NaOH, where?

[bojan]

Anybody knows where can I buy NaOH (in solid form) ?
My whole stock is no good any more, it oxidised (storage tank was not air-tight, it seems..)
I need it as photoresist developer.

[GeoffW1 (Geoff)]

Hi Bojan,

Caustic soda, or sodium hydroxide, is widely available at hardware stores. Diggers is a common brand name.

However its purity may not be perfect. Do you need a pure grade for photo work? The whiter it is the better.

It doesn't self-oxidise in the chemical sense, but it is hygroscopic (attracts moisture), and then becomes aggressive, so if its surroundings were attacked it will be contaminated.

Cheers

[bojan]

Thanks mate, I will check at Bunnings..
Something must have happened with it (apart from absorbing water), because it doesn't develop the exposed photoresist any more (I need it for PCB etching at home).
The water solution must always be fresh, after couple of days it stops being active.. so I suppose it must be oxidised (what else could happen to it?)

[bojan]

Here, it works like dream
Thanks again..

[marki]

Originally Posted by bojan

The water solution must always be fresh, after couple of days it stops being active.. so I suppose it must be oxidised (what else could happen to it?)

Bojan, as Geoff said NaOH sucks up water from the atmosphere with the greatest of ease and it is this that makes it totally useless as a primary standard. Once wet it can react with many things e.g. forming insoluble solids with just about any metal outside of Na and K. It even reacts with glass if concentrated enough. Lucky there are plentiful supplies available under the name "caustic soda" found on many hardware and super market shelves. The trick to keeping it fresh is to keep it in a well sealed container and only open it in dry warm conditions to prevent moisture being absorbed.

Mark

[bojan]

I buggered me for quite some time why NaOH deteriorates over time..
I foiund the reason for this here:

10. Stability and Reactivity

Stability:
Stable under ordinary conditions of use and storage. Very hygroscopic. Can slowly pick up moisture from air and react with carbon dioxide from air to form sodium carbonate.

So, it's not oxidation, but reaction with water and CO2 from athmosphere.. carbonation
I am happy now...