Sciencemadness Discussion Board

Determning purity of OTC sodium hydroxide

Housane - 4-5-2019 at 13:02


I have some OTC sodium hydrooxide form essential power and was wondering how to work out its purity as i hve no chemicals of known purity as mine are all synthsised by me or OTC. Any help greatly receved, BTW i did check the msds but they are too vauge. I hve much glassware and enough to do a titration if it would help.

EDIT: It is in solid form in smal 1mm diameter pellets



[Edited on 4-5-2019 by Housane]

RedDwarf - 4-5-2019 at 14:33

Without any reagents of known strength a titration won't be of any use to you (you need to know what you're measuring against!)

You also need to think what you are really trying to find out - the "strength" of your NaOH or the presence/level of a specific impurity?

In the case of NaOH all is probably not lost as it's a commodity chemical with relatively few manufacturers and a form that doesn't easily lead to adulteration - ie the specification of your NaOH is likely to be the same as similar quality products. It's no use comparing against analytical NaOH specs as that's obviously not what you bought, but a technical grade NaOH will have similar specs. This is one from a german supplier (I had their site open so apologies for it not being an english spec):

Gehalt mind. 99,30%

Natiumcarbonat (Na2CO3) max. 0,6%

Natriumchlorid (NaCl) max. 0,01%

Natriumsulfat (Na2So4) max. 0,005%

Eisen (Fe) max. 0,004%

Quecksilber (Hg) max. 0,0001%

So your major impurities are Sodium Carbonate, Sodium Chloride and Sodium Sulfate ( as you'd expect from the manufacturing process). There might also be some water as it's hydroscopic. Most adverts I see for NaOH online state 99% pure so I'm pretty sure your product will be close to the above.

DavidJR - 4-5-2019 at 15:05

I would expect at least 10% water.

CobaltChloride - 4-5-2019 at 15:27

Here you can get all sorts of NaOH grades. The cheapest one is ~70% NaOH and ~30% NaCl, while the one I use the most turned out to be ~98% after titration with oxalic acid (using phenolphthalein indicator).

kulep - 5-5-2019 at 04:51

I have some that I think is almost entirely carbonate as it came in a bag and had a lot of water

unionised - 5-5-2019 at 05:59

You can use sodium bicarbonate as a reference material. The stuff they use in cooking is of very high purity.
If you bake it in the oven at about 200C it loses water and CO" and becomes very high purity sodium carbonate.
With that you can check the concentration of some acid- pipe descaling or patio cleaning solutions are generally acids of some sort.
And that gives you an acid with a known molarity.
Then, using that, you can determine the purity of the NaOH.

woelen - 6-5-2019 at 01:48

Are you sure the baking stuff for the kitching is very pure NaHCO3? I have the impression that it is quite impure. A solution of the stuff I can buy in the supermarket remains cloudy, even at low concentration. I have the impression that maybe 30% or so is other material than NaHCO3. A fairly large part of this is insoluble matter.

Recently, I purchased sodium bicarbonate, sold to me as a chemical of 99+ % purity. Solutions of this are totally clear.

unionised - 6-5-2019 at 01:53

Do you know that baking powder is not the same as bicarbonate of soda?

Food grade NaHCO3 usually meets the spec for pharmaceutical grade. (Labeled BP, USP or P Eur, as appropriate)
That means an assay value (by titration) of 99 to 100.5%

[Edited on 6-5-19 by unionised]

CobaltChloride - 6-5-2019 at 02:15

It also depends on where you buy it from. Solutions of my supermarket NaHCO3 marketed as baking powder aren't fully clear, but the sodium bicarbonate I bought from a en-gros hypermarket in a 1kg bag as simply "food grade sodium bicarbonate" makes completely clear solutions and thus seems purer.

This seems to coincide with what unionised said.

Housane - 7-5-2019 at 04:40

Quote: Originally posted by RedDwarf  
Most adverts I see for NaOH online state 99% pure so I'm pretty sure your product will be close to the above.

Ok thanks i will assume it is 99% pure and use it to titrate my other acids and bases

Tsjerk - 7-5-2019 at 09:47

My NaOH comes as draincleaner for five euro per kilo and it stated on the bottle it is 99%, which I don't doubt.

MrHomeScientist - 7-5-2019 at 10:02

Also note that "baking powder" and "baking soda" are two very different things. Something my girlfriend discovered when she tried making pancakes using the wrong one.