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Titration of Acids and Bases

• Lab
  • Pages 117 -
    • Prelab Pages 122 –
      • Post lab Pages 126 -

Experimental Aims • To standardize (determine concentration) a

NaOH solution

• Using your standardized NaOH calculate

the molar concentration of a strong acid(HCl)

• HOW? – Through titration

Titration

Titration

is a common laboratory method

of quantitative/chemical analysis that can beused to determine the concentration of aknown reactant.

• Volume measurements play a key role in

titration, it is also known as

volumetric

analysis

Points to Note

• Your solution of sodium hydroxide will be

pre-made.

• Preparing a stock solution (Page 120) does

not need to be performed.

• Your samples of KHP are already dried.

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.

What is an acid-base indicator?

An acid-base indicator is a weak acid or a weakbase.

The undissociated form of the indicator is a different color than the associated form of theindicator.

An indicator does not change color from pure acid to pure alkaline at specific hydrogen ionconcentration, but rather, color change occurs overa range of hydrogen ion concentrations.

This range is termed the

color change interval

. It

is expressed as a pH range.

Choice of Indicator

Phenolphthalein

The color is due to the opening up of the 5 membered ring whichallows the electrons more freedom and the molecule's absorptionspectrum now transmits red light.

How is an indicator used?

• Acids are titrated in the presence of

indicators which change under slightlyalkaline conditions.

• Bases should be titrated in the presence of

indicators which change under slightlyacidic conditions

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.

This is not necessarily true of all the salts youmight get formed.

If you titrate ammonia solution with hydrochloricacid, you would get ammonium chloride formed.The ammonium ion is slightly acidic, and so pureammonium chloride has a slightly acidic pH.

That means that at the equivalence point (whereyou had mixed the solutions in the correctproportions according to the equation), thesolution wouldn't actually be neutral. To use theterm "neutral point" in this context would bemisleading.

• 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

Titration curves for strong acid v strong base HCl and NaOH are typical strong acid and strong base.

Running acid into the alkali

You can see that the pH only falls a very smallamount until quite near the equivalence point.Then there is a really steep plunge. If youcalculate the values, the pH falls all the way from11.3 when you have added 24.9 cm

3

to 2.7 when

you have added 25.1 cm

3