Commercial Sodium Hydroxide Solution Freezing Point

[smckennz]

My books indicate that a 25% solution of sodium hydroxide should have a freezing point of around -18 deg C.
However a local reputable reagent supplier tells me that their 25% solution freezes on a cold (say 0 deg C) day.

Any ideas why the difference? Stratification,impurities, book error?

I had intended to use 25% to avoid the need for heat tracing.

Cheers

Steve

 

[25362]

Freezing points (F.P.)of solutions show a minimum and then rise again with increasing concentrations.

The CRC Handbook of Chemistry and Physics gives the lowest F.P. of -16.8[sup]o[/sup]C for a 14% mass solution of NaOH. I don't have the data, however, I believe that beyond this point richer concentrations have higher F.P.

Even sodium chloride with a minimum F.P. of ~ -21[sup]o[/sup]C at ~23% mass, raises its F.P. up to zero Celsius at 26.3 % mass. So, your supplier may be right after all.

 

[TD2K]

Do a search for "Dow Caustic soda solution handbook" (there's a wealth of physical property data for caustic soda on the web), it has all sorts of information such as density, viscosity and freezing points for various caustic soda solutions

25% by weight has a freezing point of about -20C. However, the freezing point rapidly rises with increasing strength. At about 28% to 29%, you are up near 0C for a freezing point. That may be what your supplier is thinking about, possible variations in solution strength. I don't know if you are bringing in 25% caustic soda or if you are diluting a higher strength solution (for bulk, 50% is a common strength to save costs of shipping water).

 

[MJCronin]

Steve,

TD2K is correct....

Unless you live in interior Alaska, the 25% solution will be acceptable for use without heat tracing.

Everybody and thier brother, purchases 50% soulution and immediately dilutes to 25%

In addtion to DOW Chemical, PPG industries (and others) have guidelines for "caustic soda handling" available on the internet....

Try this one.....

http://www.oxychem.com/products/handbooks/caustic.pdf

Tell us more about you system, particularly the tankage and delivery volumes....

Cheers !!

MJC