Volume 84 Issue 20 | p. 6 | Letters
Issue Date: May 15, 2006

More on nitrogen in tires

Department: Letters

I wish that Joey Espinosa had taken the time in his letter to define his terms "leaking," "permeating," "diffusing," and "escaping" (C&EN, Jan. 16, page 2).

He says, "As most scientists understand, all gases will escape a 'real leak' in a tire at the same rate." I do not understand this in view of Graham's law, which states that "gases diffuse at rates which are inversely proportional to the square roots of their respective densities."

I had the privilege of verifying this in our second-year chemistry lab by measuring the rates of passage of various gases through a carefully constructed pinhole in a piece of metal foil. This was a "real leak," in my opinion, albeit not in a tire.

Charles F. Deck
Trenton, Mich.

Espinosa, while correctly pointing out the falsity of nitrogen leaking more slowly based upon erroneous physical properties of nitrogen as opposed to the physical properties of oxygen, introduces additional misconceptions in his letter.

The rate of escape of gases from a "real leak" is not the same for all gases. The escape depends upon the size of the "real leak," better described as the size of the "orifice," and the size of the "molecules" of gaseous mixture. Undoubtedly, for the punctures usually encountered in tires, the escape rates of molecules of nitrogen, oxygen, and carbon dioxide are for all practical purposes within the experimental errors of measurement.

The point of nitrogen being drier is dependent upon how pure nitrogen is manufactured and stored. Ordinary air is necessarily a mixture of nitrogen, oxygen, carbon dioxide, and the water vapor resulting from the humidity of the day and storage of the air.

To ensure nitrogen that will be drier than the dryness of air would entail a prohibitive economic cost to providing more stabilized pressures in the tires for longer periods of time. The presence of oxygen indeed hastens the oxidative process of the deterioration of the polymeric nature of tires.

The "permeation" of the gases from the tire is favored for the smaller molecules of nitrogen as opposed to that for carbon dioxide and oxygen molecules. If the liquid drops of water are formed, they will help in scavenging the carbon dioxide to some extent, but the accompanying pH changes will cause more deterioration of the tire.

Brahama D. Sharma
Pismo Beach, Calif.



Jan. 23, page 4 (Chemical Safety). A violent explosion occurred during an attempt to recrystallize 2 g of 3-iodyltoluene (3-O2IC6H4CH3), not 3-iodotoluene, from boiling water with vigorous stirring.

April 3, page 19. Nilotinib is being developed by Novartis, not SGX Pharmaceuticals. The compound is in Phase III clinical tests of its use in treating drug-resistant chronic myelogenous leukemia. The collaboration between Novartis and SGX gives Novartis access to SGX's research and preclinical compounds in this area.

April 24, page 64. The Fluorinex process requires the application of 30 milliamperes of current, not 30 amp.

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