Sciencemadness Discussion Board

Why use Mineral Oil for Alkalis?

Sniffity - 20-1-2015 at 19:18

I understand, perfectly well, that mineral oil prevents water from mixing with alkalie metals when they're stored under it.

But here's my question: In theory, a mineral oil will have two or so oxygen atoms, around the glycerol head. Isn't there a slightest possibility of some of this oxygen atoms reacting with the metals? Say, if some very small amount of water vapour was to come into contact with the alkali, form a hydroxide, this could proceed as an alkali hydrolysis?

Under this logic, isn't a hydrocarbon better for storage? Say, hexane, or even common gasoline?

subsecret - 20-1-2015 at 19:21

I believe that mineral oil is used because oxygen is very poorly soluble in it. Don't quote this.

I'd much rather use an inert material such as xylene to store sodium, because it's easier to remove. I looked up the solubility of oxygen in the xylene isomers, and it's very slightly soluble. I couldn't find the data for mineral oil. I'm sure it's close at hand, but I'm too lazy to look for it at the moment.

[Edited on 21-1-2015 by Awesomeness]

Crowfjord - 20-1-2015 at 20:16

@Sniffity: You're thinking of fat, i.e. oil from a biological source. Mineral oil is derived from petroleum and has no glycerol "backbone." Mineral oil is mostly very long alkane chains, I believe. It is indeed a hydrocarbon, but non-volatile, unlike hexane, heptane, etc.

[Edited on 21-1-2015 by Crowfjord]

IrC - 20-1-2015 at 23:25

I think the important factors are it's cheap, readily available, works very well to protect O2 reactive metals, and is fairly stable over a long time. I use mineral oil for many metals in the rare earth and alkaline series, and my U238. Never had a problem yet.

Tsjerk - 21-1-2015 at 10:35

I have some sodium under mineral oil for 5 years now, and it still looks kind of shiny.

It is under nitrogen and unopened though.

[Edited on 21-1-2015 by Tsjerk]

subsecret - 21-1-2015 at 17:32

First result on google. That wasn't hard...

I'm planning to get some sodium, so I'll put some of it under xylene in an OXYGEN atmosphere and see how it does.

Praxichys - 23-1-2015 at 07:34

Yes, storing reactive metals under triglycerides (oils from animal or vegetable sources) would be very bad. You would make soap almost immediately when the metal tears the fatty acids from the glycerol backbone.

Mineral oils, as mentioned above, contain no such backbone, and come from the ground. They are straight alkanes from C15-C60 depending on the grade.

Typically oil is used for a number of reasons. Pretty much all nonpolar oils will dissolve very little oxygen. Xylene, toluene, benzene, and lighter naphtha fractions (like camp fuel or gasoline) will all work, but the oil has several advantages:

1. The oil forms a viscous film and does not evaporate, so when handling bigger samples the protection from air is better. Xylenes or lighter fluid will eventually evaporate form the surface, leaving bare metal exposed to air during cutting and handling operations. Oil also has no odor and reduces exposure to solvent vapors.

2. Mineral oil is difficult to ignite. Accidents happen, and even a tiny bit of accidental moisture on something can create a spark. Mineral oil will not catch fire without significant heating first, so a spark here is no danger. If you are cutting sodium covered with xylenes, toluene, naphtha, etc, a small spark will create a large fire. Further, vapors of volatile solvents can travel to other ignition sources, again creating large fires involving reactive metals.

[Edited on 23-1-2015 by Praxichys]