We have a huge amount of dichloromethane but with expired date. I don't want to dispose this amount of this valuable solvent.
You might be able to extend the expiration date by testing the solvent and documenting the quality. Example is gas chromatograph analysis.
Dichloromethane is fairly stable when not contaminated, for example with keystones or aldehydes. It might disproportionate to mixed chloromethanes, but not quickly when clean and pure..
If it is found to be in need of distillation it can be distilled with safety precautions appropriate to the contents.
You should first verify if the dichloromethane (DCM) is still colourless. If so, you may shake a sample with conc. aq. H2SO4; if the acid layer also remains colourless, you may possibly consider to just dry the DCM. Drying seems particularly advisable if some noticed deviation on its physical properties suggests that it may have uptake humidity. For that, you can distil the DCM from anhydrous CaSO4, CaH2, or P2O5. If it is found that drying is not enough, you may further purify it, previously to the before mentioned distilation. For that, you can proceed to liquid-liquid extractions; successively washing the DCM with conc. aq. H2SO4 (until colourless aq. acid layer), with water, with Na2CO3 aq. (5 wt %), with NaHCO3 or NaOH dil. aq., and (again) with water. Pre-dry the washed DCM with anhydrous CaCl2, and distil as above stated, to complete drying. Store the purifyed DCM protected from bright light in brown glass bottle, preferably well-filled and with type 4A molecular sieves.
I suggest that similarly to chloroform - HCl, phosgene etc. is formed during dichloromethane decomposition process. In a simple way I would try to shake a few mL of DCM with water and after that to check whether the water layer turned acidic or not. You can also inject the old DCM into GC-MS and compare the results with chromatogram made with a newer DCM.
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