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The Wikipedia article for osmoles is pretty clear on what they are and how they are used. Right off the bat, though, it states that the osmole is not an SI unit, and indeed the usual lists of derived SI units don't mention it. This seems weird to me - it looks like a pretty reasonable thing to be talking about and it looks reasonably uniquely defined for each solute to me.

Is there some fundamental reason why this unit did not make the cut? Is it down to historical accident? Or did the unit stop being useful by the time the SI was designed? Or is it simply not that well defined, changing with respect to exactly what situation one is talking about?

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    $\begingroup$ I think the reason is ‘what the heck would I use it for?’ $\endgroup$ – Jan Jun 20 '16 at 0:43
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The unit osmole (symbol: $\mathrm{osmol}$) is a coherent derived unit with a special name and symbol like the similar unit equivalent (symbol: $\mathrm{eq}$). Both units have not been accepted for use with the International System of Units (SI).

The main reason why these units have not been adopted is probably that such use is neither necessary nor convenient. All concerned quantities have the same dimension as the amount of substance $n$ $(\dim n = \mathsf{N})$. Therefore, all values can be expressed in terms of the base unit mole (symbol: $\mathrm{mol}$).

Note that, according to the definition of the mole,

  1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is “mol”.
  2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

Thus, the amount of substance can always be expressed in terms of the unit mole, regardless of the considered elementary entities. For example,

  • amount of mercury atoms: $n(\ce{Hg})=1\ \mathrm{mol}$
  • amount of hydrogen molecules: $n(\ce{H2})=1\ \mathrm{mol}$
  • amount of chloride ions: $n(\ce{Cl^-})=1\ \mathrm{mol}$
  • amount of electrons: $n(\ce{e-})=1\ \mathrm{mol}$
  • amount of equivalent entities $\ce{1/2H2SO4}$ corresponding to the transfer of a $\ce{H+}$ ion in a neutralization reaction: $n(\ce{1/2H2SO4})=1\ \mathrm{mol}$
  • amount of equivalent entities $\ce{1/5MnO4-}$ corresponding to the transfer of an electron in a redox reaction: $n(\ce{1/5MnO4-})=1\ \mathrm{mol}$
  • amount of solute that contributes to the osmotic pressure of a solution: $n_\text{solute}=1\ \mathrm{mol}$

Nevertheless, certain coherent derived units with special names and symbols have been adopted for use with the SI. The main reason is convenience. The special names and symbols are simply a compact form for the expression of combinations of base units that are used frequently, for example “one kilogram metre squared per second squared” may be written “one joule” $(1\ \mathrm{kg\ m^2\ s^{-2}}=1\ \mathrm J)$. However, his reason does not apply to the units osmole and equivalent, since “osmole” and “equivalent” are not shorter than “mole” and $1\ \mathrm{osmol}$ and $1\ \mathrm{eq}$ are not more compact than $1\ \mathrm{mol}$.

In many cases, coherent derived units with special names and symbols may also serve to remind the reader of the quantity involved. For example, a value expressed in the unit newton may help to remind the reader that the considered quantity is a force. However, the unit symbol should not be used to provide specific information about the quantity, and should never be the sole source of information on the quantity. Therefore, even if the special name “osmole” or “equivalent” is used, the special nature of the considered quantity would still have to be explained in the text.

There are, however, a few important exceptions. For the quantity plane angle, the unit one is given the special name “radian” (symbol: rad), and for the quantity solid angle, the unit one is given the special name “steradian” (symbol sr). For the quantity frequency, the unit reciprocal second is given the special name “hertz” (symbol: Hz), and for the quantity activity, the unit reciprocal second is given the special name “becquerel” (symbol: Bq) in order to emphasize the different nature of the quantities. For the quantity absorbed dose $D$, the unit joule per kilogram is given the special name “gray” (symbol: Gy), and for the quantity equivalent dose $H$, the unit joule per kilogram is given the special name “sievert” (symbol: Sv) to avoid the grave risks of confusion between these quantities (e.g. in therapeutic work). Apparently, the concept of “osmotic concentration” (formerly known as “osmolarity”) is simply not important enough to justify a similar exception for the unit osmole.

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Maybe because "osmole" has the same measurement units as "mole" and the latter is the SI unit with that measurement unit. For example, one mole of sodium chloride gives two osmoles in water solution; the conversion between them is

$2$ osmol/mol

showing the two quantities have the same measurement unit.

The mole was included into the SI system in 1971 (https://en.wikipedia.org/wiki/Mole_(unit)).

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