Harmful Impurities in Liquid Caustic Soda

A. Since no one else has weighed in, I will give you the 5 cent sodium hydroxide spiel.

The best kind of caustic there is is mercury cell caustic. It's made by electrolyzing brine with mercury metal as the cathode, separating the Hg, then reacting it with water. Since the brine never directly contacts the product, it can be very low in chloride.

Lower grade is diaphragm cell caustic. In this process, the chlor-alkali cell is divided by a permeable divider that separates the chlorine part of the cell from the caustic part. The product is separated from the brine by "freezing out." Of course, it is loaded with chloride.

In between grade is membrane cell caustic, where the two compartments are separated by a cation permeable membrane, such as DuPont's Nafion. I think they may actually start out with some caustic soda as the catholyte, and it increases in concentration as the reaction proceeds. The sodium to make the NaOH comes through the membrane - the chloride from the brine is held back - mostly. There's still some in the product, but less than in the second kind I discussed above.

Chloride is a notorious troublemaker in many processes. It promotes pitting, etc. because it is a redox catalyst and initiator of corrosion reactions.

I hope this has been of some help.

A. The reason you are no longer getting reliable quality caustic is that mercury is subject to the ever increasing number of EU directives on chemicals.
Once again, the 'cure' is worse than the problem.

Geoff, please don't tell me that when writing the new directives, EU forgot to repeal The Law of Unintended Consequences?! They can look to the USA for model legislation; here, I'm told, laws don't have unintended consequences so we should not worry about it. Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey 2007 2007 Ted Thinking has no place in EU legislation. I believe we still lead the field here. Geoff Smith Hampshire, England