qualitative and quantitative testing Flashcards
what is a hypothesis
your predcition
what is validity
how well your experiment tests your hypothesis (includes all control variables, accurate and reliable results)
what is reliability
achieved by repeating the experiment and having reproducible data
what is accuracy
a measurement close to the true value - improved by taking a narrower range of readings within a set range
what is precision
all results are close together
what is an independant variable
the variable you change in an experiment
what is a dependant variable
the variable that is measured
what is a controlled variable
the part of an experiment in which the independant variable is not applied
what does qualititative testing help identify
the presence or absense of an ion or compound in solution
what does qualititative testing rely on
the biological molecules in the sample passing into the solution
how do we test for biological molecules
you will need to grind and squash the food samples and mix them with a small volume of water (lipids in alcohol)
wear eye protection when carrying out all of these tests
what is a positive control
positive sample, used to check that all reagents are working. allows comparison with test samples
what is a negative control
has same conditions as the test samples without the independant variable. used to check that the independant variable is causing the change
how do we test for starch
add iodine solution (in potassium iodide) to a sample
what is the positive result for starch
colour change from yellow/brown/orange to blue-black
what causes the colour change in the iodine test
when dissolved in iodide, the iodine forms a triiodide ion which slips into the middle of the amylose helix. this causes the colour change
what is the difference between a reducing sugar and a non reducing sugar
reducing sugars act as reducing agents in chemical reactions (donates electrons to other molecules). non reducing agents do not act as reducing agents
which sugars are reducing
all monosaccharides and some disaccharides such as maltose and lactose
what makes a sugar reducing
they can give electrons to other molecules
what happens if you heat a reducing sugar with Benedicts solution
there is a colour change from blue to green to yellow to orange red
what does Benedicts solution contain
copper (II) ions - Cu2+
what forms the orange red colour
benedicts solution is a blue colour. when it reacts with the sugar it donates an electron and is reduced to Cu+ forming a orange red copper (I) oxide - Cu2O
what is the orange red mixture
a precipitate because it comes out of solution and forms a solid, suspended in the reaction mixture
what happens if you use Benedicts solution in excess
the intensity of the red colour is proportional to the concentration of the sugar. its green if little precipitate is formed, orange red if lots of precipitate is formed
how else can you test for reducing sugars
commercially manufactured test strips. you simply dip the strip into the test solution and compare the colour with the calibration card supplied. this tells you whether the reducing sugar is present or absent
what are non reducing sugars
they dont act as reducing agents in chemical reactions.
which sugars are non reducing
most disaccharides (sucrose) and some simple polysaccharides
how do we test for non reducing sugars
- we have to hydrolyse the bond bond first (to free up the reducing groups)
- heat with hydrochloric acid to break the glycosidic bond and expose the reducing groups
- then heat it at 60°C with benedicts reagent
- cool solution and use sodium hydrogen carbonate to neutralise it
- test for reducing sugars again
what does a positive test in the 2nd reducing sugars suggest
that there was non reducing sugars in the first place
what can happen if a sample contains reducing and non reducing sugars
if you have a positive test for reducing sugars from your first sample, you can go test for non reducing sugars in an equal sized second sample. if its present the precipitate from this second sample will have more mass then the precipitate from the first sample. you can extract the precipitate from the mixture by filtration
how do we test for lipids
emulsion test
- take a sample and mix it through with ethanol. any lipid will go into solution in the ethanol (lipids are insoluble in water)
- filter
- pour the solution into water in a clean test tube
- a cloudy white emulsion indicates the presence of lipids
- this is made of tiny droplets that come out of solution when mixed with water
how do we test for proteins
the biuret test
- if protein is present, the colour changes from blue to lilac.
what causes the colour change in the biuret test
colour formed by a complex between the nitrogen atoms in a peptide chain and copper 2+ ions
what does the biuret test actually test
the presence of peptide bonds
what is the quantitative test for carbohydrates
colorimetry
what happens if more sugar is present
- amount of precipitate will increase
- amount of copper (II) ions remaining in solution will decrease
how does a colorimeter work
- by shining light through a sample. - we would use a centrifuge to separate the precipitate and any excess benedicts solution (supernatant)
- using a pipette we can take the supernatant and place it in a cuvette which is placed in the colorimeter
what could affect the transmission of light
leaving greasy fingerprints on the cuvette
what is transmission
how much light gets through
what is absorption
how much light has been absorbed
what are colour filters used for
greater accuracy
what should you do when using a colorimeter
zero the device between each reading by placing an appropiate ‘blank’ sample to reset the 100% transmission/absorption
the more reducing sugar there is…
….the more Cu2+ is converted to Cu+ to form a precipitate. results in more red Cu+ and less Cu2+
what happens to the red precipitate
its filtered off
the more precipitate filtered and removed…
…the paler blue the solution is
the higher the glucose concentration…
….the lower the absorbance and the higher the transmission when using a colorimeter
why is the colorimeter classes as ‘semi quantitative’
you can compare how much sugar was contained in different samples
how would you know which filter to use for absorption
depending on the colour of the solution you would use the opposite colour in the spectrum
steps for using colorimeter
1) zero colorimeter
2) using blank
3) use red filter
4) use known concentration of lactose
5) serial dilution
6) construct calibration curve
7) test unknown sample (using the same method)
8) read from calibration curve to determine the unknown concentration
