Why to Avoid Tasting Chemicals and How to Test Unknown Chemicals?
The statement about acids tasting sour represents not only a fact about acids, but also a statement about early chemical experiments and chemists. Chemists no longer taste laboratory materials. Not only has the development of modern chemical procedures made such tests obsolete, they are also clearly dangerous. However, chemists once performed these tests routinely. The following case is a description of one of these tests.
A chemist received a vial of a substance that was connected with a Civil War munitions train. The synthesis of nitroglycerine was a possible use for vial, and the chemist came across an interesting reference to tasting chemicals while examining documents about the history of nitroglycerine in the United States. “The addition of water precipitates a heavy oily looking liquid, which may be washed in water, dissolved in alcohol and separated by the addition of water. It resembles light yellow olive oil, it is heavier than water, in which it is quite insoluble – it dissolves freely in alcohol and ether, is without smell, and a sweetish pungent and aromatic flavor. It must be tasted with great caution, as a quantity sufficient to moisten the end of the finger, when applied to the tongue, produces the most unpleasant effects of nausea and headache, which last for hours. No analysis of this compound has been made”.
Nitroglycerine is known to most people as a powerful explosive. It is also a very, powerful medicine that is prescribed for people with a particular type of heart disease. Nitroglycerine is a vasodilator, a compound that causes the blood vessels to relax and expand. Since side effects include headaches and nausea, the dosage must be carefully controlled. One method is to dispense the nitroglycerine through the skin by applying a skin patch. Chemists have been able to immobilize nitroglycerine on a fabric patch that is worn like a small bandage. The amount of nitroglycerine delivered to the patient can be controlled and side effects minimized by this convenient delivery system.
Many students get an impression that professional chemists always use large, expensive instruments to perform their work. That’s an incorrect impression for many chemical problems are addressed by very simple methods and techniques as is clear from this case history. Recently, some historians found several small glass vials at the site of a Civil War explosion near Sumter, South Carolina. The vials were about 2 inches (50 mm) long and about ½ inch (13mm) in diameter, and contained a clear liquid. The obvious question was: What is it?
Records from the Civil War indicated that Sumter was a center for Confederate stores and munitions. As the Union forces commanded by General William T. Sherman swept through the state in early 1865, the Confederate army burned the railroad bridges around Sumter, leaving a fully loaded munitions train hidden in the swamp. The Confederates were unable to spare any able bodied men to guard the train, and Sherman heard rumors about the unguarded train. He dispatched Brigadier General Edward E. Potter to destroy the train and munitions. Potter set out on April 5, 1865, with about 2700 men supported by cavalry and artillery. The Confederacy was able to round up a force of 575 men from among the very young, the old, and the convalescent. A battle was fought on April 9. When the sole Confederate artillery piece was silenced, opposition was useless, and the road to Sumter was open. Ironically, April 9 was the day that Lee surrendered to Grant at the Appomattox courthouse. Potter’s troops found the munitions train three miles into the swamp and blasted 3 locomotives and 35 cars loaded with military supplies.
The historians who found the vials near the old train tracks brought them to a chemist to find out what they contained. The vials were found packed in cotton, indicating the importance (or explosiveness) of the chemical inside it. The historians speculated that the contents might be chloroform (an anesthetic), an extract of opium, or nitroglycerine. Perhaps the vials might be leveling devices used to help aim cannon. The possibility of nitroglycerine, a widely used 19th century explosive, was remote. Nitroglycerine is every bit as hazardous as people think as small vibrations can cause it to explode. Still, extreme caution was used when one of the vials was opened.
The contents, a water-clear, viscous fluid, were transferred to screw-top container. The first step in identifying the sample was to determine whether it was organic (carbon-containing), like an oil) or inorganic (mineral in nature, ionic). The chemist mixed a drop of the unknown substance with water. Organic compounds do not generally mix with water, but many inorganic compounds do. The sample dissolved in water. The chemist noticed that adding a drop of the sample to some water in a test tube caused the solution to get very hot, so hot that it became uncomfortable to hold.
The acidity was checked next; the solution was extremely acidic, indicating that the unknown sample was a concentrated acid. The chemist next added barium chloride. A white precipitate formed. The only common anions that form precipitates with the barium cation are sulfate and phosphate. Next, some ammonium molybdate was added, a compound that would form a precipitate if phosphate was present. The lack of a precipitate indicated that the sample contained no phosphates. At this point it was clear that the sample was sulfuric acid, H2SO4, the strong acid that contains the sulfate anion. As additional confirmation, it is well known that sulfuric acid gets very hot when added to water.
A few relatively simple tests were all that was needed to determine the identity of the sample. As the chemist had many years of experience, his expertise and quantum of knowledge was important in making the determination quickly. However, the presence and absence of a precipitate, and not some complex and expensive test, was central to the procedure. It is not as easy to determine the intended use of the vials of sulfuric acid as the chemical composition of the contents. Civil War historians knew that nitroglycerine was discovered about a decade before the war, and it was often manufactured on the spot because it could no be transported. Perhaps the synthesis of nitroglycerine (or a different explosive) was the intended use of the sulfuric acid.
About the Author
Dr.Badruddin Khan teches Chemistry in the University of Kashmir, Srinagar, India.
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