Post on 14-Dec-2015
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