Shrimp and Grits

A little while ago, about 384 hours if you want to be exact, I was in the lab doing an experiment with some of my students.  The experiment involved isolating proteins from milk, then doing some chemical tests to show that what the students had isolated was actually protein.

One of the tests the students used on their isolated protein was the xanthoproteic test.  This is a simple test for proteins that works on proteins containing an aromatic ring, and involves the addition of a nitro group to the ring by reaction with nitric acid.  If a protein or amino acid with an aromatic ring is present, the test will give a yellow color.

You may, if you’ve ever been in an introductory chemistry lab, heard your teacher warn you about nitric acid.  In addition to causing you some pain, it will also turn your skin or fingernails yellow.  This is the same sort of chemistry in the xanthoproteic test.  Your skin and nails, after all, contain proteins.

Now, back to 384 hours ago.  While I was cleaning up the lab after my students had left, I spilled a small amount of nitric acid onto the top of my finger.  Sure enough, my finger turned yellow, along with a small part of the fingernail.

384 hours after the spill, all the yellow skin had been replaced.  But the nail has to grow out.  We can find out how fast my fingernails are growing with a simple measurement.

So, my fingernail has been growing at a rate of (0.18 cm) / (384 hr) = 0.00047 cm per hour.  That works out to be 3.4 mm per month, which is right about what Wikipedia claims for the average rate of fingernail growth.

Now who says you never learned anything useful from chemistry?

It looks like there’s trouble in China.  Supplies of milk powder meant for Chinese infants are tainted with melamine - the same chemical found in last year’s dog-food scare.

On Sept. 17, China’s minister of health, Chen Zhu, announced that three babies had died, more than 150 were suffering from acute kidney failure, and an additional 6,000 infants had become sick after drinking milk made from milk powder tainted with melamine.

Now you might be wondering why on earth a company would add melamine to powdered milk.  Could it have been accidental?  Or was there some reason to add this compound?

Consider this:  The amount of protein in foods is often determined based on total nitrogen content.  That’s because proteins contain a fairly regular amount of nitrogen - about 16% by mass.  Take the total nitrogen content, multiply by a conversion factor, and you have a good estimate of protein content of the food.

Take a look at melamine.

Simple assay methods for total nitrogen content can’t distinguish the nitrogen in melamine - which is about 67% nitrogen by mass - from the nitrogen in proteins.

The tested substance appears to have a higher protein content than it actually does - since analysts assume that almost all the measured nitrogen comes from actual protein.  This is a reasonable assumption for uncontaminated materials, but is open to abuse by the amoral.

It amazes me that the Chinese were caught by this same ruse again.  You’d think that after finding melamine added to pet food, the Chinese government would have gone ahead and made sure nobody was adding it to food for humans.

[This is an update of an earlier post on this blog.  This version includes streaming video]

Let’s say you don’t want to do the thermite reaction, but you still want to see some flashy chemistry. The reaction between aluminum and bromine might fit the bill.

2Al(s) + 3Br₂(l) –> 2AlBr₃(s)

It’s a very simple-looking reaction - a little electron transfer from aluminum to bromine.  Like lots of these reactions, it’s exothermic.  Exothermic enough to put on an impressive show.

Enough heat is generated by the reaction to vaporize some of the unreacted bromine - throwing off orange smoke.  On top of that, the aluminum gets hot enough to melt and spark.  For obvious reasons, this reaction should only be done where you’ve got very good ventilation. I used my hood for these pictures and this video.

Here’s a still image of the reaction vessel containing only liquid bromine and its vapor.

Bromine is the dark red liquid at the bottom. Bromine is quite volatile, and you can see orange bromine vapor in the top of the beaker.

About ten seconds after adding some torn aluminum foil, things look more like this.

A little later …

And then …

… but you didn’t read all this way for still pictures, did you?  How about a video?

Get the latest Flash Player to see this player.

After the reaction’s over, you’ll want to buy a new beaker.  The melted aluminum foil fuses with the bottom of the beaker.  (The sand bath is there to catch anything that gets through the bottom of the beaker when it breaks!)

So where’s the aluminum bromide?  Some of it has stuck to the sides of the beaker.

Aluminum bromide formed will react with water, causing the release of acidic hydrogen bromide vapors, so you need to be careful disposing of the product! That reaction is also very exothermic, so touching the product or adding water to it is not recommended. Leave it out long enough, though, and it will absorb water from the air on its own.

Ain’t chemistry neat?

Disclaimer: Do not try this reaction at home. In fact, do not try this reaction at all! You were warned.