightly pink.

2 NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2 H2O(l)

Example

25.00 mL of 0.460 M sulfuric acid is titrated with NaOH, requiring 28.45 mL to reach the endpoint. What is the NaOH concentration?

Example

Example

Moles of acid

Concentration of base

Moles of base

2 4

2 4H SO 2 4

0.460 mol H SO0.02500 L 0.01150 mol H SO

L solnn

2 4

2 mol NaOHNaOH 2 4 1 mol H SO0.01150 mol H SO 0.0230 mol NaOHn

NaOH0.0230 mol NaOH 0.808 M NaOH

0.02845 LC

Some Titration Techniques• Start with burette at or beyond

0.00 mL – It doesn’t matter, just record it!– Don’t waste time getting exactly

0.00 mL

• Titrate to the ½ drop

• Add extra water if needed– How does this affect calculations?

Some Titration Techniques• Stir!

• Place on white paper

• Palest and most reproducible pink possible

Common Abbreviations for Acetic Acid

CH3COOH

HCH3CO2

C2H4O2

C2H3O2H

HOAc

Goal: Determine molar concentration and mass percentage acetic acid in vinegar

This Week’s Analysis

moles HOAc = L soln

C

mmass percentage mHOAc

soln

= 100

Units on mass % of acetic acid in vinegar:

Units of concentration of acetic acid in vinegar:

This Week’s Analysis

• Oxalic acid as a primary standard– Why?

• One oxalic acid solution per 2 groups

• Note that oxalic acid is a hydrate– How does this affect you?

• 10 mL pipettes• 250 mL volumetric flasks

Procedure ProposalHow you will prepare your oxalic acid

solution (~0.25 M)How you will determine NaOH

concentration including calculations and chemical reactions

How you will determine the concentration of the acetic acid in vinegar (and how many trials)

Road map of calculations for calculating C and mass % acetic acid from your data

Don’t even think of using C1V1=C2V2

This investigation, Author 3: Introduction and Conclusion1: Discussion2: Data/Results and Experimental

This investigation, AuthorA: Introduction, Conclusion, Data/ResultsB: Discussion and Experimental