You should adjust for Residual Alkalinity (RA) because it’s the true measure of your water’s buffering capacity. It dictates how much your mash pH will resist change, with the ultimate goal being the ideal mash pH (5.2-5.6) for enzyme activity. This makes RA the primary water target and mash pH the final quality check. High RA water makes it harder to brew pale styles. High RA water is harder to lower pH, which means you need more acids, or just brew dark beers. Low RA water is easier to adjust for pH, which is why it’s more suited to brewing pale style beers, where acidity is key. 

Residual Alkalinity is a sum of the dissolved minerals, such as calcium, magnesium, and carbonates. It affect the pH of the water. Carbonate ions, in particular, act as a buffer and can resist changes in the pH of the water.

Why Residual Alkalinity is Key:

• Buffer, Not Direct pH: Alkalinity (like carbonates) resists pH changes, but Residual Alkalinity specifically measures the excess alkalinity that isn’t neutralized by minerals like calcium and magnesium, directly influencing mash pH.
• Grain Interaction: Different grains (especially darker, roasted malts) have inherent acidity that counteracts alkalinity, so water with balanced RA helps achieve the right mash pH consistently. 

Why Mash pH is the Goal:

• Enzyme Efficiency: The mash’s pH (ideally 5.2-5.6) is crucial for saccharification, the process where enzymes convert starches to sugars, impacting flavor and efficiency. 

How They Work Together:

1. Measure Your Water: Test your brewing water for alkalinity and hardness.
2. Calculate Target RA: Use brewing software or charts to find the target RA for your beer style (e.g., lower for pale ales, higher for stouts).
3. Adjust RA: Add brewing salts (like gypsum, calcium chloride) to lower RA or baking soda to raise it, depending on your water and beer.
4. Check Mash pH: Brew your beer and measure the mash pH; adjust further with acids (lactic acid) or baking soda as needed to hit your target. 

In summary: Adjusting your water’s Residual Alkalinity is the proactive step that sets the stage, while monitoring and hitting the target mash pH is the reactive, final confirmation for great beer quality.