3.4 Testing For Carbohydrates And Other Molecules Flashcards
What tests are there
- Benedictus test for reducing sugars
- Benedict advanced test for non reducing sugars
- iodine test for starch
- biuret test for protein
- emulsion test for lipids
What is Benedictus solution chemically
-why are reducing sugars called it and what sugars are they?
1) An alkaline solution of copper (II) sulfate
2) reducing sugars are sugars that reduce the copper (II) ions into copper 1+ ions
3) all Monosaccharides are reducing sugars and some dissachairdes like MALTOSE AND LACTOSE
How is a Benedictus test carried out ?
1) Put sample in boiling tube , add equal amount of Benedictus in
2) boil in water bath for 2-5 mins, (some times temp at 70 is fine )
3) if positive forms a GREEN-YELLOW-ORANGE-BRICK RED PRECIPITATE
- if negative it is not reducing sugar could be non reducing , stays BLUE
How does a Benedictus test work (science )
- Benedictus = alkaline solution copper (II) sulfate, here these copper 2+ ions are blue
- reducing sugars will REDUCE these copper 2+ ions to copper 1+ ions , which Are RED PRECIPITATES
- these then will form a colour based on how concentrated glucose was , more then orange red less then blue green
How to get quantitative results in different ways in Benedictus
- why use distilled water
1) filter out precipitate , let it dry COMPLETELY, then weigh them for quantitative results
2) filter predominate out and with solutions less, lighter the colour = more concentrated glucose solution was there as more copper 2+ blue ions where reduced and less remain .
- put in cuvette, use distilled water to CALLIBRATE so that all values are measured to the same standard and are accurate and then measure for either absorbance or transmission
- higher glucose conc= high transmission = low absorbance
- colorimeter set to 630nm wavelentgh
How to find unknown concentration ?
1) first make series of known concentrations (can do this in serial dilutions )
2) do Benedictus tests on them and get quantitative results using colorimter and plot results , draw a calibration curve
3) now you can use unknown result and predict its concentration.
What you measure always on y axis, so absorbance
How to do serial dilutions ?
- label test tubes
- out of one beaker with full concentration add 1cm3 to other Beamer and Fill with 9cm3 water
- repeat this with all beakers to keep dilution bu factor of ten, work out concentrations.
Why cant non reducing sugars reduce (don’t need to know but)
Have 1-2 alpha link so reducing power is lost
How to do Benedictus test for non reducing
1) add dilute hydrochloric acid
2) HEAT CAREFULLY in water
3) now neutralise it with sodium
Hydrogencarbobate
4) now the 1-2 bond broken, separated into monosaccharides
5) then do the Benedictus test , if positive now it shows there was non reducing
How to do protein biuret test ?
How to do the hard way
1) add biurets solution which is an ALKALINE SOLUTION OF COPPER SULFATE
2) if proteins are present , due to the peptide bonds colour will go from blue to lilac
- 1) add same volume kf solution of 10% NaOH
2) add copper sulfohate drip by drip until it changes to BLUE
3) wait five mins to see if turns Kilmalcolm
Iodine test for starch? WHERE IS IODINE IN SOLUTION
For plant?
Add iodine solution (potassium iodide) to the sample
- if present it will turn from browny orange to blue black
(For plants Dw starch, boil to kill , add alcohol to remove chlorophyll, do the test)
Emulsion test for lipids ?
1) lipids and water won’t mix but adding ethanol will cause an emulsification
2) thus shake it with ethanol
3) pour solution into cold water
4) if positive a milky emulsion forms
Non reducing sugars
Sucrose for now - lactose and maltose not
