The Millimol and Milli-Equivalent

Of recent years, terminology in clinical chemistry has undergone a change which is based on the more rational concept of the acidbase equilibrium. Whereas, formerly, the conventional unit of measurement for the blood components (Ca, Cl, C02, P, etc.) had been the static, undescriptive absolute weight (mg. per IOO cc.) or volume (vol. 7%) of these substances, there now is rapidly developing the logical use of the dynamic, functional unit, the milli-equivalent. To the medical student of yesterday, fixed in his absolute weight concept, the milli-equivalent is a total stranger; to the medical student of today, trained to the old unit but exposed to the new, the milli-equivalent is a discomforting, unwelcome intruder; to the student of tomorrow it will be much simpler and certainly much more reasonable than the milligram has been to us. Why introduce the milli-equivalent? It serves an illuminating, graphic purpose in the understanding of the dynamic acid-base equilibria in tissue fluids, especially the blood. The old milligram unit is merely a measure of the "dead" presence of so much substance in the medium; it does not readily portray what that substance does in the equilibrium; one has to stretch a decidedly gifted imagination to interpolate from the "just is" to the "actually does". The milliequivalent, on the other hand, is a measure of what that dissolved chemical entity does in that physiological medium; it is a rational unit for the expression of the functional activity of that entity in the acid-base equilibrium. What is a milli-equivalent? There has been much confusion in the literature due to the use of "millimol" for the true expression "milli-equivalent". In this application, i.e. the study of acid-base equilibrium, the term millimol is a misnomer and, as such, misleading.* On the face of it, it implies o.0i of a molar solution,