"One drop, one drop, carefully."
Rei operated the burette.
Kana recorded. "pH 4.5"
"Still not changing," Toma observed.
"Buffering action," Rei explained. "Even adding acid, pH doesn't change immediately."
"Why?"
"A weak acid and its salt coexist. Equilibrium counteracts the shift."
Kana wrote in her notebook. "Buffer solution = solution that resists change."
"Next, one drop."
Rei carefully added it. "pH 4.6"
"Only changed by 0.1," Toma said.
"Still within the buffer region," Rei confirmed.
Kana asked. "How long will this continue?"
"Until just before the equivalence point."
"Equivalence point?"
"The point where acid and base react exactly," Rei explained.
Milia entered the room. "Titration?"
"Yeah. Titrating acetic acid with sodium hydroxide."
Milia took out graph paper. "Try drawing the curve."
Kana plotted the data so far.
"Almost horizontal..."
"In the buffer region, change is gradual," Milia explained. "But at a certain point, it rises sharply."
"The equivalence point," Rei added.
"Let's continue."
One drop, then another. pH rose little by little.
"pH 6.0"
"pH 6.5"
"Still gradual."
Toma was getting bored. "Will it stay like this?"
"No, soon," Rei predicted.
"pH 7.0"
Kana noticed. "It's getting a bit faster."
"Getting close," Milia nodded.
"pH 8.0"
"Wait, one drop raised it by 0.5," Toma was surprised.
"Near the equivalence point," Rei tensed. "Next one is important."
Carefully adding one drop.
"pH 10.5!"
All three exclaimed.
"One drop raised it by 2.5!"
Milia looked at the graph. "This is the S-curve of titration."
A sharp rise. Kana was moved.
"Why so sudden?"
Rei began explaining. "Because the buffering action disappeared."
"Beyond the equivalence point, excess base directly raises pH."
Kana understood. "Nothing left to protect it?"
"Good metaphor," Milia acknowledged. "All weak acid is neutralized, can't buffer anymore."
Toma asked. "So where's the equivalence point?"
Rei pointed to the inflection point on the graph. "The midpoint of this sharp rise."
"In pH terms?"
"For acetic acid, about 8.7."
Kana was surprised. "Not pH 7?"
"In weak acid-strong base titration, equivalence point pH is greater than 7," Milia supplemented.
"Because the salt formed is weakly basic."
"Interesting," Kana murmured. "It's not symmetric."
Rei continued. "Conversely, for strong acid-weak base, equivalence point is less than 7."
Toma asked. "Indicators use this, right?"
"Yes. Substances whose color changes with pH."
Milia showed phenolphthalein. "This changes color between pH 8 and 10."
"Right around the equivalence point."
"That's why it's suitable for acetic acid titration."
Kana tried it. Adding one drop, it changed from colorless to pale pink.
"Beautiful."
"The beauty of chemistry," Rei smiled.
Milia showed another indicator. "Methyl orange changes between pH 3 and 4."
"This is for strong acid-weak base titration."
Toma understood. "Indicators are also chosen to match the reaction."
"Ones with a color change range close to the equivalence point pH."
Kana completed the graph. "S-curve, beautiful."
"The beauty of nonlinearity," Milia said. "Barely changing for a long time, then sudden transformation at a point."
Rei supplemented. "Nature has many such phenomena."
"Phase transitions, threshold reactions, catastrophe theory..."
Kana thought. "Quiet change exploding at a point."
"Exactly," Milia nodded. "The titration curve is a small model of that."
Toma began cleaning up. "Looks plain, but it's deep."
"The essence of chemistry," Rei said. "Making invisible changes visible."
Outside the window, clouds drifted quietly. Small changes accumulate, eventually reaching a major turning point.