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Using the determined concentration of your sodium hydroxide solution, this will be titrated with a HCl solution to calculate its concentration.
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Experimental Aims • To prepare and standardize (determine
Lab Outline
The concentration of a sodium hydroxide will bedetermined by titration with potassium hydrogenphthalate, (KHC
8
4
4
Using the determined concentration of yoursodium hydroxide solution, this will be titratedwith a HCl solution to calculate its concentration.
Introduction •
Titration is a common method of determining the amountor concentration of an unknown substance.
The method is easy to use if the quantitative relationshipbetween two reacting solutions is known.
The method is particularly well-suited to acid-base andoxidation-reduction reactions.
Titrations are routinely used in industry to analyzeproducts to be sold. Many manufacturers are under strictstandards of quality control because their products are soldfor public consumption.
Background
In a titration a solution of one reactant (the titrant) is addedto a measure amount of a second reactant.
One of these reactants is called a standard (knownconcentration or molar mass) and the other is unknown(reactant).
A color change (or some distinctive change) occurs whenenough titrant has been added to consume all the reactantin the analyte.
In this acid-base titration we make use of the generalreaction:
OH
→
H
2
O + A
How is an indicator used?
Arrhenius Acid
s
acids produce H
ions in aqueous solutions
bases produce OH
ions in aqueous solutions
water required, so only allows for aqueous solutions
only protic acids are allowed; required to producehydrogen ions
only hydroxide bases are allowed
The equivalence point of a titration
When you carry out a simple acid-base titration,you use an indicator to tell you when you have theacid and alkali mixed in exactly the rightproportions to "neutralise" each other. When theindicator changes color, this is often described asthe
end point
of the titration.
In an ideal world, the color change would happenwhen you mix the two solutions together inexactly equation proportions. That particularmixture is known as the
equivalence point
Example
If you were titrating sodium hydroxide solutionwith hydrochloric acid, both with a concentrationof 1 mol dm
, 25 cm
3
of sodium hydroxide
solution would need exactly the same volume ofthe acid - because they react 1 : 1 according to theequation.
In this particular instance, this would also be the neutral point
of the titration, because sodium
chloride solution has a pH of 7.
Similarly:
If you titrate sodium hydroxide solution withethanoic acid, at the equivalence point the puresodium ethanoate formed has a slightly alkalinepH because the ethanoate ion is slightly basic.
NaOH + CH
3
3
COONa + H
2
Strong Acids and Bases
Don't confuse the words
strong
and
weak
with the
terms
concentrated
and
dilute
The strength of an acid is related to the proportionof it which has reacted with water to produce ions.
The concentration tells you about how much of the original acid is dissolved in the solution.
It is perfectly possible to have a concentratedsolution of a weak acid, or a dilute solution of astrong acid.