So I had a small bottle of NaBH4 for quite a while and it originally came to me as bigger flakes, not a powder but bigger parts which could
be easily pulverized. The overall powder from this was dry. Then, since those junks where often too big I used a mortar and pulverized all of it and
kept it in an airtight container. Some time after this the whole thing had become solid. Like when you have gypsum or similar stuff that becomes solid
after a while. This block was stuck in the bottle and extremely hard to get out. I had to use a screwdriver to drill holes in the solid and get the
powder from these holes. This lasted to a year now but now suddenly from one week to the other the bottle is filled with NaBH4 and it even reached to
the lid of the bottle. It looks like Polyurethane-sponges, as if the whole thing had started to turn into a sponge with big holes in it but still a
solid piece. I could easily tear some off and it feels really sticky and much like gum. You can form a ball from it, you can pull it to long strings
if you like to. The consistency just changed into something I have never seen before. When I dropped it into water it was still quite active. It took
longer to dissolve than usually but it would fully react and it would turn the water brown-grey. The original sample did not do that when dissolved in
water.
Does anyone have an idea what might have happened. The containers should be tightly sealed and I cannot imagine how humidity could have entered this
solid block because the sponge goes all the way down as if the whole thing had changed its modification somehow.
Schleimsäure - 16-8-2017 at 05:51
I also have a quite old but unused 100g bottle of NaBH from Merck. Maybe from the mid-2000's or so.
It is in small grains. They kind of stick together after a while but losen immediately after shaking.
It's in brown glas with typical Merck-stopper. Recently I added teflon tape around the stopper.
From your description your NaBH is not potent anymore, likely even useless.
Did you use a plastic container? fluorescence - 16-8-2017 at 10:15
Nope, Glass bottle with airtight lid, other hygroscopic compounds are unaffected in these containers...Sidmadra - 16-8-2017 at 13:41
Your Borohydride should be fine. I had about about a pound of it that essentially went through the same process as yours did, stored in a plastic
container. It went from a powder into a very hard solid mass in the container and was extremely labor intensive to get out. It required lots of
banging and chipping with a screw driver until I could get it out. I'm assuming it has something to do with moisture getting in, but I'm just
speculating.
My borohydride sat in the container like that for almost 2 years and was still very active. How I tested the activity of mine was by creating an
aqueous solution of a lewis acid metal salt that borohydride was capable of reducing. Upon addition of the borohydride, the reduced metal precipitated
immediately. In this test I didn't account for reaction of the borohydride with water, but pretty much all of my lewis acid salt was reducted
instantaneously, far faster than the borohydride reacted with the water. How much activity was lost can really only be determined through quantitative
analysis of some sort, aka, calculate moles of hydrogen, use the borohydride, and go from there.
The great thing about borohydride is that it is really cheap, and you get 4 moles of hydrogen per 37.83 grams, so about 9g per mole of hydrogen.
Because of this, you should still be able to get plenty of activity remaining out of your borohydride, on a per weight basis. A little excess goes a
long way.PirateDocBrown - 16-8-2017 at 20:08
