Environmental Chemistry 5

It is probably painfully obvious to all readers that the product of humans is typically no better than the quality of their thinking.  One common flaw is caused by jumping to conclusions before key information is known.  That said, it should also be noted that we often need a conclusion before all possible information can be gathered.  The challenges of when to draw a conclusion and what conclusion is reasonable require wisdom which unfortunately accumulates after much education, experience, and age.  At least being cautious can be taught.

In recent years there has been concern about a group of substances containing the perchlorate ion.  Ammonium perchlorate is used to produce solid rocket propellents.  The Defense Department, NASA, and their contractors are believed to be responsible for much, though not all, of the perchlorate contamination since they use 90% of the perchlorate produced in the United States.  It was discovered to have seeped into groundwater and surface water and subsequently found in foods such as lettuce and milk.  Once ingested in mammals such as humans, it inhibits the transport of iodide ions, the lack of which can result in hypothyroidism.  As a result the United State government (EPA) had proposed a concentration limit of 1 ppb (part per billion) for drinking water and proposed that industries which use perchlorate should be held accountable for the perchlorate found in the environment.  In 2005, the National Research Council established that 0.7 μg per kg of body weight per day is a safe dose, even for unborn children (the most vulnerable) carried by women who have Iodine-deficient diets or whose bodies don't make enough thyroid hormone.  Perchlorate has been found in lettuce and melons which were likely irrigated with perchlorate-tainted water, or were grown in soils where the chemical occurs naturally or that were treated with perchlorate-containing fertilizer.  Perchlorate also has been found in cows' milk, presumably because these animals drink water or eat feed contaminated with perchlorate.

Researchers at the National Center for Environmental Health detected perchlorate in urine from each of the 2,820 participants who were at least six years old in its National Health & Nutrition Examination Study for 2001-02.  They also found that concentrations of perchlorate in children were generally higher than levels found in adolescents and adults.  Researchers also discovered that perchlorate exposure was associated with a decreased level of thyroid hormone among women with lower than normal concentrations of Iodine in their urine.  Iodine is essential for the production of thyroid hormone, which regulates the body's metabolism.  Perchlorate interferes with the thyroid's uptake of Iodine.  An estimated 43 million women in the U.S. have Iodine levels low enough to put them at increased risk of developing goiter, an enlargement of the thyroid gland due to an insufficient amount of thyroid hormone.

If these women with low Iodine concentrations become pregnant, their babies could risk abnormal brain development.  The developing nervous system is sensitive to small changes in thyroid hormone levels.  During the first trimester an embryo is completely dependent on maternal thyroid hormone.  After that, the fetus begins to produce its own thyroid hormone but still receives about 30% of its total from the mother for the remainder of the pregnancy.  Nursing infants also are vulnerable to neurological problems from perchlorate exposure.  An infant can get perchlorate in breast milk and that also inhibits the movement of Iodine into breast milk, creating double jeopardy for the nursing infant.

Since NASA and the military face a potentially huge liability for cleanup if the government requires the removal of perchlorate from drinking water, they are seeking evidence that they are not responsible.  For example, fertilizer high in nitrates earlier imported from Chile might also have produced the perchlorates.

And perchlorate was discovered at unexpectedly high concentrations in areas where human caused pollution was very unlikely, evidence that nature itself can apparently also produce perchlorates.  In 2004 a group of chemists led by Andrew Jackson and Purnendu K. Dasgupta surveyed ground water under 60,000 square miles of Texas and New Mexico finding perchlorate concentrations as high as 60 ppb.  In a follow-up study they found the concentrations in the high plains correlated with concentrations of iodate known to come from the atmosphere.  To try to determine if a similar mechanism produced the percholorate, they exposed aerosols of sea salt (tiny specks of salt suspended in the air) to high voltage electrical discharges.  Perchlorates were formed.  They found high concentrations of ozone, a substance created by lightning (see lab E1 and lab E4) can produce the perchlorates.  Rain and snow samples also contain perchlorates.  They propose that lightning (high voltage electrical discharges) striking salt carried by winds from the oceans or desert salt flats is responsible for creating at least some of the perchlorates.

Investigation

The description about contains a number of related chemical names such as iodide and iodate.  Perhaps it would be appropriate here to describe the system used to name such substances.  Much of the system of nomenclature used today dates back to the new chemistry of Antoine Lavoisier (1743-1794 at right with wife, Marie), his colleague Louis Bernard Guyton de Morveau (1737-1816) and others as introduced in Méthode de nomenclature chimique in 1787.

Such new nomenclature had been proposed by Morveau since 1780 but did not garner much enthusiasm.  Lavoisier published Traité élémentaire de chimie in 1789 systematically presenting how the new vocabulary, the new Oxygen theory and the new definition of elements explained much that was previously known about substances and their reactions.  In the preface Lavoisier emphasized the importance of language to clear thinking, a point previously stressed by the philosopher de Condillac (1715-1780).  This new book was translated to most of the other European languages and finally brought about the intended revolution in chemistry.

The system was improved by Jöns Jakob Berzelius (1779-1848 at right), particularly in 1813 with one and two letter symbols based on the Latin names of the elements (such as Cl for Chlorine and I for Iodine).  Names for substances with more or less Oxygen were indicated by adding prefixes per- or hypo-.

Procedure

1. Complete the table:  If you have difficulty finding the patterns, recall that periodic charts are arranged with elements that behave similarly aligned in (typically vertical) families.

2. There is evidence that the concentration of CO₂ in the earth's atmosphere is increasing.  It has not been established that humans are the sole or primary cause of the increase.  But it has been proposed that the effect will be gradual warming of the earth, melting of the polar ice caps, and a rise in sea level.  Some nations have established the Kyoto Treaty to try to stop the increase of atmospheric CO₂ while the United States government had declared there is not sufficient evidence to necessitate major changes in the country's use of carbon containing fuels.  Use what you know, what you can learn, and your own wisdom to determine your own position on this issue.
   
   
3. A rather useful skill is to be able to read between the lines, that is, to infer additional information not directly provided by the text.  For example in the information about perchlorate measurements, one could reasonably expect the scientists did not measure the concentration in groundwater under EVERY one of the 60,000 square miles.  Drilling that many wells and extracting water would be prohibitively expensive.  More likely they tried to obtain much fewer but representative samples from wells that already existed.  But one could also infer that these chemists have a procedure that can relatively easily measure concentrations as low as 1 ppb.  Let's think about that a moment.  Other than Hydrogen ions, what is the smallest amount of substance you could measure with a method currently in your repertoire?  (...a tenth of a gram, a hundredth of a cm³ or what???)  How much water would you need from each well to obtain an accuracy of one part in a billion?  About how much volume would that be?  If you sense that these folks have a measuring procedure far better than yours, you might want to keep an eye out for clues for how they do it!  (Oh, and why did we exclude Hydrogen ions?  How could you measure smaller concentrations of Hydrogen ions?  The author hopes you appreciate how much more can be obtained from reading technical information than just grasping the surface intent.)

Communicating technical information such as observations and findings is a skill used by scientists but useful for most others.  If you need course credit, use your observations in your journal to construct a formal report.

References

• Air-Formed, Chemical & Engineering News, Vol 83, #7, 14 Feb 2005
• Purnendu K. Dasgupta,  P. Kalyani Martinelango,  W. Andrew Jackson,  Todd A. Anderson,  Kang Tian,  Richard W. Tock,  and Srinath Rajagopalan,  The Origin of Naturally Occurring Perchlorate: The Role of Atmospheric Processes Environmental Science & Technology, 29 Jan 2005
• Aaron J. Ihde, The Development of Modern Chemistry, Dover, 1964, 1984.
• Perchlorate Risk, Chemical & Engineering News, Vol 85, #20, 14 May 2007