Science Buddies: "Ask an Expert"

Hi, I'm a high school student with access to a lab and I wanted to see how much iodine I can extract from different types of seaweed in the form of black crystals.

Ultimately I am following a procedure I found on this website "http://www.nuffieldfoundation.org/pract ... ne-seaweed"
but I changed and added a couple of steps to suit my experiment:

1. Weigh evaporating basin. Record mass.
2. Weigh variable. Record mass.
3. Pile the seaweed up on the tin lid supported by a ring clamp and heat with a strong Bunsen flame.
4. Take a beaker and fill it with 20ml of deionized water. When all of variable has been reduced to ash, collect the residue and add it to the beaker.
5. Boil the ash, and filter through a funnel while hot. Collect the clear filtrate in a second beaker and allow to cool.
6. Turn off the Bunsen burner.
7. Add about 2ml of dilute sulfuric acid to the solution, then add 10ml of 1.6M hydrogen peroxide solution. A deep brown colour of iodine is formed as hydrogen peroxide oxidises the iodide ions present to iodine.
8. Transfer the mixture to a separating funnel and add 10 - 20 ml of cyclohexane. Stopper the separating funnel, secure it with your thumb, and shake vigorously for about 30 s. With the separating funnel inverted, release any pressure that has built by opening the tap briefly
9. Clamp the funnel and allow the layers to separate. The cyclohexane will form a layer on top of the aqueous layer, and be coloured purple by the iodine now dissolved in it.
10. Run off the lower aqueous layer and discard down the sink with running water.
11. Run the purple cyclohexane layer into an evaporating basin, and set aside to evaporate in the fume cupboard. DO NOT HEAT!
12. Iodine crystals will form slowly.
13. Weigh evaporating basin. Record mass.

Instead of having 1.6M of hydrogen peroxide, I have 3%, which is 0.9M. Is it still possible to do this experiment with this concentration (0.9M) of hydrogen peroxide?

Also how much dried seaweed would I need to burn to extract a recordable mass of iodine from the iodine crystals? And how would the amount I would have to use of sulfuric acid, hydrogen peroxide, and cyclohexane change if I burn a higher mass of seaweed?

I am planning to do this experiment with 4 types of seaweed: Dried Laver, Dried Konbu, Dried Nori, and Dried Wakame.

(Urgent, Please reply as soon as possible)

Hi,

Welcome to Science Buddies! This is a really great chemistry project.

The 3% hydrogen peroxide will oxidize the iodide to iodine, but it might take longer or require a larger volume. I recommend doing a small pilot experiment using a known iodide solution if this is available. Try using 50% more hydrogen peroxide for this step.

The original directions specify using a 50 cm length of kelp, and I would guess this would be about 100 grams of fresh algae, or about 15 grams of dried algae. The ratio of fresh weight: dry weight for kelp is about 6.5.

http://www.fao.org/docrep/field/003/AB724E/AB724E07.htm

Again, however, you are working on an original idea here, so the pilot experiment would be helpful to verify results before you proceed with the complete experiment.

I recommend using the quantities of reagents specified in the original experiment. If you wanted to purify a larger quantity of iodine, you could scale up and keep the proportion of reagents the same.

Scaling up the purification would actually be a chemical engineering project, so might require multiple experiments. Do complete the lab scale project first, and run your samples in duplicate if possible.

How are you going to measure the iodine?

Donna Hardy

Hi! Thank you so much for the reply. It helped rid doubts I had about the experiment succeeding.

The 3% hydrogen peroxide will oxidize the iodide to iodine, but it might take longer or require a larger volume. I recommend doing a small pilot experiment using a known iodide solution if this is available. Try using 50% more hydrogen peroxide for this step.

- Would a pilot experiment with either a dissolved solution of potassium iodate or iodized salt work?

Again, however, you are working on an original idea here, so the pilot experiment would be helpful to verify results before you proceed with the complete experiment.

- I'm still unsure on what I need the pilot experiment for. Is it just to see that the experiment does work to extract iodine?

I recommend using the quantities of reagents specified in the original experiment. If you wanted to purify a larger quantity of iodine, you could scale up and keep the proportion of reagents the same.

- I want to have extract a large amount of iodine, so I'm planning to use 50 grams of each variable.
Considering that this is the case, I should use about 3 times as much reagent specified in the original experiment correct?

How are you going to measure the iodine?

- We have a precision balance up to 4 decimal places.

After performing this experiment and receiving iodine crystals, is there any following experiment I can do with the iodine?

Thank you again for your reply.

Hi Grandtricks2,

Potassium iodate will not work; potassium iodide will work. If the salt has been supplemented with iodide, then it should be suitable, however, salt is very different from your actual sample. The best way to make sure that all of your reagents are suitable and would be by trying experiment with a small piece of the fresh kelp described in the original experiment, this verify that all of your reagents are suitable and that you are doing the experiment correctly. Or, you could try a sample of the dried kelp.

A pilot experiment will save you time in the long run. It's a quicker way to get experience in doing the experiment and find out what you need to change. It will be a lot of work to process the 4 larger samples and you would not want to find out that the experiment is not working after you've done all of the work with the 4 larger samples.

The precision balance is excellent. Be sure to weigh the sample before you start so you can calculate the percent yield for your samples.

Besides weighing the iodine and verifying the physical description of the element, you could use it to do a demonstration of the starch iodine test. You can use this as an identity test to verify that you have isolated iodine. The following project on the Science Buddies website includes a protocol; you will only need some laundry starch for this test.

https://www.sciencebuddies.org/science- ... ml#summary

Donna Hardy

Hi! I'm a 2nd year college student and I chosen this topic for my Investigatory Project. I tried doing this but after adding the hydrogen peroxide, it didn't turn into a brown color. Please help me. What do you think I got wrong?

Hi Missdeeyalong »,

The brown color forms when the iodide is oxidized to iodine. If your sample did not turn brown, there may have not been a significant concentration of iodide in the seaweed, or the hydrogen peroxide had degraded before you used it. I recommend getting a new seaweed sample and a new bottle of hydrogen peroxide to help identify the source of the problem.

Donna