LoDo sparge pH issue

[JJFlash]

This year I decided to integrate full LoDo with ale production to see if I can decern a difference.
With LoDo my sparge liquor now goes from pH 5.6 to pH 4.6
Today's brew 7.25 gallons sparge water
Using Bru'n Water I added:
Calcium Chloride 4.5 grams
Calcium Sulfate 0.4 grams
Phosphoric Acid 25% 1.3 ml
No issue here, pH now 5.6
Added LoDo:
Sodium Metabisulfite 20ppm 0.8 gram
Ascorbic Acid 20ppm 0.8 gram
pH now 4.6
I though Ascorbic Acid was a weak acid addition.
Not sure where this is going wrong.
Any LoDo brewers around to help a guy out?

 

[DonT]

This is just the water, right? Before grains are added?

 

[JJFlash]

Sparge water only.

 

[marc1]

If your sparge water has low buffering capacity, small acid additions can cause larger swings. Why are you concerned about the slightly lower sparge water pH? High pH from lots of alkalinity is the traditional concern.

 

[scrap iron]

I would suggest going no sparge if you want the full LO experience. Also don't worry about water ph it's the mash ph after adding water to the grains that needs attention. You don't say what type of water you use so try to use low alkalinity to start with or RO water.

 

[JJFlash]

Thanks for the replies.
RO water routinely for many years.
Kettle end of boil was fine at pH 5.18
Past few brews have been checking sparge water pH before and after NaMBS and Ascorbic Acid additions.
Just surprised at big drop with such a weak acid addition in 7.25 gallons.
As suggested low buffering capacity with RO water is likely source.
My measurements are precise with high quality scales.