"This pH is exactly 7.0."
Toma showed the pH meter.
Kana was surprised. "Neutral? But we added ammonia, right?"
"We did. But it's a buffer solution."
Rei explained. "Ammonia is pretending to be neutral."
"Pretending?"
"Because it's a weak base. Doesn't completely dissociate."
Milia wrote an equation. "NH₃ + H₂O ⇌ NH₄⁺ + OH⁻"
"Equilibrium state. Only part makes OH⁻."
Kana thought. "Then why is it neutral?"
"Because acid is also in there," Rei pointed to the beaker.
"We added NH₄Cl. Dissociates into NH₄⁺ and Cl⁻."
"NH₄⁺ is an acid?"
"Yes. Can release protons. NH₄⁺ ⇌ NH₃ + H⁺"
Toma organized. "Both NH₃ and NH₄⁺ are present. Conjugate acid-base pair."
"And they're in equilibrium."
"This is a buffer solution," Milia supplemented.
"Even if you add acid or base, pH doesn't change much."
Kana experimented. "Then let me add a little acid."
Dropped a few drops of HCl. pH meter: 6.9.
"Hardly changed!"
"Why?"
Rei explained. "The added H⁺ reacts with NH₃ to become NH₄⁺."
"It's consumed."
"Yes. That's why pH is stable."
"Conversely, if we add base?"
Toma dropped NaOH. pH: 7.1.
"This also hardly changed."
"NH₄⁺ releases H⁺ to neutralize OH⁻."
Kana understood. "The equilibrium shifts to absorb the change."
"Le Chatelier's principle," Milia said.
"The system resists external changes."
Rei supplemented. "But there's a limit to capacity."
"If you add too much acid, NH₃ runs out."
"Then pH changes rapidly."
Kana asked. "Why is this important?"
"Biological pH is strictly controlled," Milia answered.
"Blood pH is 7.4. Even slight deviations are dangerous."
"Buffer solutions protect it?"
"Yes. Bicarbonate buffer, phosphate buffer... multiple buffers work."
Toma gave an example. "Lactic acid forms in muscles, H⁺ increases."
"But buffers immediately neutralize it."
"So pH doesn't drop sharply."
Rei said quietly. "While pretending to be neutral, weak bases are actually working."
"Not conspicuous, but important."
Kana was impressed. "Modest guardians."
Milia continued. "Buffer capacity is determined by the [base]/[acid] ratio."
"Henderson-Hasselbalch equation. pH = pKa + log([A⁻]/[HA])"
"When the ratio is 1:1, buffer capacity is maximum."
Toma experimented. "Then let's change the ratio."
Added more NH₃. pH: 8.5.
"Shifted to basic."
"Because NH₃ is excessive."
"But still buffering?"
"To some extent. But not optimal."
Kana summarized in her notebook. "Balance is important."
"Not too strong, not too weak."
Rei nodded. "The aesthetics of weak bases."
"Protecting pH while pretending to be neutral."
Milia said quietly. "Chemistry's humility."
The three stared at the buffer solution. Looks like ordinary water. But inside, weak bases work quietly. The story of the base pretending to be neutral continues today.