The New York Times had an article on this, as I recall.

I haven't been making either pretzels or bagels lately with my wife on a low-carb diet, but this is still on my list of things to try. I've got a digital pH meter now (for our hot tub) and I think I may try several solutions made with various equal weights of both in the same volume of water to see how much difference in pH it makes.

I did talk to a chemistry professor about this when we first talked about this several years ago, to confirm that putting the baked sodium carbonate in water wouldn't turn it back into sodium bicarbonate. He wasn't sure how much difference it would make in pH, but he did think the sodium carbonate would be a stronger alkaline solution.