= Solution
Write $SA=S$ for the exposed <glass> area. If $C_e$ is the blank-corrected dissolved mass concentration of element $e$, $x_e$ its initial <mass fraction> in the <glass> and $V$ the leachate volume, then
$$
NL_e=\frac{C_e}{x_e(SA/V)}=\boxed{\frac{C_eV}{x_eSA}}.
$$
The numerator is the released element mass measured in solution. Dividing by $x_e$ converts this to an equivalent mass of the original <glass>, while dividing by $SA$ expresses that mass per exposed area. With $C_e$ in $\mathrm{g\,L^{-1}}$ and $SA/V$ in $\mathrm{m^2\,L^{-1}}$, $NL_e$ has units \b[$\mathrm{g\,m^{-2}}$ of glass-equivalent release]. The fraction $x_e$ must be used as a fraction, not an unconverted percentage.
These corrections allow comparison of specimens of different size and composition without confusing a lower concentration with better durability. State how the area was measured; geometric area and a rough powder's accessible area need not coincide. For repeated sampling, include element mass removed in earlier aliquots and the volume remaining; for an initial blank in the leachant, subtract its corresponding inventory. Surface cracking and evolving <porosity> can make a rate based on initial area differ from a rate based on the true reactive area.
Under <congruent glass dissolution>, constituents enter solution in their bulk proportions, so their correctly normalised releases coincide. Preferential <sodium> release by <ion exchange> can instead give $NL_{\rm Na}>NL_{\rm Si}$, while <silicon> retention in a gel or secondary precipitate can depress its dissolved release. \b[Noncoincident curves show nonstoichiometric aqueous release, but do not uniquely identify the mechanism.] Conversely, solution coincidence does not by itself establish that no secondary reactions occurred. A complete inference compares solution inventories with altered solids.
Normalisation removes composition and first-order area/volume scaling; \b[it does not make different <pH>, <temperature>, solution composition or contact histories equivalent]. In a batch test, even the same initial <water> changes composition and <pH> as reaction proceeds. Reporting those conditions and time is essential for a transferable comparison rather than treating $NL_e$ as a condition-independent material constant.
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