MadSci Network: Chemistry

Re: do metals become smaller after we touch them for a long time

Date: Fri Sep 24 15:29:09 2010
Posted By: Michael Kay, President and Consultant AMBRY, Inc., and
Area of science: Chemistry
ID: 1279093689.Ch

Dear K,

There are several reactions that occur in sequence to form the odor 
detected. The first is the dissolution of a very small amount of the iron 
either through corrosion to form the "green rust" or bivalent iron +2 by 
the action of the acids in sweat from the skin when handling the object.

Either of those two processes will change some of the iron atoms on the 
surface. If there is a heavy coating of "Green Rust", a change in weight 
will be noticable--the object will be very slightly heavier due to the 
addition of oxygen atoms to the object--if none of the coating has been 
lost. If the coating is removed--for example by polishing the object and 
removing the coating--then the object will weigh slightly less. These 
differences may be hard to detect without special balances. 

The second reaction is the releasing of the odorous compounds catalyzed 
by bivalent Iron +2 ions. Here the iron is not "used up", but at the end 
of the reaction will be Iron +2. The Iron +2 is not reduced back to iron 
atoms during or after the catalysis reaction.

The human nose is a very sensitive detector for aldehydes and ketones, 
the compounds released from rancid lipoprotiens. Some of these types of 
compounds can be detected at very low levels. A reference giving odor 
detection limits in parts per billion in air is: 

Since Iron +2 is required, there must be some loss of iron atoms from the 
object--that may be difficult to measure. These Iron +2 ions will not 
change in number after the completion of the odor-forming reaction. To 
attempt to measre the loss of iron atoms, the skin compounds will have to 
be removed from the object without removing any additional iron atoms 
(cleaning solutions may remove some iron atoms, and polishing definitely 
will remove any surface coatings or contamination, and and previous 
changes such as the formaiton of "Green Rust".

Since it does not take much organic material to form the odor detected, 
using a small object, such as a small coin, would give a higher 
porbability of detecting a change in weight measured on a microbalance, 
but I would not be surprised if the loss was within the uncertainty of 
the balance reading itself. 

My response is strongly based on the article you linked: 
I have used some textbook references on Catalysis from:
"Chemistry: The Centrtal Science, Eighth Edition" by
Brown, LeMay, and Bursten
Prentice-Hall, New Jersey 2000
ISBN 0-13-084090-4

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