Pique the Geek 20110612: Boron, Essential and Uncommon

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Boron, the chemical element with an atomic number (Z) = 5, is an uncommon element.  The reason is that there is no really easy way for stars to make it except through going supernova.  A more technical way of saying this is that stellar nucleosynthesis is not a viable pathway to produce boron.  As a matter of fact, it is the least common very light element except for lithium.

Tonight we shall look into some of the properties and uses of boron, as suggested by Kossack shrike Friday evening during comments on Popular Culture.  The interest that shrike has in some new medical uses for boron, and we shall go into some detail near the end of the piece.

You are likely to encounter boron in only a few ways in daily life.  Probably the most common daily life product is borax (sodium borate decahydrate), Na₂B₄O₇^.10 H₂O.  It is an effective detergent booster, but does not do a very good job in overcoming hard water where it is not nearly as effective as phosphates.  However, it does not support algal blooms like phosphate does, so is permitted in parts of the country where phosphates are not allowed for laundry purposes.  Another use for borax is as a flux in hard soldering and brazing but lots more people wash clothes than solder and braze.

Another fairly common product is boric acid, H₃BO₃.  This is used in medicine as a mild antiseptic (mild enough to use in eye drops), but the major consumer use is as a relatively nontoxic insecticide.  It kills insects by two mechanisms:  one is a mechanical action whereby it gets into the joints of an insect’s exoskeleton and abrades the softer connective tissue, allowing water to be lost from the insect.  It is also directly toxic to insects when eaten.

A less common consumer use of boron is in the form sodium perborate, NaBO₃·H₂O.  (Sometimes there are three rather than one water molecules in the crystal lattice).  This is used as a bleach in some non chlorine preparations for laundry use and also in some teeth whitening preparations.

The last encounter that consumers are apt to have with boron is in borosilicate glass.  This was formerly marketed under the brand name Pyrex from the Corning Glass Company.  Sometime in the mid 1980s Corning sold the trademark Pyrex to another company for consumer cooking utensils only, keeping the trademark for its glass used in industrial (especially chemical) settings.  The company that bought the trademark soon quit using borosilicate glass for kitchen utensils, to any new Pyrex kitchenware is actually the much cheaper soda lime glass (the same used for drinking glasses, window glass, and other low cost items) that has been heat treated to resist the effects of thermal shock.

Borosilicate glass is resistant to thermal shock because it has a coefficient of thermal expansion of around 3 x 10^-6 K^-1 whilst soda lime glass has a value around three times that.  Since glass is by its nature brittle and also a poor heat conducter, when a hot piece is put in, say, cold water, the surface contracts much faster than the center, causing stresses that can lead to it breaking.  The surface gets smaller and just has no place to go because the center is still larger.  Thus, the glass breaks.  Many metals have comparable coefficients of thermal expansion, but most metals are not brittle and are good heat conductors so they do not break.  Obviously, for glass, the smaller the coefficient of thermal expansion the more resistant it is to thermal shock.  However, the champ is fused silica, with a coefficient of thermal expansion only a little over one tenth that of borosilicate glass.  Molten lead poured into a bowl of fused silica frozen into a block of ice will not break it!  However, it is extremely hard to fabricate, being a pure substance and thus with a sharp melting point.

One more thing about borosilicate glass:  it is stronger mechanically than soda lime glass and much more resistant to attack by bases, so it it the preferred material for laboratory glassware.  It is easy to fabricate and relatively inexpensive, but certainly not as cheap as soda lime glass.

Boron gets it name from the Farsi word burah, the word for borax which has been known for thousands of years.  It has been used in glazes for ceramics for a very long time, and as a flux for also a very long time.  However, pure boron is a recent achievement.  The brilliant English chemist, Sir Humphrey Davy (one of my personal heroes) first called it an element in 1808, and in 1824 the equally brilliant Swedish chemist Jöns Jakob Berzelius confirmed it.  Incidentally, my academic lineage can be traced to Berzelius.  What we would call relatively pure boron was not produced until probably 1909.

There are only a few uses for pure boron.  My personal experience with it is as a fuel in pyrotechnic delay trains from my days as a civilian working for the Army.  Boron is a good fuel, and when mixed with oxidizing agents in suitable proportions forms a relatively gasless delay train, and gasless is good.  That is beyond the scope of this piece, but it you are interested bring it up in the comments.

Boron is used to “dope” semiconductors for specific electrical properties, and boron fibers have high strength and are used in demanding environments for reinforcement that can tolerate high temperatures, such as aerospace applications.

That is about all of the uses for the pure material, but boron compounds are widely used.  First, we should take a look at where boron is found.  As stated earlier, pure boron does not occur naturally, but several different oxides are found in large deposits.  Currently, Turkey is the largest producer with the United States second.  These oxides are the remains of ancient seabeds that have dried and deposited the oxides.  For the most part they are found in arid regions, because boron oxides are extremely water soluble and would wash away in wet areas.  In the US, most boron oxides are mined in the Mojave Desert in California.  The trademark 20 Mule Team is a reminder of how boron oxides were transported in the old days.  Now the process is highly mechanized.

The crude oxides are pulverized and treated to produce, normally, either borax or boric acid, which can be further purified for either consumer use or to be converted into other boron compounds or to elemental boron.

One important use for borax is in the production of insulation grade glass fibers.  This is a different composition than borosilicate glass, but does have a considerable amount of boron in it to raise its melting point and to strengthen it.  Nearly three quarters of boron produced in the United States is used in one way or another in glass and ceramic applications.

Another important industrial use is for the production of hard materials for abrasives.  One of these, boron nitride, (BN), is truly amazing.  Do you remember when we discussed carbon a few weeks back?  Boron nitride is very much like carbon in a couple of ways.  Now, remember that boron (Z=5) has one fewer electron than has carbon (Z=6) and that nitrogen (Z=7) has one more electron per atom than has carbon.  This means that BN polymers have the same number of electrons that carbon polymers have.  This is called isoelectronic.

Sure enough, BN occurs both in a very hard cubic crystal lattice, just like diamond, and also a very soft hexagonal crystal lattice, just like graphite.  Remarkably, these materials are used in much the same way that diamond and graphite are, but are more stable to oxidation at high temperatures.  I find this fascinating.

Another important material is boron carbide, with an approximate formula of B₄C.  Not only is this material hard, but it is also extremely tough.  There is a difference.  Hardness is the property of scratching other materials or being scratched by them.  Thus, glass is harder than aluminum (you can scratch a soda can with a broken piece of glass).  However, glass is not tough at all, and is actually quite brittle.  On the other hand, aluminum might not be as hard as glass, but it is much tougher.

Boron carbide is used to make the ceramic inserts in body armor.  It is also used in nuclear applications to absorb neutrons, since boron-10 is an efficient neutron “sponge” and does not become dangerously radioactive under conditions of neutron irradiation, and neither does carbon.  This, and with the high strength of the material, makes is uniquely suited for these applications.

Now let us look at biology a bit.  Boron is an essential ultra trace mineral for both plants and animals, but the biochemistry is poorly understood.  There is some evidence that nutritional supplements of very small amounts of boron are useful in treating osteoporosis, but the results are far from clear.

There is even an antibiotic, boromycin, that contains boron.  There is some fairly strong evidence that it is also useful as an antiviral agent against HIV.  At present the mechanism of action is poorly understood.

The topics that shrike was referring to, I am fairly sure, involve the use of boron compounds to treat disease.  One of these is boron neutron capture therapy (BNCT).  In this experimental therapy,  compounds enriched in B-10, the isotope that absorbs neutrons, are given that bind to tumor tissues much more strongly than to healthy tissue.  Then the patient is exposed to thermal (slow-moving) neutrons .  The B-10 absorbs the neutrons and immediately decompose to a Li-7 atom and and an alpha particle (a helium nucleus).  These particles are energetic, and damage cells by transferring this energy, causing them to die.  Since the starting material binds to tumor cells much more tightly than to healthy ones, and since the neutron beam can be aimed with accuracy, the risk of tissue damage to healthy tissue is minimized.  This is still experimental, but research continues.

Another use of boron in therapeutics is the new drug bortezomib, a drug that has been shown to be effective against multiple myeloma, a particularly nasty white blood cell cancer.  The drug binds to the 26S proteasome (proteasomes are involved with destruction of damaged or misfolded proteins in cells) and thus, but a mechanism that is not completely understood, deprives the tumor cells of the ability to replicate.  It it possible that the reduction in the ability for a tumor to develop a rich blood supply might be involved, but this is not clear yet.

Well, you have done it again!  You have wasted many einsteins of perfectly good photons reading this boring material.  And even though John Stossel quits whining when he reads me say it, I always learn much more writing this series than I could possibly hope to teach, so keep those questions, comments, corrections, and other feedback coming.  Tips and recs are welcome too.

Warmest regards,

Doc