agar cube diffusion experiment results

Practical Biology

A collection of experiments that demonstrate biological concepts and processes.

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Published experiments

Effect of size on uptake by diffusion, class practical.

Set up cubes of agar jelly and see how far liquid penetrates them by diffusion over five minutes. Calculate surface area to volume ratio for cubes of different sizes and consider the problems faced by large organisms .

Lesson organisation

While the cubes are soaking, students can try the calculation of surface area to volume ratio for themselves. Many students struggle with these calculations and their implications, so it’s worth taking the time to go through the calculations methodically.

Apparatus and Chemicals

For each group of students: :.

Beaker, 100 cm 3 , 1

White tile,1

Paper towel, 1

Stopclock/ stopwatch

For the class – set up by technician/ teacher

Agar cubes 2 cm x 2 cm, 1 per group

Agar cubes 1 cm x 1 cm, 1 per group

Agar cubes 0.5 cm x 0.5 cm, 1 per group ( Note 1 )

Hydrochloric acid, 0.1 M, 20 cm 3 per group

Health & Safety and Technical notes

The low concentrations of sodium hydroxide and hydrochloric acid are below the concentration that needs to be labelled IRRITANT. Wear eye protection and rinse splashes off the skin.

Universal indicator is dissolved in ethanol, so it is highly flammable in the stock bottle, but not once added to agar. See CLEAPSS Hazcard and Recipe card.

Read our standard health & safety guidance

1 To make good solid agar, stir 2 g of plain (technical) agar powder into 100 cm 3 of water. Heat, in an aluminium water bath filled with boiling water, with stirring, until the agar solution boils, then allow to cool. Make the agar blocks in straight-sided dishes or ice cube trays. If students cut cubes themselves, each group will need a block 2 cm x 3 cm x 2 cm. If you have chosen to use hydrochloric acid to soak the cubes, make up the agar with 0.01 M sodium hydroxide and colour the agar with universal indicator (Hazcard 31, Recipe card 36) or phenolphthalein ( Note 2 ).

2 Phenolphthalein is described as LOW HAZARD on the CLEAPSS Hazcard. Refer to the CLEAPSS Recipe card (acid-base indicators): Dissolve 1 g in 600 cm 3 of IDA then make up to 1 litre with water.

Ethical issues

There are no ethical issues associated with this procedure.

SAFETY: Take care with the solutions used: wear eye protection and rinse splashes off the skin.

Preparation

a Make up plain (technical) agar with sodium hydroxide and universal indicator, or with sodium hydroxide and phenolphthalein. ( Notes 1 and 2 .)

b Cut agar cubes for the students, or provide them with a larger block to cut up. ( Note 1 .)

Investigation

a Collect agar cubes of different sizes – 2 cm x 2 cm, 1 cm x 1 cm and 0.5 cm x 0.5 cm – or cut cubes from the larger cube provided.

b Place the cubes in a beaker and cover with the diffusing solution.

c Start the stopclock.

d Leave the cubes for 5 minutes

e While you are waiting, complete the table with calculations for each cube. (See student sheet.)

f Pour off the solution. Rinse the cubes in a little water and blot the surfaces of each cube dry with a paper towel.

g Time how long it takes for the acid to change the colour of the indicator in each agar block. If the acid does not penetrate the largest block in the time available, cut the block and measure how far it has penetrated in the time.

Teaching notes

If this investigation is familiar to the students, some groups could investigate the effect of different shapes.

Health and safety checked, September 2008

Related experiments

Evaluating Visking tubing as a model for a gut

Australian school science information support for teachers and technicians

Laboratory Notes: Phenolphthalein/NaOH agar cube experiment

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Investigating the effect of cell size on diffusion (absorption) rate (speed) is a common investigation carried out by secondary students. It is modelled with a simple experiment using different-sized cubes of agar containing a pH indicator.

These laboratory notes provide a method for producing Phenolphthalein - sodium hydroxide agar and directions for conducting the experiment. Safety considerations and waste disposal instructions are also included.

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Laboratory notes: Phenolphthalein/NaOH agar cube experiment

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Diffusion and Osmosis

Instructor prep, student protocol.

• Diffusion in Agar
• Movement of Molecules Across a Semi-Permeable Membrane
• NOTE: In this exercise, you will be given agar containing an indicator chemical called phenolphthalein. When phenolphthalein is exposed to the normal alkaline conditions in the agar, it will look pink. But when it is exposed to neutral or acidic conditions, it changes from pink to clear. You will make different size and shaped agar cubes as a model for cells to study the impact of cell size and shape on diffusion rate.
• To make the first set of cells, measure out and cut a small cube of agar where each side measures one centimeter.
• Next, measure and cut out a medium cell cube with sides of 2 cm and a large cell of 3 cm on each side.
• Knowing the length of the sides of your cube cells, calculate their surface area using this equation, where lower case a represents the length of the sides: Surface Area = 6a^2
• Record these values in the appropriate column in Table 1. Click Here to download Table 1
• Then, use the same length value and the equation below to calculate the volume of each cube and add these to the table: Volume = a^3 HYPOTHESES: The experimental hypothesis might be that the acid will diffuse completely to the center of the small cell faster than the medium and large cells. The null hypothesis could be that the acid will diffuse to the center of the small and two larger cubes at around the same time.
• Add 100 mL of 0.1 M HCl to each of the three 400 mL beakers to make the diffusion baths.
• Working in a team, have one experimenter ready with the timer and the second and third experimenters ready to drop each cube into one of the beakers.
• When the first experimenter says go, simultaneously drop all three cubes into their respective beakers and start the timer.
• Observe carefully until one of the cubes becomes completely clear or 10 min have passed.
• Stop the timer, remove the agar cubes from the beakers and place the cubes into a Petri dish.
• Make a note of which of the three cells became clear or had the smallest remaining pink area. Then, also note which cell had the most remaining pink agar.
• Next, in Table 1, calculate the surface area to volume ratio for each cell. Surface Area: Volume Ratio = (surface area)/volume
• As the cell size increases, note whether the surface area to volume ratio increases or decreases. Also consider whether this correlates with your observation of the depth of diffusion into the agar cells. If cells rely on diffusion to deliver essential nutrients and molecules to the whole cell, discuss with your group if it would be better to have a smaller or larger surface area to volume.
• Now, with the remaining agar, cut three rectangular shaped blocks of different sizes and record their length, width, and height. This will test what happens when the shapes of cells are different.
• Calculate the surface area of your rectangular cells using the formula below, where length is l, width is w, and height is h. Surface Area = 2lw + 2lh + 2wh
• Then, calculate the volume of your rectangles using this formula: Volume = l * w * h
• Repeat the experiment by dropping the new shapes into the hydrochloric acid solution for 10 min or until one cube becomes completely clear.
• Remove the cell shapes from the solutions and observe the depth that the hydrochloric acid diffused into each of these cells, and which shapes have the smallest and largest remaining pink areas not reached by the solute.
• Using the surface area and volume data you recorded for your rectangular shapes, calculate the surface area to volume ratio of these cells. Surface Area: Volume Ratio = (surface area)/volume
• Consider whether these values correlate to which cells had the most and least complete diffusion. Additionally, discuss with the group whether these rectangular cells displayed a similar or different pattern of diffusion to that observed with the cube shaped cells, and what this might mean.
• Before beginning the experiment, add 250 mL of distilled water to each of four 1 L beakers.
• Then, label the beakers from 1-4, and add 0.5 mL of iodine to the first beaker. HYPOTHESES: In this experiment, the experimental hypothesis is that some of the solutes will be able to pass through the dialysis tubing membrane and others will not. The null hypothesis is that there will be no difference in the ability to diffuse through the dialysis tubing membrane between the solutes.
• To prepare the dialysis tubing, remove the pieces one at a time from the distilled water bath and tie a tight knot at one end of each tube. These tubes, when filled, will act as model cells with the dialysis tubing acting like the semipermeable membrane.
• Add 10 mL of starch solution to the first tube and tie off the open end, making sure to leave space in case the tubing expands during the experiment.
• Then add 10 mL of the NaCl and dextrose solutions to the second and third pieces of tubing, respectively, and tie off both tubes, again, leaving space in case of expansion.
• After adding 10 mL of distilled water and tying off the fourth tube, weigh each of your model cells.
• Record the initial weight values in grams and the colors of the starting solution in each tube in the appropriate columns of Table 2. Click Here to download Table 2
• After quickly rinsing the outside with tap water, place each piece of tubing in its corresponding beaker for 1 h at room temperature. NOTE: For example, the starch solution tube should be placed into the beaker containing the iodine.
• At the end of the diffusion period, weigh the tubes again.
• Then, observe the tubes carefully, noting any color changes.
• Record all of these data in Table 2.
• Next, to perform a Benedict's Reagent test for simple sugars, make a water bath by adding 250 mL of water to a 600 mL beaker and placing it onto a hot plate.
• Set the plate to high, to boil the water.
• Label two new glass test tubes as H 2 O and dextrose, respectively.
• Use a graduated cylinder to transfer 1 mL of solution from the water and dextrose beakers into the corresponding test tubes.
• Then, add 2 mL of Benedict's Reagent to each tube.
• Once the water is boiling, place each test tube into the water bath for 3-5 minutes.
• After this time, note the color of the solution in each tube.
• Then use this key to assess whether the test is positive or negative and record these data in the appropriate column in Table 2. Click Here to download Figure 1
• First, look at the mass of your four dialysis tube cells at the beginning versus the end of the experiment. Calculate the change in mass for each of the four cells and plot it onto a bar chart.
• Note which cells demonstrated the most change, and whether any of the cells appeared visibly different in size.
• For the experiment with the starch and iodine indicator, note whether there was a color change in the fluid in the artificial cell. Also consider whether there was a color change in the water in the beaker, and what both of these observations say about the properties of the dialysis tubing membrane.
• Finally, in the Benedict's Reagent test for dextrose, note whether this simple sugar was able to pass through the semipermeable membrane of the “cell” into the water in the beaker. Discuss with the class which of the molecules you think could and could not pass through the semipermeable membrane.

Caitlin's Blog

My riverside rapid digital portfolio, diffusion in agar cubes.

What determines the efficiency of diffusion throughout the model “cells”?

As the substance diffuses further inwards, more work will be required. The substance diffuses quickly near the surface but the further inwards it needs to go, the slower the process will be. This means that smaller cells will be more efficient. The percent diffusion of a small cell will be higher than the percentage of a large cell. In a small cell, the substance will have to travel a smaller distance in order to completely diffuse within the cube.

Data Table 

1) In terms of maximizing diffusion, what was the most effective size cube that you tested?

• The most effective size cube was the 1cm cube with a percent diffusion of 75%.  As we can see from our data as well as photos, the amount of space that was left untouched by the NaOH (as indicated by the lack of pink colouring) in the small cube, is much less in comparison to the two other bigger cubes.

2)Why was that size most effective at maximizing diffusion? What are the important factors that affect how materials diffuse into cells or tissues

• The smallest cube has a lot more surface area than it does volume. This allows for the NaOH to diffuse more effectively into the cube. The increase of surface area, provides lots of space for the NaOH to penetrate the cube.  The low volume of the cube means that the NaOH doesn’t have to penetrate as deep into the cube. In conclusion, surface area and volume are important factors in how materials diffuse into cells or tissues.

3)If a large surface area is helpful to cells, why do cells not grow to be very large?

• It’s true that if cells were to grow to be very large, they would have a large surface area. However, in order for efficient diffusion to occur, a large surface area  and a low volume must be characteristics of the cell. It is not the amount of surface area that matters, it is the ratio of surface area to volume. A bigger cube would have a large surface area, however the volume would be increased as well, thus making the ratio less ideal. Therefore, cells remain small in order to be more efficient.

4)You have three cubes, A, B, and C. They have surface to volume ratios 3:1, 5:2, 4:1 respectively. Which of these cubes is going to be the most effective at maximizing diffusion, how do you know this?

• Cube C would be the most effective at maximizing diffusion. Cube C, much like the 1cm Agar Cube used in the lab, has a large surface area and low volume and in turn has the highest ratio of surface area to volume. Since it has the highest ratio, it will be the most effective.

5)How does your body adapt surface area-to-volume ratios to help exchange gases?

• In our respiratory system, we have tiny balloon-like structures known as alveoli. The alveoli are responsible for moving oxygen and carbon dioxide in and out of our blood stream. They accomplish this task through diffusion. In order for this process to be done quickly and efficiently, they have adapted to have a large surface area and low volume.

6)Why can’t certain cells, like bacteria, get to be the size of small fish?

Bacteria are made of only one cell. As we have observed in this lab, in order for efficient diffusion to occur, there must be a large surface area and low volume. If bacteria were to get to the size of a small fish, that would mean the cell would be that large. A large cell of that size would not support efficient diffusion, thus greatly hindering the cell’s ability to function and survive.

7)What are the advantages of large organisms being multicellular?

For single-cell organisms, the size of their cell determines their overall size. As we have learned, a cell must maintain a large surface area and low volume to function effectively enough to survive. However, large organisms are made of lots and lots of cells. Despite the organism being of a large size, the cells themselves remain small enough to maintain the correct ratio to function and maintain life. It’s also worth noting that by having many different cells,  a large variety of functions can be performed simultaneously. This allows for the multi-cellular organism to have more capabilities than a single-cell organism.

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• CIE A-level Biology Revision Notes

Investigating Transport Across Membranes (A-level Biology)

Investigating transport across membranes, investigating diffusion.

We can investigate how diffusion occurs in biological cells by using cubes of agar jelly. The basic concept of this experiment is outlined below:

Table of Contents

• The agar jelly contains a pH indicator. We can make up agar jelly with an alkaline solution (e.g. sodium hydroxide) and add a few drops of phenolphthalein to it before the jelly sets. Phenolphthalein is a pH indicator which turns pink in the presence of alkaline solutions, thus, the jelly will have a bright pink colour.
• The agar jelly is placed in an acidic solution. Once the jelly has set, we can cut it up into cubes and place it in an acidic solution, such as dilute hydrochloric acid.
• The agar jelly is neutralised by the diffusion of the acid. The acidic solution will slowly diffuse into the agar jelly and neutralise the alkaline solution. As it does, the jelly will lose its pink colour and become colourless, as phenolphthalein turns colourless in non-alkaline environments.

We can alter different parts of this experiment to model how different factors affect the rate of diffusion.

Investigating the effects of surface area on diffusion

• Cut the agar jelly into different sized cubes to investigate the effects of surface area . Cut the jelly into cubes of different sizes and work out each cube’s surface area to volume ratio . For example, a cube with 2cm edges will have a surface area to volume ratio of 3:1.
• Place the cubes in the same volume and concentration of acid. Put the cubes into containers which hold the same volume and concentration of hydrochloric acid. Then measure the time it takes for the different cubes to go colourless.
• The cube with the largest surface area: volume ratio will go colourless the quickest. The cube with the largest surface area: volume ratio has the greatest amount of space available for the hydrochloric acid to diffuse into the jelly so it will be neutralised the fastest.

Investigating the effects of concentration on diffusion

• Place the agar jelly cubes in different concentrations of acid. Cut the agar jelly into equal sized cubes and put them in different containers, each with a different concentration of hydrochloric acid. Measure the time it takes for the different cubes to go colourless.
• The cube placed in the highest concentration of acid will go colourless the quickest. The cube placed in the container with the highest concentration will have the greatest concentration of acid being diffused into the jelly per minute. As such, it will go colourless the quickest.

Investigating the effects of temperature on diffusion

• Place the agar jelly cubes in different temperatures. Cut the agar jelly into equal sized cubes and put them in different containers, each with the same concentration of hydrochloric acid. Put the containers in water baths heated to different temperatures. Be careful not to heat the water baths over 65° as the agar jelly will melt.
• The cube placed in the highest temperature of acid will go colourless the quickest. As high temperatures speed up the rate of diffusion, the cube in the hottest container will be neutralised the quickest.

Investigating Osmosis

Osmosis is the movement of water molecules from an area of high water potential to an of low water potential by osmosis. Water potential is determined by the concentration of solutes in the solutions on either side of the cell membrane.

Investigations using plant tissue

This experiment involves placing plant tissue, e.g. potato cylinders, in varying concentrations of sucrose solutions to determine the water potential of the plant tissues.

• Prepare the different concentrations of sucrose solutions . Using distilled water and 1M sucrose solution, prepare a series of dilutions such that you now have 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0M sucrose. Place 5cm 3 of each dilution into separate beakers.
• Prepare equal sized pieces of potato chips. Using a cork borer, cut out 18 pieces of potato chips, all of equal sizes.
• Weigh the mass of the potato chips. Dry the potato chips gently with a paper towel. Divide them into groups of three and weigh each group.
• Place each group of potato chips in each solution . The potato chips should be left in the solutions for a minimum of 20 minutes.   All groups should be left in the solution for the same amount of time.

• Weigh the mass of the potato chips again. Once your desired amount of time has passed, remove the chips from the solutions, and dry them gently using a paper towel. Reweigh each group again.
• Calculate % change in mass. Using the mass of the potato chips before and after being placed in the solution, calculate the % change in its mass.
• Plot the % change in mass on a calibration curve. The calibration curve helps us determine the water potential in the potato sample. Plot the % change in mass against concentration of sucrose solution.   The point at which the curve crosses the x axis is when the sucrose solution is isotonic with the potato samples i.e. the water potential of the sucrose solution is the same as the water potential of the potatoes. At this point, there is no movement of water in or out of the potato. Overall:
• The potato samples in the dilute solutions will have a net increase in mass – the water potential is greater in the potato than in the sucrose solution, so water moves into the potato samples via osmosis.
• The potato samples in the concentrated solutions will have a net decrease in mass – the water potential is lower in the potato than in the sucrose solution, so water moves out of the potato samples via osmosis.

Investigations using Visking tubing

Visking tubing is an artificial membrane that is selectively permeable as it has many microscopic pores. This allows smaller molecules such as water and glucose to pass through it, while larger molecules such as starch and sucrose are unable to cross the membrane.

• Prepare three equal-sized pieces of Visking tubings. Run the tubing under tap water to soften it and knot each tubing on one end to create a bag.
• Place a rubber bung at the open end of the Visking tubing. Find rubber bungs with an opening in the centre that will fit the open end of the Visking tubing. Then seal the tubing using the bung and fix it in place using a rubber band.
• Prepare sucrose solutions with concentrations of 0.5M and 1.0M. You may wish to add a food dye to the 0.5M solution so that it is easier to see later on.
• Pipette in the 0.5M sucrose solution. Using a pipette or a syringe, fill each tubing through the opening of the rubber bung with the 0.5M sucrose solution. Make sure it is filled completely to the brim with no air bubbles.
• Insert capillary tubes into each of the tubings . Insert a capillary tube through the rubber bung’s opening. Mark the level at which the sucrose solution has risen to in the capillary tube.
• Place each Visking tubing into containers of different solutions. Prepare three beakers, each containing distilled water, 0.5M sucrose, and 1.0M sucrose. Place each Visking tubing into each of the beakers and leave them in for the same amount of time.
• Measure the change in liquid level. Mark the new liquid level on the capillary tube before removing the Visking tubing from its beaker. Measure the change in the liquid level. Overall:
• The liquid level of the Visking tubing placed in distilled water will have risen as the sucrose solution in the tubing is hypertonic to the water i.e. the sucrose is more concentrated. Thus, there is net movement of water into the Visking tubing via osmosis.
• The liquid level of the Visking tubing placed in 0.5M sucrose will remain the same as the solution inside the tubing and outside the tubing are isotonic i.e. the solutions are the same concentration.
• The liquid level of the Visking tubing placed in 1.0M sucrose will have decreased as the solution inside the tubing is hypotonic to the solution outside the tubing i.e. the solution inside the tubing is less concentrated.

Transport across membranes is the movement of substances such as ions, molecules, and fluids from one side of a biological membrane to the other. This process is crucial for maintaining cellular homeostasis and allowing cells to exchange materials with their environment.

Investigating transport across membranes is important because it helps us understand the mechanisms by which cells regulate the flow of substances in and out of the cell. This is essential for understanding cellular processes such as metabolic reactions, waste removal, and communication between cells.

There are several methods used to investigate transport across membranes, including: Diffusion experiments to study the movement of substances through the lipid bilayer Osmosis experiments to study the movement of water across a semi-permeable membrane Active transport experiments to study the movement of substances against a concentration gradient with the use of energy Electrochemical experiments to study the movement of ions across the membrane

Factors that can affect transport across membranes include the size of the substance being transported, the charge of the substance, the concentration gradient, and the presence of specific transport proteins.

Transport across membranes can be measured in a variety of ways, including measuring changes in substance concentration, changes in electrical potential, and changes in fluid movement.

The limitations of investigating transport across membranes include the difficulty of obtaining pure and intact biological membranes, the potential for damage to the membrane during experimentation, and the limitations of experimental techniques.

In A-Level Biology, knowledge of transport across membranes can be applied to understand cellular processes such as the movement of nutrients and waste, the regulation of cell volume, and the communication between cells. This knowledge is also important for understanding diseases and disorders related to the malfunction of transport processes, such as cystic fibrosis and diabetes.

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Practical: Surface Area to Volume Ratio

Practical: surface area to volume ratio.

In this experiment you will explore the relationship between surface area to volume ratios and diffusion. This must completed in your logbooks.

BACKGROUND INFORMATION

This experiment will model cells using agar cubes. The agar cubes have been prepared with an acid-base indicator called phenolphthalein. Phenolphthalein turns pink in the presence of bases, but is colourless when exposed to acids.

The phenolphthalein agar cubes will be prepared with a basic solution of sodium hydroxide (NaOH). This experiment will model diffusion into a cell using a solution of hydrochloric acid (HCl).

EXPERIMENT: THE EFFECT OF SURFACE AREA TO VOLUME RATIO ON THE EFFICIENCY OF DIFFUSION

To investigate the effect surface area to volume ratio on the efficiency of diffusion.

State your hypothesis for how the surface area to volume ratio for each cube will affect the efficiency of diffusion. Make sure you justify (provide a reason) for your prediction.

State the variables for this experiment. List at least three controlled variables.

Independent:

Controlled:

Phenolphthalein indicator agar cubes ( 0.5 cm 3 , 1 cm 3 , 2 cm 3 ) - you should have three cubes in total

50 mL 0.1M sulfuric acid per beaker

1 x 25 0 mL beaker

White cutting tile

Measure the dimensions of each agar cube and cut them down to the appropriate size.

Use these measurements calculate surface area and surface area : volume in Results Table 1 .

Place the agar cubes in a beaker. Cover the cubes with the sulfuric acid solution and use the stopwatch to time 5 minutes.

While waiting for the cubes to diffuse, calculate the surface area to volume ratio of each agar cube.

After 5 minutes pour the acid solution into the waste container. Rinse the agar cubes with water and then transfer them to the tile. Use a paper towel to blot them dry.

Measure the distance the acid diffused into each cube, and calculate the volume occupied by the acid.. Record your results in Results Table 2 .

Use Results Table 3 to calculate the percentage of each cube diffused.

Draw a graph of cube size vs. percentage diffused on the graph paper provided.

Table 1: Calculate the surface area to volume ratio for each cube.

Table 2: Record the distance of diffusion into each cube.

Table 3: Calculate the percentage volume diffused.

Based on your results, what can you conclude about the size of an object and its surface area to volume ratio? Why is this important?

Based on your results, what can you also conclude about cube volume and the rate of diffusion? Why does this occur?

A student stated the same shape scaled down should retain the same surface-area-to-volume ratio, the student’s reason being ‘the shapes stay the same’. Do you agree with this student? Explain your decision.

Describe one source of experimental error for this experiment and how this error affected your results.

Describe one way to improve this experiment’s procedure?

Please write a brief conclusion for your experiment.

Your conclusion should:

link back to the aim briefly re-state your results,

state limitations of the experiment and how they affect the results and,

provide suggestions for extending the investigation.

Cell Size Lab: Examining Surface Area to Volume Ratios

I’ve seen cell size labs that use different sized agar cubes prepared with a pH indicator. The cubes start pink and lose their color as they soak. (Here is a free version from Flinn if you are feeling ambitious!) Frankly with 3 preps a day this year, I didn’t have the time or energy to pour agar cubes. Instead I found a quick and easy way for students to see the same concept- using beets and bleach.

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Phenolphthalein agar: size and diffusion:

The original prac is in Biological Science, The Web of Life Teacher’s Guide Part 2, Exercise 10b.4 1. Approx 25g agar (increase from 18g) 4g NaOH Dissolve NaOH in 100ml water. Heat 900ml water in a beaker almost to boiling Add agar and continue boiling while stirring till dissolved. Allow to cool until temperature drops below 60C then add NaOH, still stirring. Add phenolphthalein until very deep pink – 6 droppers full. Allow to set then cut unto 1cm, 2cm and 3cm cubes, leaving some for cylinders. Do NOT refrigerate (it fades). Can be made 2-3 days in advance if you take it out of container and leave loosely covered with plastic wrap on glass Petri dish and store in chemical store shelf. 2. Calculate amount of agar required to make gel at 1 cm, 2 cm & 3 cm deep Recipe: Dissolve 18 g agar per litre in distilled water. Microwave in 1L beakers, with caution Add 4 g NaOH per litre after agar solution has cooled below 60C then add – phenolphthalein and pour into containers. Can last for 2 weeks in the fridge 3. We provided kits for students:- Kit set up in large plastic tray containing – 500 ml 0.1M Sulphuric Acid 2 pairs gloves plastic ruler plastic spoon paper towel large crystallizing dishes. These make removing the agar blocks easier than using beakers which are much deeper. knife small white plastic dish for cubes of phenolphthalein agar -1 cm3, 2 cm3 & 3 cm3 To make up 500mL of jelly for the cubes, weigh out 9g of agar and 2g solid NaOH. Dissolve the NaOH in 50mL of water. Heat 400mL of water in a beaker almost to boiling, add the agar and stir until dissolved. Allow to cool then, when the temperature falls below 60C, add the NaOH solution, stirring continuously, until the jelly is deep pink. Allow to set. The agar can then be cut into blocks of the appropriate sizes. 4. 36gm agar 1600ml water, enough for 24 groups with 1 x 3cm,1 x2 cm,1 x 1cm block each 9gm NaOH in 100mls water Phenolphthalein in dropper bottles Make agar with water & boil till dissolved until clear solution Dissolve NaOH in the water stirring constantly as this is exothermic Cool agar to 60 deg C then add NaOH soln – this is important Add phenolphthalein – a lot at first then drop by drop till agar is a deep pink colour Pour into containers to set. You can use the rectangular “Unitray” which has square corners. 5. 1800mL boiling water + 40g Agar, stir until dissolved Turn hot plate off, but leave stirrer on and wait until it drops to less than 60oC. Mix 6g NaOH in 150mL water and add to above Squirt in 200mL Phenolphthalein. Yes, 200mLs! Pour it into plastic dropper bottle trays, the “Unitray”. Leave overnight to set and the students then cut them to sizes 6. We changed the prac because the agar tends to fade. Make the cubes ahead of time. Students place them in 0.5% potassium permanganate solution for 5 minutes then slice as per the Student Prac book. It works really well.

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Agar Cube Lab Report

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