Adding sulphites before or after racking

[Rice_Guy]

* potassium metabisulphite is K2S2O5 it is a salt where potassium with a positive charge balances a negative charge coming from an oxygen. It is extremely soluble in water, in the ppm levels we talk about it will dissolve!. It has an ionization constant around pH 2.5 which is why we add citric acid to our sanitizers, , , we want to force as much as possible to react with H+ contributed by water to produce SO2. This reaction is reversible. Therefore if we let the pH approach 7 it will stay in the metabisulphite salt form which really isn’t a good sanitizer. Wine at pH 3.5 is a reasonable good solvent to push SO2 release , , (most up us have seen a table of free SO2, which was calculated based on pKa, and says how much is free)
* Metabisulphite is some what reactive to humidity. I wouldn’t call it extremely reactive like a water activity salt which will pull moisture out of desert air at 5% humidity. ie it has a good shelf life, , , BUT keep it in a sealed jar so humidity doesn’t get at it. (As a lab chemical I would keep it in the sealed jar 10 years)
* If I was making a tablet I have put the powder from the chemical jar in a tablet press with about 10,000 pounds of force to create the dense, reduced surface area tablet. Simple force without binders holds everything together and! because there is less surface area there is slower solubility. If I walked down the hall to Wyler’s bouillon their patent has sodium bicarbonate in it which is a neat trick to break down the rectangular tablet. I like the INDOSE trick, , , Good technology!
INDOSE is probably a large uniform tablet so I would expect the same chemistry on half or eighth or whole. The potassium bicarbonate has a higher water activity so the shelf life should be slightly reduced from the powder in my chemical jar. The water activity is significantly lower than bouillon cubes so , , , all in all the shelf life of meta is good (Wylers will suck moisture out of 40% humidity and form a soft blob)

Based on experience and a few chem textbooks I do not toss 5 year old meta which has been stored in a humidity tight jar. If it loses the burn in the nose when mixed into citric acid sanitizer sure I would toss it

 

[CDrew]

* If I was making a tablet I have put the powder from the chemical jar in a tablet press with about 10,000 pounds of force to create the dense, reduced surface area tablet. Simple force without binders holds everything together and! because there is less surface area there is slower solubility. If I walked down the hall to Wyler’s bouillon their patent has sodium bicarbonate in it which is a neat trick to break down the rectangular tablet. I like the INDOSE trick, , , Good technology!
INDOSE is probably a large uniform tablet so I would expect the same chemistry on half or eighth or whole. The potassium bicarbonate has a higher water activity so the shelf life should be slightly reduced from the powder in my chemical jar. The water activity is significantly lower than bouillon cubes so , , , all in all the shelf life of meta is good (Wylers will suck moisture out of 40% humidity and form a soft blob)

Not sure what your point is.

Indose tablets (and other brands) come as a tablet, and as granules. They are a mix of KMBS and K Bicarb. The granules come in 2 gm and 5 gm individual dose packs that are foil lined. For all intents and purposes, the dose pack would have an infinite shelf life. For all I know, the granules are tablets first and then crushed into granules by the manufacturer. But at the home wine making scale, the granules are extremely convenient. Dump in the wine, cap and job done. No pre-mixing or other prep.

But since the granules are more expensive, I would only use for adding sulfite to wine, and not for making sterilizing solutions.

 

[Rice_Guy]

@ cdrew; The point on metabisulphite is it is extremely stable unless in an acid like pH 1. This is good since the break down product of meta is SO2 which is toxic/ burns the lungs. , ,
I scratch my head with the folks in this thread who toss meta or meta tablets at 12 months and ask WHY? , , are they poisoning themselves?