Sciencemadness Discussion Board

How hard can it be to purify hardware store chems?

cnidocyte - 9-4-2011 at 08:29

I found out that hardware store caustic soda is sometimes contaminated with mercury due to the process used to manufacture it. Although I rarely let toxic or corrosive chemicals come in contact with my skin, I still don't like the idea of the reagents I'm working with being contaminated like this. I found a good few threads on cleaning hardware store chemicals and noticed a lot of negative opinions concerning the purification of them. How hard can it be to bring hardware store chemicals up to laboratory or reagent grade?

I'll start with NaOH. Firslty I'd dissolve it in distilled water then gravity filter through an activated charcoal filter. This will remove vast amounts of insoluble impurities and I don't see why it wouldn't remove any traces of mercury. Then I would recrystallise the NaOH from a suitable solvent to remove any water soluble impurities that weren't removed by the charcoal. Wouldn't this result in reagent or at least laboratory grade NaOH?

Secondly I've seen youtube videos of people using brown or green H2SO4 based drain cleaners in chemistry. If this crap doesn't interfere with whatever reactions you're doing then why not but at the same time how hard can it be to clean the H2SO4? Again gravity filtering through an activated carbon filter would remove a large amount of that crap and to futher purify it an extra step such as freeze concentration or distillation can be used. Am I right in thinking you can get these compounds up to reagent or laboratory grade like this or am I missing something?

peach - 9-4-2011 at 11:02

NaOH is made from salty water. If that water came from the sea, the amount of mercury in it will be minute when compared with the sodium, potassium and lithium present. If it was made in a mercury cell, it could be higher. I have zero idea how many plants still use mercury cells for NaOH and chlorine production but, in the west, it is likely low due to the problems with handling mercury; particularly due to the legal problems with pouring mercury down domestic drains (which is what most of the domestic NaOH goes towards). Unless you're planning on eating the NaOH, there are bigger problems in terms of practical chemistry, such as water content and it absorbing CO2 from the atmosphere; which it will do at a decent rate (I posted a graph of the mass change for NaOH in the 'make sodium from magnesium' thread, in which you can see the percentage gain in mass it makes over the hours).

Link to graphs <--------

Distilling sulphuric, yes it's possible. But you certainly need to have a decent glassware and heating setup. It is not at all nice or easy; to the point that you should avoid it (given how hot the acid is, any water dripping back will produce extremely violent pops, the entire setup can lift it's self apart and blow the 300C+ concentrated contents all over the place). I would far rather recommend you work from the other end. Buy some battery sulphuric. This is about 30%. Because it is less concentrated, it is less likely to have rotted whatever touched it. Because it's for use in batteries, it won't have any dyes or other muck added to it. Boil off the water and you should have a clear or yellow tinted concentrated result. This is far easier than trying to distil the acid it's self.

You can use shitty drain acid for some things, but I wouldn't recommend it for anything that isn't a test on something you've already tried with clean acid. I use drain cleaning acid for cleaning glassware or generating gases (where the impurities matter very little) but nothing else; e.g. I wouldn't try making a sulphate salt using drain acid directly.

In the UK we rate everyday chemicals as CP; commercially pure. This means it is mostly what you want with not easily removed impurities. E.g. the supplier hasn't put any extra effort into purifying it beyond basic distillation and so on. For technically pure reagents, that means someone has specifically looked at the remaining impurities and used 'excessive' methods to remove them. You can certainly obtain CP purities at home with enough drying, filtration, distillation and so on. As this is what the suppliers themselves are using.

There are lots of different methods to the same thing. E.g. you could make cleaner than drain acid by burning sulphur and bubbling it through H2O2, or dripping concentrated hydrochloric onto metabisulphite and bubbling it through nitric. <---- wiki

[Edited on 9-4-2011 by peach]

smaerd - 9-4-2011 at 11:31

I've made some sulphate salts with drain acid, and they turned out visibly clean. As for purity it's hard to say.

Also used drain acid for TsOH and it seemed to be reasonably pure crystals judging solely by appearance. I'm certain a recrystallization in HCl would suffice to improve purity of the product.

I wonder if the acid couldn't be filtered through activated carbon to remove some of the discoloration.

[Edited on 9-4-2011 by smaerd]

ScienceSquirrel - 9-4-2011 at 12:02

I have used the pale violet drain cleaner acid to make ferrous sulphate, Mohr's salt, ferric ammonium sulphate and potassium chromium sulphate.
The colour and crystal form were good and I did a melting point on the ferric ammonium sulphate and it was in agreement with the literature.
Recrystallisation gets rid of a lot of sins and the main impurity in the acid is water, the steel wool has a few per cent of carbon, etc. Say that everything is 95% pure, most of the impurities do not matter or can be filtered out. I think if you know what you are doing your home made reagents could be at least 99% which is not half bad! :-)

chemrox - 9-4-2011 at 15:17

I used red devil lye as NaOH for years. I have some "food grade" NaOH now and I feel better about using it. The specs were actually superior to acs for the material I got. However one must ask-what are the concerns? You're not making things for human consumption, surely? I ran into red dyed H2SO4 too. The drain cleaner of old was kind of beige-brown. I only used it for generating HCl and for drying wet gasses. I'm not sure I'd want to use the red shit for either purpose but ScienceRody has enhanced my confidence in it. What is the red color anyway? Has anyone tried to find out?