[Alexey]

Air-sensitive reagents can be stored as a dispersion in paraffin ( mp = 48-50 °C) with a known concentration (mg/g). When you need it - just cut a piece an weigh it.

There is a commercially available Li suspension in solid paraffin. However it seems that tthis trick has not been often used for organic compounds.

Grubbs’ Catalyst in Paraffin: An Air-Stable Preparation for Alkene Metathesis
Douglass F. Taber* and Kevin J. Frankowski

J. Org. Chem. 2003, 68, 6047-6048
http://dx.doi.org/10.1021/jo030005p

"Abstract: A homogeneous mixture of Grubbs’ catalyst in
paraffin wax was shown to catalyze three important types
of metathesis reactions: ring-closing metathesis, alkene
dimerization, and alkene cross-metathesis (e.g., 11 f 13).
This catalyst preparation demonstrated no loss in catalytic
activity after 22 months of storage open to the air."

"Grubbs’ Catalyst 1 in Paraffin. In a nitrogen atmosphere
(grade 4. drybox, Grubbs’ catalyst 1 (1.27 g, 1.50 mmol) was
combined in a 25 mL beaker with paraffin wax (paraffin wax
for canning, mp = 48-50 °C, 8.73 g). The mixture was warmed
to melting in a heating mantle and stirred with a glass rod to
apparent homogeneity (one min). After cooling, the solid was
removed from the drybox, broken into useful-sized pieces, and
stored in a brown glass bottle without further protection."