1.9 Titrations

What is a titration

Technique used to find concentration of unknown solution- volumetric analysis

What is a pipette used for

Acid/base titration, into conical flask and indicator added.

Why measuring cylinder not used

Conical flask

Measuring cylinder less accurate only used when volume to measure does not affect outcome of analysis and conic flask can be squeezed to mix reactants

What is burette used for

Other solution added slowly until indicator just changes colour called end point.

Why indicator used

Show when base neutralised completely by acid.

what is a standard solution

A standard solution is a solution for which the concentration is accurately known.

• calculate the required mass of the solid/liquid required to make the solution

• weigh the solid/liquid in a beaker on a top pan balance

• dissolve the solid in a small volume (50-100 cm3) of deionised water

• transfer the solution to the appropriately sized volumetric flask using a filter funnel

•  rinse the beaker and glass rod with deionised water and add washings to the volumetric flask

• make up to the mark by adding deionised water until the bottom of the meniscus is on the mark

• stopper the flask and invert to mix

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how to

Pipette 25 cm3 of the standard solution into a clean conical flask

Add deionised water to the flask until the water is just below the line

Using a dropping pipette add deionised water until the bottom of the meniscus is on the line

Stopper the flask and invert to mix thoroughly.

Dilution factor

Dilution factor is the amount the original solution is diluted by

Dilution factor equation=

Dilution factor equation= new volume over original volume 

how to carry out titration

• rinse the pipette with the solution to be transferred to the conical fask using a pipette and pipette filler place (25.0cm3) of the solution in the conical flask 

• rinse the burette with the solution to be placed in it

• Fill the burette with this solution and record the reading on the burette to one or two decimal places; the second decimal place should be a zero or a five

• Add 3-5 drops of a suitable indicator to the conical flask

• Add the solution from the burette, with swirling, until the indicator just changes colour

• Record the reading to one or two decimal places reading to the bottom of the meniscus (the
  second decimal place should be 0 or 5)

• Repeat the fitration to achieve 2 concordant results. (within 0.2 cm3 of each other)

Safety:

• Use of a pipette filler

• Use of gloves (only for harmful substances)

Accuracy:

• Rinse the apparatus with the appropriate solution

• Add the solution dropwise near the endpoint

• Swirl the flask and wash down the sides of the flask with deionised water

• Read the burette/pipette at the bottom of the meniscus

Reliability

• Repeat the titration two or three times

• Ensure concordant readings are obtained (within 0.1 cm° of each other)

Titre volume equals 

End volume - Start volume

expalin why a rough titration was carried out

to obtain an estimate of the end point enabling faster subsequent titrations\

titration of strong acid in concial flask against strong base in burette

HCL(aq) + NaOH (aq)

Methyl orange: red to yellow

Phenolphthalein: colourless to pink

titration of strong base in a conical flask against strong base in burette

KOH + H2SO4

Methyl orange: yellow to red

Phenolphthalein : pink to colourless

titration of weak acid a conical flask against strong base in burette

CH3COOH (acetic acid) + NaOH

Methyl orange: NOT SUITABLE

Phenolphthalein : pink to colourless

titration of weak base in a conical flask against a strong acid in burette

Na2CO3 + HCL

Methyl orange: yelow → red

Phenolphthalein : not suitable

CONCENTRATION FORMULA

CONC mol/dm³ = moles over volume dm³

C = N DIVIDED BY V

Uncertainty def

Uncertainty estimate attached to measurement which characterises the range of values within which the true value is thought to lie.