I found two different types of formulas for Pyrocat HD. One says Sodium Bisulfite the other Sodium Metabisulfite. Is there any difference? Which one is right?
There's a whole thread about Bisulphite/metabisulphite. In the US they are sold as one and the same, although there is a difference. In Europe Metabisulphite is cheap & common, true Bisulphite is expensive as it's unstable. US Bisulphite is usually a mixture of Bisulphite & Metabisulphite.
There's a whole thread about Bisulphite/metabisulphite. In the US they are sold as one and the same, although there is a difference. In Europe Metabisulphite is cheap & common, true Bisulphite is expensive as it's unstable. US Bisulphite is usually a mixture of Bisulphite & Metabisulphite.
I mix Pyrocat HD stock A in propylene glycol. A small amount of ascorbic acid takes the place of sulfite or bisulfite for catalyzing the Phenidone-catechol interaction. The B part is the same for all the Pyrocat series.
Pat is correct. The purpose of the bisulfite or metabisulfite is two fold, to preserve the stock solution, and to release a small amount of sulfite into the working solution when mixed with water.
When mixed in glycol there is no need for a preservative, and the ascorbic acid plays the same role as sulfite in the phenidoen/catechol interaction, and it also has some advantage in slowing down oxidation. I will probably at some time in the future ask the folks at Formulary to start mixing the glycol version of Pyrocat-HD with ascorbic. But then there would be another question to ask about different formulas!!
I mix Pyrocat HD stock A in propylene glycol. A small amount of ascorbic acid takes the place of sulfite or bisulfite for catalyzing the Phenidone-catechol interaction. The B part is the same for all the Pyrocat series.