Sciencemadness Discussion Board

Caustic soda or potassium hydroxide

CHRIS25 - 19-7-2012 at 06:44

for titration purposes I have caustic soda which "contains" NaOH. I wanted to know how much is actual sodium hydroxide but could find anything on the web except an msds stating that sodium chloride and sodium carbonate were added (but this was industry name specific)

I added 1 gram of Caustic to 20mL of water and it dissolved and remained clear. I then added a total of 4 grams to the 20mL water and it dissolved and remained clear so I presume this tells me that the Sodium content is very high (unlikely that the caustic that I have is under 75% pure NaOH) ok so this was the best idea I could come up with.

Nevertheless I assume that for pure titration purposes Potassium Hydroxide would be best since it's only the H ions that are important for my purpose, (I have pure grade flakes). Or is there a specific test I could run to ascertain more information about my Caustic soda? I have only basic equipment, not a Lab. thankyou.

[Edited on 19-7-2012 by CHRIS25]

Hexavalent - 19-7-2012 at 07:50

More information about your caustic soda;

You could theroretically do a titration using, say, 4g of your product (theoretically 0.1 moles) against some HCl or a similar acid (monoprotic acids are simpler to calculate for). From this, ascertain the number of moles of the hydroxide present and compare it against the supposed 0.1. The Na2CO3 would, of course, take away from accuracy, but if you want just a rough estimation then it should be fine; sodium carbonate is not caustic and thus would probably not work for unblocking drains (I assume your commercial product was sold for this purpose), thus it is unlikely to be there in significant quantities.

You could also look into using universal indicator, which may help you decide how much of the carbonates, chlorides and hydroxides are present as the first is moderately alkaline, the second neutral and the third strongly alkaline.

For titrations;

Hydrogen ions come from the dissociation of acids, not bases...you do realise that, right?

For most purposes, yes - KOH would do the job, if, of course, you used the right Mr data when calculating the results of your titrations (which I have a video of here; http://www.youtube.com/watch?v=58NArANp0kE&feature=plcp )

In these procedures, the metal ions are what are called spectator ions. This is because they are not involved in the main reaction (in this case, H+ + OH- → H2O, rather like spectators in a stadium not taking part in the event they are viewing itself.

[Edited on 19-7-2012 by Hexavalent]

Magpie - 19-7-2012 at 08:36

You have to be careful when weighing solid NaOH and KOH as they are both quite hygroscopic. In fact commercial KOH is usually considered to be 15% water, I believe.

I agree with Hex that your best bet would probably be to "standardize" your NaOH or KOH solutions using HCl. In my locale HCl is sold as muriatic acid. The concentration is specified on the label as a %. Using a table this can be converted to a molarity. This would certainly be accurate enough for your work. Check your hardware store. It's used to cleanup concrete work.

CHRIS25 - 19-7-2012 at 08:41

Ok great video, learn a lot from that one, thanks. Let's have a look at my caustic soda, just too stingy at the moment to buy the 100% grade stuff including postage.....Ok just a quickie, what does MR mean please?

Oh, ====Hydrogen ions come from the dissociation of acids, not bases...you do realise that, right?==== Yes thankyou, I need the H+ in the hydroxide ions to ascertain how much free HCL there is in my etching solution, (a much discussed topic in my previous posts) If I am using H+ and "ions" incorrectly please correct me. I know what I mean but my use of terminology might be off key...

[Edited on 19-7-2012 by CHRIS25]

CHRIS25 - 19-7-2012 at 08:43

Quote: Originally posted by Magpie  
You have to be careful when weighing solid NaOH and KOH as they are both quite hygroscopic. In fact commercial KOH is usually considered to be 15% water, I believe.

I agree with Hex that your best bet would probably be to "standardize" your NaOH or KOH solutions using HCl. In my locale HCl is sold as muriatic acid. The concentration is specified on the label as a %. Using a table this can be converted to a molarity. This would certainly be accurate enough for your work. Check your hardware store. It's used to cleanup concrete work.


Hi Magpie, no problem with HCl I have easy access to all the concentrated acids except acetic, pity, about that one as I use it the most and always need the concentrated stuff, only able to get 40%. Never mind. My KOH is 99%, and it melts very fast when I take it out of the container, one flake is dissolving in the air within 3 minutes!

[Edited on 19-7-2012 by CHRIS25]

Hexavalent - 19-7-2012 at 08:47

Surely you realise that when chemicals say they are "100%" pure, this is 100% rubbish? It is extremely unlikely that any compound ever made didn't have some kind of trace impurity, perhaps even present in extremely tiny amounts. Usually insufficient to count them, but enough to make the true purity less than 100%.

Mr means relative molecular mass, and can be calculated by taking the atomic masses of all the elements in the compound (found on a periodic table) and adding them together appropriately, or just look them up online. A good textbook should have a list of some common ones as well.

CHRIS25 - 19-7-2012 at 09:01

Quote: Originally posted by Hexavalent  
Surely you realise that when chemicals say they are "100%" pure, this is 100% rubbish? It is extremely unlikely that any compound ever made didn't have some kind of trace impurity, perhaps even present in extremely tiny amounts. Usually insufficient to count them, but enough to make the true purity less than 100%.

Mr means relative molecular mass, and can be calculated by taking the atomic masses of all the elements in the compound (found on a periodic table) and adding them together appropriately, or just look them up online. A good textbook should have a list of some common ones as well.


Ok thanks for that clarification. And well yes I agree with you about the 100% pure, but that applies to the supermarket!!! If a reputable chemical company says that the KOH flakes are quote 99%+ then for me I believe that they can not or rather dare not lie, especially since they supply to schools and labs. Off of ebay I would have my doubts, would never purchase from there unless at least 100 chemists had purchased before me!

[Edited on 19-7-2012 by CHRIS25]