CPrac B - Titration

% error with pipette

• 0.1 / 25.00 x 100% = +-0.8%

• NO x2!

% error with burette

• 0.05 x 2 / 26.00 x 100% = +-0.5835

Describe % error and total % error

• Percentage apparatus error = (uncertainty / value measured) x 100%

• % error decreases for a larger value measured

• Total percentage error: Just add together, NOT over 200 x 100%

For non-standard dilution factors, use burette instead of a pipette (Normal: 4,10, 25 times) 

Why is the result accurate?

Since the overall % experimental error is less than the total % apparatus error -> result is accurate

State precision pipette, burette, mass readings

State measuring cylinders readings (10, 50, 100cm³)

Describe which indicators to use during SA-SB, WA-SB, SA-SB, SA-WB

1. Phenolphthalein, thymolphthalein and thymol blue indicators can be used for SA–SB and WA–SB titrations

2. Methyl orange indicator can be used for SA–SB and SA–WB titrations

Describe MnO4 titration and colour changes

• Usually Fe2+ or hot C2O42– is titrated against KMnO4

Describe idometric colour change

Describe idometric explanation

• It involves the liberation of iodine from the reaction of an oxidising agent (Eg. IO3–, MnO4- or Cu2+) and excess KI

• I2 liberated is then titrated against S2O32–

• 1 cm3 (half dropper full) of starch solution is added near the end point when [I2] is low (solution is pale yellow)

• I2 + 2S2O32– → 2I– + S4O62–

Double indicator titration

Eg. Titrate FA2 (NaOH and Na2CO3) (conical flask) against HCI (burette) 

• NaOH + HCI -> NaCI + H2O thymolphthalein 

• Na2CO3 + HCI -> NaHCO3 + NaCI thymolphthalein (1st titre volume) 

• NaHCO3 + HCI -> NaCI + CO2 + H2O methyl orange (2nd titre volume)

2nd-1st titre volume = volume of HCI reacted with NaHCO3 = volume of HCI react with Na2CO3 

Describe titration errors

1. Impurities present in conical flask may react with the solution inside -> volume of solution needed is higher than expected 

2. Methyl orange changes colour at pH 3-4 -> titration value smaller than expected as methyl orange change colour before acid or alkali is completely neutralised at pH 7

2. Measuring cylinder is not precise for the measurement of 25.0 cm3 of FA 1 leading to inaccurate concentration of FA 3

3. Pipette of a smaller capacity -> percentage error in measuring 10.0 cm3 will be larger

Be SPECIFIC with titration error modifications

• Use a pipette or burette to measure out 25cm^3 of FA1 to ensure more accurate volume -> more accurate concentration of FA1 

• Add only 3-4 drops of indicator to obtain more accurate titration results