Wouldn't the recipe for brine be better unlocked by the electrolysis-2 tech that unlocks saltwater electrolysis? To have to go through all the RTG research to be able to mix salt and water seems odd.

(Edit: brine electrolysis recipe is unlocked by electrolysis-2. The recipe for brine isn't.)

Using Brine in electrolysis is an alternate recipe.
I mean... Yeah, I put the recipe to use it on electrolysis, rather than up in RTGs, but the idea is basically just to allow you to use Brine in electrolysis now that its added. it was added specifically for the RTG recipe chain, otherwise you'd need more fluid inputs than the chemical plant has.
(I mean, you don't need brine specifically, because salt is an item, but the chemistry does say that first step is to put salt in the water, so, it had to come first.)

The recipe for brine is unlocked by the RTG tech. The recipe for brine electrolysis is unlocked much earlier, but is useless without the brine recipe.

(I mistakenly thought both were locked initially, my mistake)