PRACTICALS Flashcards
How do you test for reducing sugars
- Add 2cm of the food sample (in liquid form) into a test tube
- heat with 2cm of Benedict’s reagent in a water bath at 95°C - 100°C for 5 minutes
- if reducing sugars are present there will be a colour change from blue to green, yellow, brown/orange to brick red depending on the concentration of reducing sugars
Why does Benedict’s reagent change colour
Benedict’s reagent is an alkaline solution of copper (ii) sultate. When a reducing sugar is heated with Benedict’s solution, the reducing sugar reduces the blue Cu 2+ ions in the copper sultate, to produce a brick red precipitate called copper oxide
→ when cu 2+ accepts electrons from a reducing sugar it forms copper oxide which is red
The more reducing sugar is present, the more copper oxide precipitate is formed and the less blue Cu 2+ ions are left in the solution
If the colour is —— the concentration of reducing sugars is ——
Stays blue → none
Green → very low
Yellow → low
Brown/ orange → medium
Brick red → high
How do you test for non-reducing sugars
- Heat the sample with Benedict’s reagent in a water bath at 95°C - 100°C for 5 minutes
- if no colour change add 2cm of the food sample to a different, clean test tube
- next add 2cm of dilute HCL to the test tube in order to hydrolyse the disaccharide into its monosaccharide components
- place in a water both at 95°C - 100 °C for 5 minutes
- slowly add some sodium hydrogen carbonate (enough until it stops fizzing) to neutralise the HCL because Benedict’s reagent won’t work in acidic conditions
- use a pipette to take some of the sample into a dipping tile and add some universal indicator to check that its neutral (it should go green)
- again, heat with Benedict’s reagent in a water bath at 95°C to 100°C for 5 minutes
- if a non reducing sugar is present it will go from blue to orange-brown
What is an eyepiece graticule
eye piecegraticule is a glass disc fitted into the eyepiece of the microscope. It is marked with a scale from 0 - 10. It will be calibraled to the stage micrometer. It allows you to measure organelles on any magnification.
What is a stage micrometer
The stage micrometer is used to calibrale the eye piece graticule. A stage micrometer consists of a microscope slide on which is engraved a fine and accurate scale.
How do you test for starch
- Place a sample of the food into a dipping tile
- use a pipette to place a few drops of iodine onto the sample
- if the sample contains starch it will turn black
PAG 9 Serial dilution
- Label six test tubes with the permanent marker pen as 0.32 M, 0.16 M, 0.08 M,
0.04 M, 0.02 M, and 0.01 M, and place into a test tube rack. - Using the 10 cm’ measuring cylinder, measure out 10 cm’ of the 0.32 M glucose solution. It is important to be as accurate as possible, so use one of the dropping pipettes to make sure the meniscus of the solution is exactly on the 10 cm’ line of the measuring cylinder
- Add the 10 cm of 0.32 M glucose solution to the test tube labelled 0.32 M.
- Using the other 10 cm measuring cylinder and a dropping pipette, add 5 cm’ of distilled water to each of the other five test tubes.
- Using a 5 cm syringe, take out 5 cm’ of the solution in the 0.32 M test tube and add it to the test tube labelled 0.16 M.
- Invert the test tube three times. You do this by putting a test tube bung into the test tube and turning the test tube upside down.
- Using a new 5 cm’ syringe, take out 5 cm’ of the solution in the 0.16 M test tube and add it to the test tube labelled 0.08 M. Invert the test tube three times.
- Repeat step 7 for the 0.04 M, 0.02 M, and 0.01 M test tubes
PAG 9 carrying out reducing sugar test on serial dilution
- Take 2 cm from the solution in the 0.32 M test tube using a 2 cm’ syringe, and add it to a new test tube. Label this as 0.32 M B. Using a 1 cm’ syringe, add 1 cm of Benedict’s quantitative solution.
- Do the same with the solutions in the 0.16 M, 0.08 M, 0.04 M, 0.02 M, and 0.01 M test tubes, labelling them as 0.16 M B, 0.08 M B, 0.04 M B, 0.02 M B, and 0.01 M B.
- Add these six test tubes to a water bath set to 95 °C for 5 minutes.
- Remove the test tubes and place back into a test tube rack. Allow the precipitate to settle to the bottom of the test tube OR filter the solution to remove the precipitate and only be left with the colour of the Benedict’s solution.
PAG 9 measuring absorbance with a colorimeter
-Fill a cuvette almost to the top with the solution from the 0.32 M B test tube. Do the same with the solutions from the 0.16 M B, 0.08 M B, 0.04 M B, 0.02 M B, and 0.01 M B test tubes.
- Fill another cuvette almost to the top with distilled water. This will be your blank.
- Zero the colorimeter by setting it to 0.0 and the filter is set to 680 nm (red filter).
- Place the blank cuvette into the colorimeter and press the reading button. This should give an absorbance of 0.0.
- Place the 0.32 M cuvette into the colorimeter and press the reading button.
Record the absorbance. Do the same with the other cuvettes.
PAG 9 Finding the concentration of glucose in a urine sample
- Take 2 cm of the urine sample and add to a test tube. Label this as U.
- Add 1 cm’ of Benedict’s quantitative solution to the test tube.
- Place the test tube in a water bath set to 95 °C for 5 minutes.
- Remove the test tube and place it into the test tube rack. Allow the precipitate to settle to the bottom of the test tube.
- Fill a cuvette almost to the top with the solution from the test tube.
- Use the blank cuvette and repeat steps 3 and 4 from ‘Measuring the absorbance with a colorimeter’.
- Place the U cuvette into the colorimeter and press the reading button. Record the absorbance.
- On the calibration curve graph, draw a line from this absorbance on the y-axis to the line of the graph, and down to the x-axis. The point on the x-axis where this line touches will be the concentration of glucose in the urine sample.
PAG 9 Drawing a calibration curve graph
Plot the absorbance of each concentration of glucose (x-axis) against absorbance (y-axis). Join the points together using a sharp pencil and a ruler, or draw a line or curve of best fit.
