Suspected Carcinogen To the Editor:
The article "Gas Chromatographic Determination of Fatty Acid Compositions" (1985,62,449) suggests the use of the reagent N-nitrosomethyl urea for the preparation of diazomethane to be used in the conversion of fatty acids to their methyl esters. Though themethod given is very convenient for the smallscale preparation of methyl esters of carboxylic acids, Nnitrosomethyl urea is a cancer suspect agent [see Sax, "Dangerous Properties of Industrial Materials"; Van Nostrand Reinhold: New York, 1984, p 18961. The article does not name it as a potential carcinogen, but it hints that there might be some risks involved if the substance is misused by students. There also is a contradiction in the article which states (p 4501, "Due to the use of less than 10 pg of N-nitrosomethyl urea per standard preparation, it is highly improbable that risksituations will arise in afairly organizedlab". The procedure given for the preparation of the methyl esters calls for 30-40 mg. For many years we have used 14% boron trifluoride in methanol for the preparation of fatty acid methyl esters, either from the free acids or after the saponification of the fat. The procedures are described in Bulletin 721, Supelco Inc., Bellefonte, PA 16823-0048. Solutions of boron trifluoride in methanol may he purchased from a number of commercial sources. Since boron trifluoride is toxic, preparation of the esters should be carried out in a hood. Richard A. Hendry Westminster College New Wilmington, PA 16172
Fundamental and Descrlptlve Unlts To the Editor:
In this Journal (1983,60,942) Wadlinger recommended the adoption of descriptive units in addition to the fundamental units. The descriptive units, atom, electron, cycle, and so on, relate to the physical entity being measured in contrast to the fundamental units, kilogram, meter, and so on, which are quantitative. Thus, he advocated the use of such expressions as 18 g HaOImol, 1015 wavels, and k JIKmolecule. Nevertheless, ambiguities do not occur if the physical quantity and the entity of interest are clearly described as recommended by McGlashau.' It is not necessary to impose on the unit the burden of describing the physical entity
being measured. The business of measurement is aided if physical entities, physical quantities, and units are carefully distinguished from each other. Wadlinger argued that Dalton could not have made his deduction of the ratio of the masses of a hydrogen atom and an oxygen atom without considering the atom as a descriptive unit of measurement. But this isnot necessary, although it is essential to consider the atom as a discrete physical entity. Dalton knew that the mass ratio of oxygen to hydrogen in water was 8, hence number uf v x y gt n atumt.X-mass of m e oxygen atom =8 numtrer uf hydrogen atoms X mnss of hvdroym atom Dalton's assumption that there is only one atom of hydrogen and one atom of oxygen in a molecule gives quite simply that mass of one oxygen atom - 8 mass of one hydrogen atom
Wadlinger calculated the mass of one water molecule thus 18 g H,O/mol 6X
loz3molecules H,O/mole
-
3x 1
0 H ~~O
1 HsO molecule
~
(1)
This does not name the physical quantities used, but the reader is expected to deduce them from the units. Generally, this is not a good procedure because different physical quantities sometimes have the same units. It would be clearer to write molar mass of H,O - 18 g mol-' Avogadro's constant 6 x loz3mal-'
=
(2)
I t follows from the definitions M = mln and NA = Nln that the left-hand side of (2) equals the mass per entity, here the water molecule. Wadlinger suggested thattbe Boltzman constant k has the meaning of energy per kelvin per single gas molecule, the units being joule/kelvin-molecule (JIK-molecule). But k can be clearly defined by the relation
whereR is the gas constant and NAis the Avogadro constant. Then
Wadlinger suggested that the classical light velocity1 wavelength frequency relationship
Volume 64
Number 2
Februaw 1987
191
