Required Practical 7 - Thin Layer Chromatography of Chlorophyll

Question 1

Suggest what is meant by Rf values

Answer 1

Ratio of distance travelled by spot to distance travelled by solvent front from origin line

Question 2

Suggest a hazard in this investigation and how it is managed

Answer 2

• propanone (volatile solvent)

- prevent inhalation by using a bung and working in a well ventilated environment

Question 3

Suggest why pigment spots are measured from the centre

Answer 3

• average distance
• standardise readings
• allows comparisons to be made

Question 4

Suggest why pencil rather than pen is used to mark the chromatography paper

Answer 4

• ink in pen may dissolve in solvent

- interferes with pigments on chromatogram

Question 5

Suggest why veins of leaves should be avoided when taking leave sample

Answer 5

Do not contain much chlorophyll

Question 6

Suggest why pigment is applied several times

Answer 6

To get a strong enough colour in order to accurately identify pigments (tend to be faint)

Question 7

Suggest why it is important to allow each pigment to spot to dry before applying the next

Answer 7

Drop could expand too much and blur separation

Question 8

Suggest why it is important that the pigment spot is dry before running the chromatogram

Answer 8

Composition of solvent could be altered compromising its effectiveness

Question 9

Suggest why it is important the pigment spot is above the solvent and the chromatogram is not moved

Answer 9

• pigment does not wash off into solvent

- pigment moves up chromatogram rather than spreads

Question 10

Suggest why it is important chromatogram is straight

Answer 10

Ensures solvent runs straight so that distances run by pigment can be measured accurately

Question 11

Suggest why solvent front is marked immediately after chromatogram is complete

Answer 11

Solvent front is impossible to see once solvent has evaporated

Question 12

Suggest why a picture should be taken of the chromatogram

Answer 12

Pigments fades over time

Question 13

Suggest why it is better to compare Rf values rather than distance moved by pigment

Answer 13

• Rf value is a ratio so remains constant for pigments
• allows comparisons to be made
• solvents can travel different distances

Question 14

Suggest what is meant by chlorophyll

Answer 14

chlorophyll a + b

excludes accessory pigments