Tom, being this thread originated from a post revolving around root beer it can be a slight bit confusing to follow I’ll admit that.
Here’s some clarity:
The OP on the root beer thread was adding 1 tab per gallon to remove chloramine/ chlorine which we all know is too high as that one tab is meant for 20 gallons. So this is what sparked my curiosity about the MANY things such as the following.
a. If dosing just plain water (mine for example/ SEPT report) 8.7 PH/ 51 ALK/ 24 CA/ 3 MG/ 17 NA/ 35 CL/ 21 SO4/ 3.65 ppm chlorine what would be the final effect for total/bound/ free SO2 using 1 campden tablet that weighs .60 gram/ 150ppm per tablet to a gallon of the following water? And is it the high PH that still binds portions of the sulfite in solution. ( Now this was preliminarily answered by tsans so I have some better understanding that it is mostly the volatiles in solution of must and that was exactly what I wondered because some of the cider, wine making sulfite calculators/ documents show this in their models, I have included a copy of one or two of the documents I’m referencing to. I fully understand that must itself will be within 2.8 - 4.0 in most cases so the water PH is what I wanted to understand because of how the root beer OP dosed his water. As said somewhere in this thread you would never need to dose water this high its just what spurred the whole question pattern.)
b. Being that tsans raised the issue about the water PH causing the Kmeta/ campden to cause rates of H2S more than the binding of the sulfite then sparked even more curiosity now regarding H2S to become an off aroma/ flavor in the finished root beer as in this case alone. Which brings more understanding of the kmeta/ campden as a whole for me. Please understand this is all worst case scenario learning curve questions not actual process.
c. As you can see in my example above I am in the heart of the St Paul, MN watershed district and feel blessed to have the water I do. I find no issue with it whatsoever other than treating it at .25 tablet per 5 gallons for chloramines/ chlorine and then minor salt manipulations for CA and SO4 and I use a slight amount of phosphoric to lower PH when mashing light SRM beers. Otherwise as stated to new brewers in my area without adjustment most 7-15 SRM beers will hit 5.2-5.5 PH in the mash.
d. I actually just got off the phone with a St Paul watershed lab tech as they have not posted OCT/ NOV water #'s so here are those if anybody is interested.
OCT: 32 CA/ 2 MG/ 17.5 NA/ 40 CL/ 21 SO4/ 55 ALK/ 67 HCO3
NOV: 25 CA/ 5 MG/ 18.1 NA/ 38 CL/ 21 SO4/ 55 ALK/ 67 HCO3
Cant attach the .xls file so here is the link and the I will link the page it comes from if any would like to browse it also it has some good info also about cider.
http://www.cider.org.uk/sulphite_binding.xls
webpage:
http://www.cider.org.uk/sulphite.html
Another good read about this topic from Moorbeer:
http://morewinemaking.com/public/pdf/so2.pdf
e. Like you just mentioned Tom that at/ above 4 PH in a must you either have to use so much as to be unreasonable and also you lose antioxidant properties that you should use acid to reduce the PH to under 3.8 first.