Needed purity of Causic Soda?

[bongbudda]

I have found a DIY store who sells 'drain unblocker' which is 99-100% pure sodium hydroxide.
Now my questions are;
For the conversion how pure does it need to be? If its 99% pure whats the rest and will it harm me?
How do I know it is that pure?
Any info would be great.

 

[killarava2day]

It needs to be 99.9% pure at least, the impurities in it can be VERY nasty. Unless it is of a very high grade it is likely to contain heavy metals such as lead and mercury which can build deposits in your brain, most undesirable to say the least.

 

[day_for_night]

this is not a cooker's web site. there are other pages for that.