Volumetric Analysis: Titration Techniques and Standardization of Solutions, Slides of Chemistry

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Volumetric Analysis

Lecture 5

Experiment 9 in Beran page 109

Prelab = Page 115

Experimental Aims • To prepare and standardize (determine

concentration) a NaOH solution

• Using your standardized NaOH calculate

the molar concentration of a strong acid(HCl)

• HOW? – Through titration

Lab Outline

The concentration of a sodium hydroxide will bedetermined by titration with potassium hydrogenphthalate, (KHC

8

H

4

O

4

= KHP)

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

• HA

→

H

2

O + A

How is an indicator used?

• Weak acids are titrated in the presence of

indicators which change under slightlyalkaline conditions.

• Weak bases should be titrated in the

presence of indicators which change underslightly acidic conditions

Background Information

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

COOH

CH

3

COONa + H

2

O

• The term "neutral point" is best avoided. • The term "equivalence point" means that the

solutions have been mixed in exactly theright proportions according to the equation.

• The term "end point" is where the indicator

changes color, and this isn't necessarilyexactly the same as the equivalence point

Alkali to Acid Titration

• This is very similar to the previous curve

except, of course, that the pH starts off lowand increases as you add more sodiumhydroxide solution.

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.