Hold on, let's emphasize several statements in this thread so that neither the op or future readers get confused.
1) Hydrogen sulfide is H2S, not SO2
2) Time will not dissipate the H2S and can make the problem worse by allowing it to form disulfides/mercaptans (as stated already by @bernardsmith) which are harder resolve.
3) Copper is the solution to the problem but excess copper is toxic, so you must carefully make copper-sulfate additions so you don't poison yourself. See below for details.
4) Prevention is truly the key. Timely addition of yeast nutrients (as stated already by @BoozeMedic ) will largely prevent H2S formation.
5) H2S is an odor we are very sensitive to but it is also fairly soluble and does not come out of solution easily. So attempts to purge it out of solution with CO2 or splash racking are ineffective.
6) Be cautious with adding yeast nutrients toward the end of fermentation or you may end up feeding spoilage organisms with left-over nutrients.
To solve your H2S problem, get a bottle of 1% copper sulfate and add 0.75ml of it per gallon of your cider. Stir it in and smell it the next day. Repeat until the odor is gone. This slowly steps you toward removal of the H2S without introducing excess copper. It may take several doses. The copper will react with the H2S to form copper-sulfide which will precipitate out. Good luck and brew on.
See if you can find copper sulfate packed for wine makers. I would be very wary about using this chemical in a haphazard manner. Copper Sulfate is not harmless. Try using copper wool (used for scrubbing pots and pans) first.
I think I read that using yeast hulls and bentonite after a copper treatment will absorb and drop out all the copper, but don't quote meI found some 1% copper sulfate solution (made from reagent grade copper sulfate pentahydrate). I’ve also reviewed the MSDS. Yes, I’m very wary of using this solution, and will be very precise and methodical while making additions. I’ll be adding 0.1 ppm at a time. Using copper wool in such an acidic liquid I couldn’t be precise as to the amount of copper ending up in the solution, so I’m committed to using the copper sulfate instead.
Hey! Sorry to jump on this thread many years after it was posted! But I have splash racked a cider (4 times back and forth) and added some SO2 to the carbuoy to prevent oxidation, and will check it a couple of days, maybe splash rack it again. But if I 'need' to go down the copper route, how would I use copper wool instead of copper sulfate solution. I have 1GL batches, so I read 0.5 - 0.75ml for that, but would love to know how to use copper wool instead if its better, less hazardous (and also to learn a new technique...!)See if you can find copper sulfate packed for wine makers. I would be very wary about using this chemical in a haphazard manner. Copper Sulfate is not harmless. Try using copper wool (used for scrubbing pots and pans) first.
Not so certain that it is the copper, itself that is toxic, but for sure even a small quantity of copper sulfate can kill you dead. Using copper pots or copper wool to scrub pans would seem to be harmless even if you cook with lots of acidic foods or you are very aggressive when you clean with the wool.Use Reduless for all the reasons @bernardsmith mentioned above. Copper is toxic, so you need a way to control the amount that you release into your cider.
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