Hydrogen Peroxide Purification, July-03
PURITY OF INDUSTRIAL GRADE HP
Generally speaking, industrially produced hydrogen peroxide ? like the quality I use to buy here in Sweden - can be considered to be a quite pure chemical. Impurities are present on the ppm-level only. (ppm = parts per million).
The exception is two substances that are added to the HP by purpose, at production. Unfortunately at least one of these ?some times both- are poisonous to the decomposition catalyst!
One of the substances is a mineral acid ? nitric acid or phosphoric acid - that is added to the product in the production process to lower the pH and thus make it more stable. Nitric acid has not been shown to be poisonous to decomposition catalysts, but the phosphoric acid has. Unlucky for me, the product on the market in Sweden has phosphoric acid added to it.
Prior to distribution, the manufacturers add stabilizing agents to the product, so that it better tolerate contamination at handling and storage by the customers. An industrial standard stabilizer since almost 20 year is an organic phosphonic acid. It is called Dequest 2010 and is manufactured by Solutia (former Monsanto). It act as a complex binder and is quite efficient for its purpose, but again ? it is poisonous to decomposition catalysts.
Other, older stabilizers, like sodium stannate are not known to be poisonous.
PURIFICATION METHODS
Vaporization and re-condensation
The most common industrial method is to vaporize most of the crude HP prior to feeding it to the distillation column. The impurities will stay in the non vaporized part and is separated and sold as a lower purity technical grade quality. The vapors are condensed in the distillation column and will end up as a higher purity ?chemical grade? product in the bottom of the column.
Typically only about 80 % of the impurities are removed with this method. The reason why it is not more efficient is that some liquid feed, with high impurity level, carries over to the column as a mist.
The method requires two boilers to be practical: one crude feed boiler and one re-boiler in the bottom of the distillation column. As there is only one boiler in my distillation system at this time I would need to run the product two times in the equipment ? a double distillation procedure. This is time consuming so I have not practiced the method yet ? even if I could.
Freeze Crystallisation
One other method that can be used to both concentrate HP and to purify it, is freeze crystallisation.
The HP is cooled until ice crystals are formed. If starting with HP that is higher in concentration than 61%, the ice crystals will be higher in concentration than the starting material. The ice will also be more pure. By separating the ice from the mother liquid one receive a higher concentrated and more pure fraction than the starting material.
The method can be illustrated with help of the phase diagram on the upper part of this page:
If cooling 70% conc. HP, ice will start to form when hitting the freezing point curve at ?41 oC. The first ice will have a concentration of 88%. This concentration is found by moving horizontally in the diagram to the melting point curve. When cooling to lower temperatures, more ice will form, but it will be somewhat less concentrated than the first ice. The quantity of the ice fraction can be calculated, using the ?lever rule?.
In practice I have managed to receive a yield of separated high concentrated ice fraction of about 30% from the mother liquid, when starting with HP70. The level of impurities was typically reduced with about 70%.
The procedure may be repeated, receiving higher concentrations and higher grades of purity.
You can read more about freeze crystallisation in a patent by FMC, US2724640, and a patent by Laporte, GB889421.
Decomposition Catalysts, Aug.-03
This article was updated on January 2nd, 2006