Chemical Tests and Chromatography Flashcards
State the chemicals used in the biuret test for proteins. (F)
- sodium hydroxide (NaOH)
- copper (II) sulfate (CuSO4)
State the chemicals used in the Benedict’s test for reducing sugars. (F)
- copper (II) sulfate (CuSO4)
- alkali
State the chemicals used in the Benedict’s test for non-reducing sugars. (F)
- copper (II) sulfate (CuSO4)
- alkali
- dilute hydrochloric acid (HCl)
- sodium hydrogencarbonate (NaHCO3)
State the chemicals used in the iodine test for starch. (F)
- iodine water/potassium iodine solution
State the chemicals used in the emulsion test for lipids. (F)
- ethanol
- water
Outline the method for the biuret test for proteins. (F)
- add the biuret reagent drop by drop to the liquid sample
- mix and leave to stand for a few minutes
Outline the method for the Benedict’s test for reducing sugars. (F)
- add an equal volume of Benedict’s reagent to the liquid sample
- heat in boiling water for 5 minutes
Outline the method for the Benedict’s test for non-reducing sugars. (F)
- do the Benedict’s test for reducing sugars and continue if negative result
- with a fresh sample, boil with dilute hydrochloric acid
- boil for a few minutes
- cool down and add sodium hydrogencarbonate
- when it has stopped fizzing, add Benedict’s solution
- warm for 5 minutes
Outline the method for the iodine test for starch. (F)
- add a few drops of iodine water to the sample
Outline the method for the emulsion test for lipids. (F)
- mix the liquid sample with ethanol
- mix the solution with water and shake
State the appearance of a negative and positive result for the biuret test for proteins. (F)
-ve: remains blue
+ve: turns purple
State the appearance of a negative and positive result for the Benedict’s test for reducing sugars. (F)
-ve: remains blue
+ve: turns green, yellow, orange, red, brick red
State the appearance of a negative and positive result for the Benedict’s test for non-reducing sugars. (F)
-ve: remains blue
+ve: turns green, yellow, orange, red, brick red
State the appearance of a negative and positive result for the iodine test for starch. (F)
-ve: remains brown
+ve: turns blue/blue-black
State the appearance of a negative and positive result for the emulsion test for lipids. (F)
-ve: remains clear
+ve: white emulsion forms
Define the terms “qualitative test”. (F)
Shows whether a substance is present or not.
Define the terms “semi-quantitative test”. (F)
Shows a substance is present, and can give a rough indication of how much is present.
Define the terms “quantitative test”. (F)
Shows a substance is present, and how much is present.
Explain how the Benedict’s test for reducing sugars can act as a semi-quantitative test (include the colour range that can be seen with different concentrations of glucose).
- the more reducing sugar present, the more precipitate is formed
- solution is a mix of blue Cu^2+ ions and brick-red Cu^+ ions
- if green, low concentration of glucose
- if yellow, medium concentration of glucose
- if red, high concentration of glucose
List 5 examples of reducing sugars.
- glucose
- fructose
- galactose
- lactose
- maltose
State one example of a non-reducing sugar.
Sucrose
Explain why reducing sugars are called “reducing” sugars. (S+C)
They act as a reducing agent by taking an electron from the Cu^2+ ions to reduce them and form Cu^+. This oxidises the reducing sugar.
Describe what “reagent test strips” are with an example of how they are used.
They can be used to test for the presence of reducing sugars, and have a colour-coded chart to show the concentration.
Can be used in urine tests for diabetes.
Describe, in principle, how a colorimeter works. (F)
It measures the absorbance or transmission of light by a coloured solution.
Define the terms “percentage absorbance” and “percentage transmission” in relation to data provided by a colorimeter.
Percentage absorbance: the light absorbed by a coloured solution
Percentage transmission: the light transmitted by a coloured solution
Explain how to convert percentage transmission into percentage absorbance.
Percentage absorbance = 100% - percentage transmission
Explain how a colorimeter can be used in a quantitative test for reducing sugars.
- do the Benedict’s test on a sample
- filter to remove the precipitate
- use a colorimeter to find the percentage transmission
Define the term “calibration curve” and explain how they are used to identify the concentration of glucose in a solution.
Calibration curve: a graph for determining the concentration of a sample by comparing to known concentrations
- use serial dilution to make several solutions of different concentrations
- do the Benedict’s test on them and a sample of distilled water
- filter to remove the precipitate
- use a colorimeter to find the percentage transmission and make a calibration curve
- do the Benedict’s test on a sample of unknown concentration, filter and find the percentage transmission and compare to the curve
Describe, in principle, how biosensors work.
- Molecular recognition: interaction with specific molecule
- Transduction: causes a change in transducer that detects change.
- Display: the visible, quantitative/qualitative signal
Describe the purpose of chromatography. (F)
Separation of individual components of a mixture.
Describe a step by step method for conducting chromatography to identify the components of an unknown mixture of dissolved substances (e.g. proteins, carbohydrates, amino acids vitamins or nucleic acids). For each step explain why it is necessary and why it is done the way it is. (F)
- Draw a pencil line on the surface about 2cm from the bottom
- prevents the mixture from being washed away
- can calculate how far the component travelled - Place a spot of the mixture on the pencil line and allow to dry
- can see how far the component travels - Place the surface in the solvent below the pencil line
- doesn’t wash mixture away - When the solvent almost reaches the top, remove and allow to dry
- can see how far solvent travels - Measure distances from the line and calculate the Rf value
State what “Rf” stands for in “Rf value”.
Retention value
Explain how to use Rf values to identify the molecules present in a solution. (F)
Rf = distance travelled by component / distance travelled by solvent
Compare the Rf value to a database of known values to find the component present.
Explain what determines how far a particular molecule travels in chromatography.
Attraction to the mobile/stationary phase.
Mobile phase = liquid state.
Stationary phase = solid state.
- If it is more attracted to the mobile phase, it travels further.
- If it is more attracted to the stationary phase, it doesn’t travel as far.
