Power of Purple

[guitarpic]

I have tried the experiment many times and it just wont work. I have my oil and add three drops of iodine into each one. The iodine just settles or floats and does nothing. Am I doing something wrong and is there anything that you can tell me that might help.

[deleted-71417]

Hi,

I have no personal experience with this experiment, but the most likely issue is this statement from the Science Buddies Project writeup:

“7% tincture of iodine; available at some drug stores and animal care stores; also available from online stores, such as www.amazon.com.
Note: The more commonly available 2% tincture of iodine will not work for this experiment.”

I presume we are discussing this experiment:
https://www.sciencebuddies.org/science- ... p062.shtml

I hope this helps. If not then post back and we will flag Science Buddies professional staff to see if they can help.

Good luck!!

Barrett L Tomlinson

[guitarpic]

Yes I have tried using what they told me and the iodine does not turn purple. Now What?

[deleted-71588]

What color is your iodine to start with before you add it to anything? It is supposed to start out purple.
Try getting a raw potato and cutting a slice and dropping a couple of drops of your iodine solution on the potato. What happens to the potatoe slice in terms of color changes?

What is the solute in your iodine solution (need to look at the chemical list on what you purchased)?
If you iodine solution isn't mixing with your oils, then you probably have a situation where your test solution and oil are not mixing which probably means they are incompatible.

[guitarpic]

Thank you for gettingback to me. That must be the problem my iodine is a reddish brown. I will order the one from your website and see what happens.

[deleted-71932]

Hello,

I think you may be using the wrong indicator for testing the presence of oils.
Iodine is a starch-indicator, which means that it turns dark purple or black when there is starch present.
If you are testing for oil, the indicator you need is Sudan IV, which will turn red when oil is present.

[deleted-71712]

There is some confusion here, perhaps because we've had several posts recently about two different projects which use two different iodine indicators.

"Power of Purple" uses tincture of iodine to identify the presence of double bonds. The literature refers to the "iodine value" or "iodine number" of various oils. Some background info is here:

http://www2.chemistry.msu.edu/faculty/r ... ddene1.htm
http://en.wikipedia.org/wiki/Iodine_value
http://www.britannica.com/EBchecked/top ... dine-value
http://old.iupac.org/publications/pac/1 ... 2x2339.pdf

"Which Orange Juice Has the Most Vitamin C?" is the other project, utilizing Lugol's iodine. Lugol's is indeed a starch indicator, but it is NOT the same solution as the tincture required for "Power of Purple". The chemistry of iodine is complicated, and the other substances present in each of these two solutions alter its reactivity.

http://en.wikipedia.org/wiki/Iodine#Iodine_chemistry
http://en.wikipedia.org/wiki/Tincture_of_iodine

Tincture of iodine should work properly for the detection of unsaturated carbon-carbon bonds.

Amanda

[guitarpic]

I bought a potato and placed the iodine on it. it was a reddish brown color and in some areas of the potato it turned purple. Why is it not working in the oil. we are using the oils from the list given on the website.

anna

[deleted-71588]

in some areas of the potato it turned purple. Why is it not working in the oil.

A potato is starch. The iodine solution you are using has proved to be a great starch indicator and a poor indicator of double bonds. I originally suggested testing on a potato to help determine the behavior of your iodine test solution.

[amyC]

Hi Anna - After reading your questions and the input you've received from several of our Experts here at Ask an Expert, I talked with our lead staff scientist to see if she had additional input. She writes:

You've chosen a fun project that has a lot of good chemistry, not to mention important human health implications! Sorry to hear you're having trouble with it. There are 4 critical things to make sure you're doing in this project:

1) Make sure you've bought (or been sent...sometimes a vendor can pack the wrong thing!) the right tincture of iodine. Read the label of the bottle carefully, it must be at least 7% iodine, a lower concentration (like the more typical 2%) will not work because there will not be enough iodine in your solutions. The iodine solution may not be purple in the bottle, but it will show its purple color once mixed in with the oils.

2) Make sure you are heating the oils to 65 degrees Celsius (that is 149 degrees Fahrenheit). The oils needs to be in the bath long enough to reach that 65C temperature.

3) Once you add the 7% tincture of iodine to the oils - stir. You need to mix the oil and tincture together well.

4) Build positive controls and negative controls in to your experiment. You should have one oil that you KNOW from your background reading is LOW in unsaturated fats (this one should turn purple and stay purple with very few drops of iodine), and another you KNOW from your background reading is HIGH in unsaturated fats (this one should take many more drops of iodine before it stays purple).

I hope things are on track with the project, and we look forward to hearing how it goes!

Amy
Science Buddies

[magicron]

Having trouble with this, as well. My daughter did a trial run tonight, but was unsuccessful.

Using 7% tincture of iodine, ordered from Amazon, as recommended in the list of materials.

Heated water bath to 60C. Perhaps not heating the oil to 65C is the problem. That begs the question as to why is this temp important? I assumed it had to do with just making sure that oils solid at room temp would be liquid (eg coconut oil).

If anyone who had done this experiment could chime in, that would be great.
Thanks!

[deleted-71588]

The iodine and oil chemical reactions are complex and temperature sensitive. If the lead Science Buddies scientist is recommending that the oil reach at least 65C before adding the iodine, then there is probably a different behavior below 50C that phases out somewhere above 60C. If you want the indicator behavior described in this experimental proceedure, you need to be doing it at 65C or slightly above and probably below 75C.

[magicron]

Craig-

Thanks for your reply. We tried again, heating the oil to at least 65F. The main difficulty is that the iodine solution is more of a dark yellow/amber. Since the oils themselves are already a light yellow, it can be difficult to determine 'the' drop that causes the change, as it seems each subsuquent drop makes the solution a darker yellow, or amber.

The next attempt will likely be to compare two 'opposites': one oil known to be highly unsaturated and one oil known to be highly saturated. Each tested side by side, drop by drop so as to be able to decern color difference between the two. This is somewhat described above having positive and negative samples in play during the test.

One comment from the science teacher is the concern of this experiment mearly showing something already known, ie one can merely read the label on the bottle to determine saturated fat levels, etc. So, we're wondering how to introduce some unknowns into the experiment, or somehow take this to a more advanced level.

My contention is that most science fair projects involve things that are already known. IMO, the scientific method and application are keys to success of a science fair. Very few people are going to 'invent the wheel' at a science fair.

We have found some biochemistry experiments online that refer to using water/bromine solutions, along with other chemicals, rather than an iodine solution, alone. In these cases, heating the oils is not mentioned.

Thanks againg for your input. If you have anything to add to my comments above, we would appreciate it.

[magicron]

Meant to say that we heated to 65C, not 65F.