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I had probably 2-3 grams of calcium hydride leftover in a schlenk flask from predrying a distillation of benzyl chloride. For the disposal, I diluted the flask contents in about 30mL isopropanol and then, over a water bath (flask open but under heavy stream of argon), added distilled water every few minutes until well after the bubbling had stopped, which I assumed was exothermic evolution of hydrogen gas. I also thought any leftover benzyl chloride would just make trace toluene, benzyl hydroxide, and calcium chloride.

I then figured the milky-white solution was okay to pour down the sink with lots of water running and removed about half of it in this manner, although I noticed a brief but strong smell from the vapors as I did so and stopped. I checked and noticed some sites place it into a basic waste container. Was it a mistake to pour down the sink? I thought limewater would be harmless with enough water. Were there any other errors in this process? I may have to repeat this in the future with calcium hydride again or phosphorous pentoxide (seen various methods on P$_2$O$_4$, but one mentioned sink disposal as well).

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    $\begingroup$ Pfft, most students wouldn't probably be so careful, I think you done it quite OK. $\endgroup$
    – Mithoron
    Jun 14, 2017 at 17:34

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The safest way to get rid of calcium hydride is to pour over ice. The water quenches the reagent itself, with the ice helping to control the exotherm (if you just add water, a lot of heat and gas is liberated, having a tendency to spray hot chemicals over the whole hood).

Once everythings quenched, it's helpful to add some hydrochloric acid before disposing of down the sink to prevent it clogging the drains.

Note that whilst clean calcium hydride is safe (once quenched) to be disposed of via the normal drainage, you must ensure any chemicals are washed off first (hexane works well), especially odorous things such as amines which you may have been drying with the calcium hydride.

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    $\begingroup$ Okay, I guess the isopropanol was too miscible with water and must have been carrying whatever organic material became of the leftover benzyl chloride to cause the smell. Next time I will use a less polar solvent and take care to separate the organic phase into waste, and some acid to neutralize the hydroxide. Thank you for the thorough reply. $\endgroup$
    – Blaise
    Jun 15, 2017 at 6:32

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