PAG 9.2 Flashcards
Equipment
- Distilled water
- Ethanol
- 12 test tubes
- 2 x 5 cm3 syringes
- 8 dropping pipettes
- Liquid samples to be provided in labelled beakers:
o Tap water
o Vegetable oil
o 0.5 mol dm-3 sucrose solution [Note: To make 100 cm3 of 0.5 mol dm-3 sucrose solution dissolve 17.12 g sucrose in distilled water to a final volume of 100 cm3]
o Protein suspension [see note below]
- Solid samples to be provided in labelled beakers:
o Cheese
o Pasta softened by pre-soaking
o Bread
o Apple
- Glass rod
- Paper towels
- Black paper
method
Method
- Put a few drops of each liquid sample into its own labelled test tube.
- Add 2 cm3 ethanol to each of the four samples and shake thoroughly.
- Next, add 2 cm3 distilled water to each sample and shake gently to mix.
- Observe the appearance of each sample. You may find it helpful to place a piece of black paper behind the sample when viewing it.
- Record your observations and your conclusions about the presence or absence of lipids in these samples.
- Next, place a small piece of each solid sample into its own labelled test tube.
- Add 2 cm3 ethanol to each of the four samples.
- Using the glass rod gently mash each sample. Ensure you clean the rod between samples to avoid cross-contamination.
- Shake each tube thoroughly.
- Allow the solid to settle to the bottom of the tube.
- Carefully pipette the ethanol from each sample into a fresh, labelled tube, leaving the solid sediment behind.
- Add 2 cm3 distilled water and shake gently to mix.
- Observe the appearance of each sample. You may find it helpful to place a piece of black paper behind the sample when viewing it.
- Record your observations and your conclusions about the presence or absence of lipids in these samples.
- An effective qualitative test must give a clear positive result whenever the substance being tested for is present. This means it avoids ‘false negatives’. It must also not give a positive result due to the presence of some other substance (this would be a false positive). Explain, in terms of their solubility in different solvents and your knowledge of the emulsion test, why you do not see false positives due to: - Monosaccharides and disaccharides - Starch - Nucleic acid - Protein
Monosaccharides and disaccharides dissolve in water and so will not form a precipitate (or emulsion).
Starch is not soluble in ethanol, so even though it could form a visible emulsion in water it will not have been extracted by the ethanol. Nucleic acids are insoluble in ethanol (so they will not be extracted) and soluble in water (so they would not precipitate to give an emulsion). Proteins are insoluble in ethanol so they will not be extracted.
- The emulsion test is qualitative. How could it be made the basis for a semi-quantitative test (giving an indication of whether lipids are present at high, medium or low concentration) or even a fully quantitative test for lipids?
- For a semi-quantitative test students should be attempting to both control the quantity of each sample tested and identify a way in which the resulting positive result (if any) can be scaled low/medium/high. For example students might specify that an equal mass of each solid sample should be taken, to ensure comparability of results, and that the height of the emulsion formed could be used to give an indication of the quantity of lipid.
For a quantitative test the requirement to standardise the sample is equally important. In addition some way should be suggested to produce quantitative output. Students might suggest filtering and weighing the precipitate. An alternative could be measuring the density of the emulsion in a well-mixed sample using a colorimeter.
