[EccentricHeather]

This is an age-old problem, but a quick search didn't turn up much here (although I certainly could have missed something).

Imagine this...  You have a bottle containing about a pound of sodium hydroxide that has been around for a while.  You open it up intending to use some and, instead of finding powder, you find a few very large rocks securely bonded to the sides and bottom of the bottle.

I'm sure something like this has happened to many of us; it is a common problem.  I got to deal with it today.  Grinding it up and extracting it with a variety of tools occupied quite a bit of time, but I finally got out what I needed.  It probably would have been more cost effective to buy a new bottle and use the old stuff to make a large quantity of sodium hydroxide solution.  And of course, it would be a dangerous chemical, so the necessary safety precautions required even more time.  (While I was going through my chemicals, I found that my sodium nitrate had also caked up, but it was easier to break apart.)

So has anyone found any good ways to prevent chemicals from caking like this (perhaps some way of controlling moisture), or good ways to extract them once it has happened?  And which ones are most notorious for becoming as hard as a rock?  Certainly, sodium hydroxide must be near the top of that list.

Heather

[woelen]

I have a special way of using my chemicals. When I buy a new chemical (I usually buy them in 100 or 250 gram quantities, I only do testtube scale experiments, so I certainly don't need much more), then the first thing I do is transfer 7 grams or so to a small high quality vial and keep the rest of the chem in its original container if it is OK, or I put it in a good well-sealed container.

The larger (usually original) container, I put in a small plastic bag together with a little amount of air, and I tightly close that plastic bag. Around that plastic bag, I put another tightly closed plastic bag. I use these plastic bags because they allow expansion and shrinking of the air, contained inside them. These bags hence will expand and shrink, according to temperature, such that their main effect is that air inside the bags and containers is not exchanged with outside-air after each thermal cycle. This works great (provided the bags are not too porous, that's why I use two bags).

In this way, I have kept a very air-sensitive chemical like Na2S (even more sensitive than NaOH, because it is besides being hygroscopic also easily oxidized) good for many many years, while some Na2S in a simple container is partially oxidized. The bottle in which I received it told that it only has a shelf life of 1 year, but with my way of storing it, I can keep it almost indefinitely.
In the same way I kept NaOH and KOH when I was a boy. I still had some amounts lying around at my parent's house, which I had forgotten, sealed 20 years ago, and still they were as nice and dry (little prills) as when I purchased them  .

So, I do my experimenting with the small amounts in the 7...10 gram vials, while keeping my stock very well isolated from the air. This combines convenience of working with keeping my expensive chemicals good. It sometimes has happened to me that the small quantity in my 7 grams container went bad after a year or so, but then I still have my main stock, which is good. I had this with my NaCN. The main stock of almost 100 grams still was perfect, my small experimenting-stuff had turned into Na2CO3 for a large part after 2 years of storage.

Of course, if you use larger quantities or are doing larger scale synths then this method of working is not good for you, but for any small-scale experimenter, I really advice to work like this.

The only chems I have, for which I do not use this method of working are HAuCl4, IrCl4, RuCl3, and NH4ReO4. Of those chemicals I do not have stock amounts, I just have a few grams of them.

[jdurg]

Also, I hope that the solution of NaOH you eventually made didn't need to be standardized.  NaOH will not only absorb water from the air, but CO2 as well making NaHCO3.  As a result, if you have an old bottle of NaOH (powder or solution) it will likely have a VERY high percentage of sodium bicarbonate in there.  So if you think you're weighing out 100 grams of NaOH, you may only be weighing out 50 grams or so.

[EccentricHeather]

Yes - I know it is unlikely to be very pure at this point.  I made a solution out of it by putting too much powder in and letting it become very concentrated and then decanting it.  There is nothing precise about the way I made the solution, but given the nature of the raw material, there would be no purpose in doing it carefully.  At least I got a reasonably strong solution this way.  This sort of solution has its uses anyway, so I don't mind having some around.

I finally went to the chemical shop and bought a fresh bottle of sodium hydroxide.  It's dirt-cheap, so there is no reason to spend any more time dealing with the old bottle.  I'll keep the old stuff around, but I will only use it for making imprecise solutions or for household uses (drain cleaner, general cleaning, etc.).

I'll try the plastic bag trick to slow this process in the future.  Perhaps I could also put the bottles in air-tight boxes.  Thanks for the advice.

Heather