Sodium hydroxide splits into two ions the instant it hits water. One does the work and disappears. The other does nothing — and stays permanently. Here's what's really happening every time we inject caustic.
Narrated overview of this module. The article below has the full procedure and the numbers.
Caustic soda is sodium hydroxide. In water it dissociates completely — no leftover “NaOH molecules” floating around. You now have two separate, independent ions, and they behave nothing alike.
From this point on, the sodium and the hydroxide have completely separate stories.
The OH⁻ is the reactive half, and it's the reason we dose caustic at all: it neutralizes the acidity in the water. In most of our source waters that acidity is dissolved CO₂ / carbonic acid, and the hydroxide converts it up the carbonate ladder:
pH climbs because you've knocked out the acid. But look at where the hydroxide ends up: it turns into bicarbonate, carbonate, or plain water. At normal treated-water pH there's essentially no free hydroxide left. It did its job and transformed into something else.
Meanwhile the Na⁺ just sits there. It doesn't react with hardness. It doesn't precipitate. It doesn't get stripped out. It doesn't leave. Every pound of caustic we inject drops its sodium into the water permanently — sodium up, TDS up. And to keep the charge balanced, that sodium is now paired with the fresh bicarbonate you just made:
Net result: you've effectively dosed the water with sodium bicarbonate. The pH you wanted came bundled with sodium you may not have.
Keep injecting and watch which numbers move. pH creeps toward a ceiling as the buffer builds — but sodium climbs with every single dose, and the “removed” line never budges off zero.
Illustrative — directional behavior, not a real dosing calculation. Actual mg/L depends on caustic strength, dose rate, and the water's starting chemistry.
Caustic is a pH / alkalinity tool — a good one. Just remember it only ever adds. Keep it front of mind wherever sodium or TDS is what the customer actually cares about:
Sodium taste and sodium-restricted diets. EPA guidance sits around 20 mg/L for restricted intake. Every caustic dose nudges that number the wrong way.
More sodium = more load on the membranes and more sodium in the permeate. You can end up treating a problem the caustic helped create.
Sodium drives up SAR (sodium adsorption ratio), which degrades soil structure. Adding sodium to ag or reuse water can quietly cause damage downstream.
Caustic pushes TDS up. If a site is near a discharge cap, dosing caustic eats into the margin you have left.
If the goal is really just to knock down CO₂ or lift pH, ask whether stripping the CO₂ (aeration / degasification / a decarbonator) gets you there while adding no ion at all. Other pH chemicals simply swap the trade — lime adds calcium and hardness, KOH adds potassium. Pick the trade that fits where the water is headed.
Caustic trades acidity for sodium. The hydroxide clocks out — the sodium never leaves.
