jamit - 5-11-2014 at 01:49
It is said that sodium tetraborate can be used as a primary standard to titrate hydrochloric acid. Sodium tetraborate can be purchase as borax
(laundry). I can recrystallize it to purify it. But how pure can I get after maybe 3 recrystallization? 99.9%? enough to be used as a primary
standard?
Also, how can I tell if my purified crystals of sodium tetraborate is the decahydrate or pentahydrate? How can I tell the different crystal water
content? I can't do a melting point to determine it since the decahydrate melts at too high a temperatue - 743C.
If I heat it at about 300C, can I convert it to the anhydrous form?
Lately, I've been into analytical chemistry and into purifying OTC. At what point is re-crystallization ineffective at further purifying a chemical?
Sorry for the many questions, but if you can point me in the right direction, I would appreciate it.
thanks
Chemosynthesis - 5-11-2014 at 12:24
If you have a known purity and molarity of acid, and a good balance, you could back titrate the borax prior to using it as a standard.
"4389. To Test the Purity of Borax. Its strength is best ascertained by the quantity of sulphuric acid required to neutralize a given weight of borax.
(See Alkalimetry.) The impurities in borax are common salt and alum, which are mixed with it to lower the value.
Common salt may be detected by a solution of the borax in hot water yielding with nitrate of silver a curdy white precipitate which is soluble in
ammonia; this must be distinguished from the white pulverulent precipitate of borate of silver which will be thrown down from pure borax.
The presence of alum is determined by addition of ammonia water to a solution of the borax giving a bulky white precipitate."
http://chestofbooks.com/reference/Encyclopedia-Of-Practical-...
unionised - 5-11-2014 at 12:46
If you recrystallise it from water you will get the decahydrate.
A few crystallisations will get you a very pure product.
On the other hand, heating bicarbonate of soda in a clean container in the oven to 250 C will get very high purity sodium carbonate which you can also
use as a standard for titration.
Recrystallised sulphamic acid is also a useful "standard".
jamit - 5-11-2014 at 15:23
@chemosynthesis and @unionised
thank you for that helpful info. I'm assuming that you have tried recrystallization for these products (borax, sodium bicarbonate and sulfamic acid)
from personal experience?