I forgot to add baking soda to my boil and my low sodium water seems to benefit from a 75ppm level. Any reason I can’t add it at kegging time to get the sodium in there?
I would dissolve it in some hot water first to make sure it gets fully incorporated, but otherwise I don’t think there would be any problem. I’ve had to do the same thing with other salts before.
To me, the question seems to be “do yo normally add it to the boil or to the mash?”. If normally added to the boil, then I’d think adding it to the keg would be fine. If it’s normally added to the mash, you’ll miss the interaction with the grain that would happen if you add it to the keg. Does that make any sense?
This is very interesting. So it’s never too late to add brewing salts?
What I was trying to get at is that depending on the intent, it might or might not be. For instance, if you add CaCO3 to your mash, would it have the same effect adding it to the kettle or finished beer? In that case, I’d think not.
I agree with the mash-water salts addition. Would adding salts destined for the boil have the same effect or make that much of a difference by adding to the finished product instead? Why not the fermenter?
Not trying to stir stuff up here but truly curious. 
Yeah, I’m curious, too. This came up once (somewhere) in a discussion about adding gypsum post boil. I didn’t think it would be the same, but there was a convincing discussion that you could ad it later and it would still be the same. I haven’t tried it to know for myself.
In this case I used calcium chloride and and epsom salt to get my mash chemistry adjusted. I’ve found that adding baking soda really adds drinkablity to my beers. I didn’t want to add plain table salt to the keg since that might alter my chloride/sulfate ratio.
In my mind there are 2 reasons to add salts to the brewing water: 1. mash pH adjustment for good conversion and/or to decrease tannin extraction. Obviously those salts need to be added during the mash.
2. flavor - some use salts to mimic a local brewing water or use the Cl/sulfate ratio to effect the bitterness. These can be added anytime before drinking the beer.
In this case I think dee uses it for flavor, so I woud go ahead and add it anytime. It might cause a good bit of foam when adding it to the keg, so I would boil it in some water and add when cool.