PAG 05 - Colorimeter OR Potometer

How long should a colorimeter be left to stabilize after switching it on?

• 5 minutes.

Which filter should be selected for a Benedict’s test and why?

• A red filter; it is the complementary color to the starting blue solution, ensuring maximum light absorbance/sensitivity.

What must be done to a Benedict's sample before placing it in a cuvette?

• Centrifuge the solution or allow it to sit to precipitate out the copper(I) oxide solid.

How is the colorimeter "zeroed" or calibrated?

• By using a cuvette filled with distilled water.

What are the requirements for the cuvette and the solution inside it for an accurate reading?

• The light must pass through the clear sides.

• The sides must be clean.

• There must be no bubbles in the solution.

• The cuvette should be filled about 3/4 full.

What is the relationship between solution paleness and light transmission?

• A paler solution absorbs less light, resulting in a greater percentage transmission.

Define a "standard solution" in a colorimetry context.

• A solution of a known concentration.

When creating a calibration curve for reducing sugars, what must be true about the Benedict’s solution?

• It must be added in excess.

What are the axes for a reducing sugar calibration curve?

• X-axis: Reducing sugar concentration.

• Y-axis: Percentage transmission.

In a Benedict’s test, does higher sugar concentration lead to more or less transmission?

• More transmission (because the resulting supernatant is paler/less blue).

What does a potometer actually measure vs. what is it used to estimate?

• It measures water uptake, which is used to estimate the rate of transpiration.

List three limitations of using a potometer to measure transpiration.

• Some water is used to maintain cell turgidity.

• Some water is used in photosynthesis.

• The plant may take up less water as it begins to die after being cut.

Why is the plant shoot cut at a slant/angle?

• To increase the surface area available for water uptake.

How do you prevent air from entering the xylem during potometer setup?

• Cut the shoot underwater and assemble the entire apparatus underwater.

How do you ensure the potometer apparatus is airtight and watertight?

• Use screws or apply petroleum jelly to the joints.

What two things must be done after the potometer is assembled but before recording begins?

• Dry the leaves.

• Allow time for the plant to acclimatize.

How is a single air bubble introduced into the potometer?

• Remove the end of the capillary tube from the water beaker until one bubble forms, then replace it in the water.

How is the rate of bubble movement calculated?

• Divide the distance traveled by the air bubble by the time taken.

Describe how to assemble and use a potometer to estimate the rate of transpiration. (6 Marks)

• Assembly: Cut a leafy shoot underwater and at a slant to prevent air entering the xylem and increase surface area.

• Setup: Submerge the potometer and insert the shoot while underwater to ensure no air bubbles are trapped.

• Sealing: Remove from water and ensure the apparatus is airtight and watertight using petroleum jelly or tightened screws.

• Preparation: Dry the leaves to ensure a steep water potential gradient and allow the plant to acclimatise before starting.

• Execution: Introduce one air bubble by briefly removing the capillary tube from the water; record its starting position.

• Measurement: Use a stopwatch to record the distance the bubble moves over a specific time interval (e.g., 30 mins) and divide distance by time to calculate the rate.

Describe how to use a colorimeter to produce a calibration curve and find the concentration of an unknown reducing sugar. (6 Marks)

• Preparation: Switch on the colorimeter for 5 minutes to stabilise and select a red filter (complementary to Benedict’s blue).

• Calibration: Set the colorimeter to zero absorbance (100% transmission) using a cuvette of distilled water.

• Standard Solutions: Prepare a range of known concentrations (standard solutions) of the reducing sugar.

• Benedict’s Test: Perform the test on each standard (using excess Benedict's); centrifuge the resulting samples to remove the precipitate.

• Graphing: Measure the percentage transmission of each standard and plot a graph of transmission (y-axis) against concentration (x-axis).

• Analysis: Measure the transmission of the unknown sample, then use the calibration curve to find the corresponding concentration.