Planning Flashcards
3 parts of a procedural step
- Verb (action word)
- Quantity and identity of chemical to be measured and recorded
- Instrument/ apparatus (specifying their capacity) to be used
Volume of titrant and volume of solution recommended
Ratio of volume of solution pipetted for titration to volume of solution added from burette should be around 1:1
Titrant volume should be between 10.00cm3 and 30.00cm3, while solution should be around 20.00cm3 to 25.00cm3
If titre/ titrant volume is too low, the percentage uncertainty associated with the volume would be high, leading to problems of accuracy of measurement
If titre/ titrant volume is too high, tirtration process would be time-consuming as careful refilling of burette would be needed
Acid-base titration uses
Strong acid-strong base [4-10]:
Thymolphtalein (colorless (acid) to colorless (if coming from acid)/ light blue (if coming from base) to blue) [9.4-10.6]
Thymol blue (yellow to green to blue) [8.0-9.6]
Methyl orange (red to orange to yellow) [3.2-4.4]
Screened methyl orange (violet to grey to green) [3.2-4.4]
Strong acid-weak base [3.5-6.5]:
Methyl orange
Screened methyl orange
Weak acid- strong base [7.5-10.5]:
Thymolphtalein
Thymol blue
Weak acid-weak base: NONE
To ans qns:
Titration involves reaction between [strong/ weak] acid and [strong/ weak] base
___ is a suitable indicator since its pH range coincides with region of rapid pH change in this titration.
Types of redox titrations
Manganate titrations
- Potassium manganate as oxidising agent
- Turn from purple to colorless (reduced to Mn2+) in acidic medium
Dichromate titration
- Potassium dichromate as oxidising agent
- Turn from orange to green (reduced to Cr3+) in acidic medium
Iodine thiosulfate titration
Colour change at end point for manganate titration
For products that are colourless:
IF KMnO4 was placed in burette- from colourless to pale pink::
Just before end point, conical flask contains Mn2+, which is colourless in dilute solution and very little unreacted [reactant] only
At end point, the addition of one drop of excess KMnO4 which remains unreacted and this gives a pale pink colour (die to very dilute purple)
IF KMnO4 was placed in conical flask- from pale pink to colourless
Just before end point, conical flask contains very little unreacted KMnO4, which is pale pink due to low concentration and Mn2+ which is colourless in dilute solution only
At end point, addition of one drop of excess [reactant] which remains unreacted and the solution will be colourless
For products that are coloured (eg: Fe2+ to Fe3+)
If KMnO4 was placed in burette- from yellow to pale orange::
Just before end point, conical flask contains very little unreacted Fe2+ (almost colourless due to low concentration), Fe3+ (yellow) and Mn2+ (colourless in dilute solution) only,
At end point, addition of one drop of excess pale pink colour (pale pink due to very dilute purple)
Mixture of yellow and pink gives an orange colour
If KMnO4 placed in conical flask:
Just before end point, conical flask contains Fe3+, very little unreacted KMnO4 (pale pink due to low concentration) and Mn2+ (colourless in dilute solution) only
Mixture of yellow and pink gives an orange colour
At end point, addition of one drop of excess Fe2+ which remains unreacted and colour will be yellow due to Fe3+ present
Choice of acids
Dilute H2SO4 (aq) is commonly used to provide acidic medium for redox titrations and is usually added in excess using a measuring cylinder
Colour change at end point for iodometric titration, why should starch indicator not be added at start of titration
With iodine and starch indicator placed in conical flask: from blue-black to colourless
With iodine in conical flask: from pale yellow to colourless
Starch indicator should not be added right at start of titration
- Concentration of iodine is relatively high
- If starch indicator is added at start of titration, many iodine molecules is trapped in starch molecules
- Liberation of iodine molecules from starch molecules takes time, titration results will be inaccurate
Preparation of standard solution using 250cm3 graduated flask
- Using an electronic balance, weigh out accurately about ___g of impure ____ sample in a clean and dry weighing bottle
- Dissolve the sample in some deionised water in a 100cm3 beaker. Transfer the sample quantitatively from the weighing bottle to the beaker by rinsing the weighing bottle a few times with deionised water and transfer all the washings into the beaker.
- Transfer the solution in the beaker quantitatively into a 250cm3 graduated flask with the aid of a funnel and a glass rod. Rinse the beaker a few times with deionised water and transfer all the washings into the graduated flask.
- Fill the graduated flask to the 250cm3 mark with more deionised water. Use a dropping pipette to add the deionised water drop by drop when nearing the mark.
- Stopper the graduated flask and shake the solution thoroughly to ensure that it is homogeneous. Label the solution FA3.
Acid-base titration
- Fill the burette with the [concentration] [acid]
- Pipette 25.0cm3 of [base] into a 250cm3 conical flask. Add 2 drops of [indicator]. Titrate this solution in the conical flask against the [acid] placed in the burette.
- Stop the titration when the end-point is reached, when one drop of the [acid] added from the burette changes the colour of the solution in the conical flask from [color change]
- Record the titration results using an appropriate table
- Repeat the titration until at least two consistent results are obtained, where 2 titre volumes are within 0.10cm3 of each other.
Dilution of a given solution
- Using a burette, add ___cm3 of [solution] into a 250cm3 graduated flask.
- Fill the graduated flask to the 250cm3 mark with deionised water. Use a dropping pipette to add deionised water drop by drop when nearing the mark.
- Stopper the graduated flask and shake the solution to ensure that it is homogeneous. Label the solution __
