Sciencemadness Discussion Board

NaOH attacking glass

sbreheny - 12-5-2019 at 22:05

Hi all,

I've often heard cautions about keeping strong bases in glass (either for storage or for reactions) because many of them (such as sodium hydroxide) will "eat" glass.

I did an experiment to see for myself how quickly this happens. I used 50% NaOH in water and a 5mL Wheaton-style pyrex ampoule (open) as the "victim".

First, I filled the ampoule with the NaOH solution and left it inside an outer closed glass bottle to prevent CO2 from getting in and reacting with the NaOH solution. I left it this way for a total of two months and I changed the NaOH solution twice. The temperature was 22 deg C +/- 3 deg C for the whole time. I measured a total mass loss of 2.55mg during this time, corresponding to an average rate of 1.65 micrograms per hour.

Then, I took the same ampoule and filled it with fresh NaOH solution once more and heated it to 105 deg C (on a hotplate with remote temperature sensing/control using a Pt100 temp sensor immersed in the NaOH solution) for 6 hours. The total mass loss this time was a whopping 106.8mg!!! This gives a rate of 17.8mg per hour or just over ten thousand times faster than at 22 deg C.

This implies that the "Q10" rate constant for this reaction is about 3 - in other words, every 10 deg C increase in temperature triples the rate of reaction.

I just wanted to share this as an interesting datapoint. To me it says that the NaOH attacking glass is only really a problem for (a) very accurate volumetric glassware, (b) hot reactions, (c) very long-term storage (>5 years), or (d) solutions where silica contamination cannot be tolerated even at the milligram level.

Herr Haber - 13-5-2019 at 03:31

Very interesting data !

I dont think a lot of people experiene problems while storing NaOH but rather using it...
Boiling anything with a strong base will attack the glass and leave visible marks. It may not affect the outcome of the experience but no one wants to damage their glassware.

There is just one point where you forgot Na or KOH being a problem: when it's in contact with ground glass (=more surface).
Bubbling acid gases in NaOH to neutralize them can sometimes lead to stuck glassware.

NaOH cracks glass

sodium_stearate - 13-5-2019 at 11:06

I've got a fair amount of practical experience in this area.

A process I use quite often demonstrates that even
a rather weak NaOH solution does tend to crack
cheap glass if the solution is left in the cheap glass
for very long.

OK, by cheap glass I mean old pickle jars...

But it will crack them.

As a result, I always remove the solution from the
glass jars soon after that portion of the process is done,
and transfer it into a plastic jar for long term storage.

nimgoldman - 14-5-2019 at 21:31

I have a beaker made of thickened borosilicate glass in which I make concentrated (50%) NaOH solution periodically. It etched the glass considerably but it is just the very surface, not really eating the glass.

I agree the temperature is a big factor here.