Titration

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General

 

Titration is a quantitative analysis technique which involves pipetting a solution (solution 1) into a conical flask and then adding a second solution (solution 2) from the burette until solution 1 is just completely reacted.

 

To perform a titration accurately, you will need to learn the following skills:

• Washing a burette
• Filling up a burette (including removing air gaps)
• Washing a pipette (including using pipette filler)
• Using a pipette to transfer 25.0 cm³ of solution into conical flask (including tapping out solution stuck at tip)

 

 

Things to note: Remove funnel after filling up burette. Ensure that burette is upright and take readings at eye-level. Ensure there is no air gap at the burette tap. Keep the pipette tip immersed in the solution when adjusting the liquid meniscus to the required height. Rinse the side of the conical flask with deionised water when reaching end-point.

 

Acid-base Titration

• titration involves an acid, an alkali (or carbonate) and a pH indicator.
• most common indicator used in O-levels is methyl orange. Methyl orange is red in acidic, yellow in alkaline and orange in neutral solutions.
• end-point:
  (acid in conical flask) when solution turns from red to orange
  (alkali/carbonate in conical flask) when solution turns from yellow to orange

 

Redox Titration

• titration involves an oxidising agent [usually potassium manganate(VII) (in burette)] and a reducing agent [usually an aqueous iron(II) salt].
• since many reducing/ oxidising agents are already coloured, no additional indicator is needed.
• most oxidising agents need to be acidified so titration will involve adding an acid. The end-point may not be seen if student forgets to add the acid.
• end-point: when solution turns from colourless to pink.

 

Redox Titration (Iodometric Titration)

• titration involves iodine solution (in conical flask), sodium thiosulfate and starch indicator.
• aqueous iodine is brown which will fade to pale yellow as the titration proceeds and eventually becomes colourless when all the iodine is reacted.
• when the solution is pale yellow, it is difficult to detect the point where it becomes colourless. Hence, starch indicator is added to give a dark-blue colour.
• end-point: when solution turns from dark-blue to colourless.

 

Recording Data

 

Titration data is recorded in a table as shown (example):

	1	2	3
final reading/ cm3	22.50	42.50	22.20
initial reading/ cm3	0.00	22.50	0.10
volume of titrant used/ cm3	22.50	22.10	22.00
best titration results		✔	✔

• all data must be recorded to 2 d.p. (precision of burette).
• titrant = solution in burette; titre value = volume of titrant used.
• best titration results = titre values that are consistent i.e. within ±0.10 cm³ of one another.

 

Calculations

• all titration calculations will involve the chemical calculation approach (3C).
• challenge lies in understanding the background (story) of the question.