Chemistry - Titrations

Pipette

• should be rinsed with deionised water followed by the solution it is going to contain

• pipette filler should be used

• reading taken from the bottom of the meniscus (except for KMnO4)

Burette

• should be washed with deionised water followed by the solution it is going to contain

• reading taken from the bottom of the meniscus

• solution allowed to run through the tap, ensuring the volume below the tap is full of chemical

Conical Flask

• washed out with deionised water only

• swirled during titrations

• white tile placed underneath

Volumetric Flask

• washed out with deionised water only

• stoppered and inverted 20 times

Standard Solution

a solution whose concentration is known exactly

Primary Standard

a substance that can be weighed out precisely and used to make up a standard solution

Features of Primary Standards

• easy to weigh out (high molecular mass)

• available pure

• dissolves well in water

• does not react with the oxygen / water vapour in the air

Examples of Primary Standard Solutions

• anhydrous sodium carbonate

• ammonium iron (II) sulphate

Dilute Sulphuric Acid

iron (ii) sulphate must be acidified using this acid, in order to stop the iron in it from being prematurely oxidised by oxygen in the air or dissolved in water

Concentration (g/L)

molarity × molar mass

Number of Moles

volume in litres × molarity

Number of Grams

no. of moles × molar mass

Parts Per Million

the same concentration as expressed in milligrams per litre (water titrations)

Methyl Orange

indicator used in strong acid/ strong base and strong acid/weak base titrations

Yellow to Red

colour change of methyl orange

Phenolphthalein

indicator used in strong base/weak acid titrations

Pink to Colourless

colour change of phenolphthalein

Methyl Orange

indicator used in sodium carbonate (Na2CO3) titration

Phenolphthalein

indicator used in vinegar titration

Ammonium Iron (ii) Sulphate

is used to standardise Potassium Permanganate

Dilute Sulphuric Acid

is added twice during the KMnO4 and Iron (ii) Sulphate Titration:

1. prevents the Fe (ii) from oxidising to Fe (iii), due to oxygen in the air and water

2. allows Potassium Permanganate to work at full oxidising power - ensures the full reduction of Mn (vii) to Mn (ii), and prevents the formation of a brown precipitate Mn (iv)

KMnO4

• is an oxidising agent

• is self-indicating

First Permanent Pale Pink

colour change in Potassium Permanganate and Ammonium Iron (ii) Sulphate titration

KMnO4

“indicator” used in the iron tablet titration

Iron Tablets

are dissolved using H2SO4 and water

Potassium Permanganate, Sulphuric Acid, Potassium Iodide

iodine solution is liberated by mixing…

Red/Brown, to Straw Yellow, to Blue/Black, to Colourless

colour change in sodium hypochlorite titration and sodium thiosulfate titration

Stronger Bleach

turns the solution more red/brown

Starch

indicator used in sodium hypochlorite titration, is only added at the straw yellow stage

Potassium Iodide

is added to the sodium hypochlorite titration:

1. to ensure there are enough I+ ions for the active ingredient (hypochlorite) in the bleach to be used up

2. excess KI keeps the I2 in solution

Potassium Iodide

provides a source iodide (I-) ions, to make iodine soluble in water

Deionised Water

has no ions but may contain impurities, such as Chlorine, which would affect results as Chlorine is an oxidising agent

Ethylenediaminetetraacetic Acid

EDTA

Eriochrome Black T

indicator used to measure total hardness in water

Red to Blue

colour change of eriochrome black t

pH 10 Buffer Solution

needed to allow eriochrome black t to work

Brown Precipitate

forms in Winkler titration, if a white precipitate forms no oxygen is present

Red/Brown, to Straw Yellow, to Blue/Black, to Colourless

colour change of Winkler titration

Starch

indicator used in Winkler titration

MnSO4

added as a source of Mn2+ ions in Winkler titration