Volumetric

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An acid with a known concentration (a standard solution) is slowly added to a base with an unknown concentration (or vice versa). A few drops of indicator solution are added to the base.

The indicator will signal, by colour change, when the base has been neutralized

i.e. when [H+] = [OH-].

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At the end point

At that point - called the equivalence point or end point - the titration is stopped. By knowing the volumes of acid and base used, and the concentration of the standard solution, calculations allow us to determine the concentration of the other solution.

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Volumetric apparatus

Pipette

Burette

Conical flask

beaker

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Titration Procedure

Rinse 20 or 25cm3 pipette with the base solutions.

Using the pipette, accurately measure 20 or 25cm3 of the base into a clean conical flask.

Add 2 or 3 drops of a suitable indicator to the base in the flask.

Pour the acid into the burette using a funnel.

Adjust the tap to expel air bubbles and then take the initial burette reading.

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Titration Procedure

Place the conical flask on a white tile under the burette.

Run the solution gradually from the burette into the conical flask and swirl the flask along.

Continue the addition with swirling until the end point is reached.

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How do you know when you are reaching the endpoint?

The indicator will begin to show a change in colour. Swirling the flask will cause the colour to disappear.

ENDPOINT IS REACHED AS SOON AS THE COLOUR CHANGE IN PERMANENT.

ONE DROP WILL DO IT - once the colour change has occurred, stop adding additional acid

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Warning!

Do NOT continue adding until you get a deep colour change - you just want to get a permanent colour change that does not disappear upon mixing.

NOTE:

If a pH meter is used instead of an indicator, endpoint will be reached when there is a sudden change in pH.

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Then,

Record the burette reading. The difference between the final and the initial burette readings gives the volume of the acid used.

The titration should be repeated two or more times and the results averaged.

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Precautions during titration