If you were asked to name the three most important molecules in
your life, what would your response be? I personally would
forego one of the obvious choices, DNA, and go back to basics
with three of the simplest molecules, water, carbon dioxide and
oxygen. I think the vital importance of oxygen and water to our
continued existence is obvious. My choice of carbon dioxide,
with its atom of carbon and two atoms of oxygen, may be more
controversial. But hear me out. We humans, along with most of
our other animal cousins, breathe in oxygen and breathe out
carbon dioxide. In turn, the green leaves of the plants of the
world mix that carbon dioxide with some light from the sun and
make glucose.
In the process of making glucose, oxygen is formed and released
back into the atmosphere for us to continue breathing. We also
eat the plants that eat the carbon dioxide. Or we eat the animals
that eat the plants. Ok, it”s a bit more complicated. Some of the
animals or seafood that we consume eat other animals or species
that eat the plants, etc. Whatever, this photosynthesis is what has
promoted and sustained life on this earth for many hundreds or
thousands of millennia.
Carbon dioxide does have a darker side. It”s a greenhouse gas
and the amount of carbon dioxide we generate these days is
exceeding the plants” ability to recycle all of it. With the highly
publicized continuing loss of plant life, not only in the rain
forests but all over the world, the excess carbon dioxide builds
up more and more. Of course, the world”s increasing burning of
oil and other fuels also generates carbon dioxide and doesn”t help
the situation.
Carbon dioxide has beneficial practical uses too. And it”s not just
a gas. When I was a kid, we had frequent contact with the solid
form of carbon dioxide, known as dry ice. We used the dry ice
to keep our ice cream frozen if we were taking it on a picnic or a
long car trip. Did I ever tell you that I literally owe my life to
Breyer”s ice cream? A block from our house when I was
growing up in Mechanicsburg, Pennsylvania, was Rakestraw”s
ice cream factory. Virtually every Sunday, it was our practice to
have at least half a pint of ice cream each for dessert. In the
summer, we usually saved the week”s supply of cream from the
tops of the bottles of milk. We didn”t have homogenized milk
then. We saved enough cream to use in our hand-cranked 2-
quart ice cream freezer that Sunday.
When we didn”t make the ice cream, we”d buy either Rakestraw”s
or Breyer”s hand-packed ice cream. One day in 1938, when I
was 10 years old, I was assigned to get the ice cream and we
debated our selection. Breyer”s was the choice and I walked to
the little store a few blocks away. When I returned, we learned
there had been an explosion of an ammonia tank at Rakestraw”s.
The explosion killed at least one little girl. I would have been in
the factory at the very same time! Though a devoted fan of
Rakestraw”s ice cream, I”ve had a special place in my heart for
Breyer”s ever since. (My thanks to Paul Heeter, current Vice
President and General Manager of Rakestraw”s for providing
some details of the explosion.)
When my wife and I moved to New Jersey we would often visit
her relatives near Pittsburgh. On our way back to New Jersey,
we usually stopped in Mechanicsburg and would pick up a quart
of Rakestraw”s French vanilla and a quart of White House ice
cream with those great Bing cherries. In what was then a five to
six hour trip we had the ice cream packed in an insulated bag
with plenty of dry ice. According to the February 2001 issue of
National Geographic, at the south pole on Mars the surface is
covered with dry ice, while the north pole is covered with
ordinary ice and dust. (No connection with ice cream but, hey, I
just thought I”d throw in that tidbit.)
I was told in my science or chemistry classes that carbon dioxide
only exists in two forms, solid and gas, but not as a liquid. What
our teachers didn”t reckon with is what”s called “supercritical”
carbon dioxide. This supercritical carbon dioxide, let”s call it
SCDO for short, exists only at a slightly elevated temperature,
and under a high pressure, somewhat over a thousand pounds per
square inch (psi). (Normal atmospheric pressure is about 15 psi.)
A supercritical substance is a liquid that has some of the
properties of a gas and is really a sort of cross between a liquid
and a gas. It flows very easily and will fill in nooks and crannies
just like a gas.
You might think that such an exotic form of matter wouldn”t
have a practical use, given that it”s only stable at high pressures.
Wrong. For one thing, SCDO turns out to be a pretty good
solvent for various substances. It”s especially useful as a
substitute for organic solvents, which often are toxic and/or
carcinogenic or otherwise undesirable. Sometimes these organic
solvents can be quite nasty indeed. For example, in the days of
my youth, carbon tetrachloride was a common solvent used in
the dry cleaning business. Carbon tet was really an awful choice
but people didn”t know any better. In the semiconductor
industry, organic solvents have been the norm in various
cleaning operations.
I imagine that you”ve been reading and hearing about all the
health benefits of green tea, as well as ordinary tea. I was just
reading my Harvard Medical School newsletter and noted that
someone had written asking about the benefits of decaffeinated
tea. It was a reasonable question. The flavinoid antioxidants are
supposedly the good guys in the tea and the question is what
happens to the flavinoids when the tea is decaffeinated?
One method used to decaffeinate tea involves the use of the
solvent methylene chloride. Methylene chloride is a good
solvent for caffeine and does a nice job of removing it from the
tea. But methylene chloride is also carcinogenic and one
certainly hopes that none is left behind in the tea. Presumably,
no significant amount is left behind or I would assume that some
action would have been taken. I was surprised to see that SCDO
was also used to decaffeinate tea, as well as coffee.
What about the flavinoids? We don”t want them to go along with
the caffeine. In answering that questioner, the Harvard response
was that tests had been run on decaffeinated teas made with
either methylene chloride, SCDO or ethyl acetate, another
solvent. It was found that of the three choices, the most
flavinoids were left in the tea when SCDO was the solvent. The
article pointed out that you can”t tell from the label which solvent
was used. However, the newsletter contacted Lipton, which said
that they use SCDO. (I logged on the Lipton Web site and found
an article citing a study showing that black tea also provides
health benefits.) Aside from the disadvantage of having to
maintain a high pressure, SCDO has a number of great features.
Carbon dioxide is very cheap. When the pressure is released the
carbon dioxide is gone without nary a trace. It”s also nontoxic.
However, see the following disclaimer.
Disclaimer: Carbon dioxide is nontoxic. You might find that
residents of Cameroon, West Africa, would take strong exception
to that statement. They would say, “What about our killer
lakes?” Back in August of 1984, Lake Monoun “burped” a
substantial volume of carbon dioxide and 37 people died. But
that event pales before the Lake Nyos incident on August 26,
1986. A huge volume of carbon dioxide was released from that
lake and about 1700 people died! I logged on a University of
Michigan Web site that cited a 1987 publication by George Kling
and coworkers in Science. Their conclusion was that the victims
died of carbon dioxide asphyxiation. Apparently, more than 10
percent of carbon dioxide in the air can be lethal.
The source of the carbon dioxide, as I understand it, is magma
deep in the earth. The magma contacts ground water, which
dissolves the carbon dioxide and then feeds into the lakes
through underground springs. Hence, the bottom layer of water
in the lake can be saturated with carbon dioxide. Normally, the
layers of water in these lakes don”t mix much and the carbon
dioxide stays at the bottom. The speculation is that a relatively
recent trend towards cooler temperature in the region combined
with a predictable decrease in the stability of these lakes during
August to bring the bottom layers up to the surface. This was
thought to cause local supersaturation of the water with carbon
dioxide and the sudden burping of the “nontoxic” gas.
I suppose that these incidents show quite dramatically that too
much of a good thing can be dangerous. If you drink too much
water it”s not good but at the same time you should have your
eight glasses a day. Nitrogen is certainly nontoxic, comprising
80 percent of the air we breathe. However, if you should flood
the room with nitrogen and squeeze out the oxygen, you”d soon
be just as dead as those victims of the killer lakes.
When I started writing this column, I had planned to discuss
several other uses of supercritical carbon dioxide but got diverted
when I remembered the Cameroon incidents. SCDO will
probably return in a later column. Carbon dioxide does indeed
have many faces!
Allen F. Bortrum
