"Drop by drop, carefully."
Toma was adding base from the burette.
Kana stared at the flask. "The color hasn't changed yet."
"A little more," Rei said. "We're approaching the equivalence point."
Toma added one more drop. The liquid suddenly turned pink.
"Whoa!"
"Equivalence point," Rei quietly declared. "Acid and base have completely reacted."
Kana recorded in her notebook. "23.5 mL. Can we calculate the concentration from this?"
"Yes. The principle of titration. Measuring unknown acid concentration with known base concentration."
Toma looked at the phenolphthalein bottle. "Why does it turn pink?"
"pH indicator," Rei explained. "The molecular structure changes with pH, changing the color."
"Interesting."
"Phenolphthalein turns pink above pH 8. It's colorless around neutral."
Kana thought. "So it changes color at pH 8, not 7."
"Correct. That's why it's suitable for strong acid-strong base titrations. The equivalence point is pH 7 with a sharp change."
Toma picked up another indicator. "What about methyl orange?"
"Changes color from pH 3.1 to 4.4. Used for weak base titrations."
Kana drew a diagram in her notebook. "The titration curve looks like an S."
"Yes. It changes gradually, then rises sharply at the equivalence point. Choose your indicator in that steep region."
Rei drew on the whiteboard. Horizontal axis showing base amount, vertical axis showing pH.
"This is the buffer region," Rei pointed to a flat section in the middle. "pH doesn't change much."
"Why?" Toma asked.
"Weak acids like acetic acid only partially dissociate. Additional base first neutralizes the undissociated acid."
Kana wrote the equation. "CH3COOH + OH- → CH3COO- + H2O"
"Right. Acetate ion increases, undissociated acetic acid decreases. When both exist, it becomes a buffer."
Toma understood. "That's why it's flat."
"But when all acid is neutralized, pH suddenly rises. Can't buffer anymore."
Kana gazed at the graph. "Beautiful curve."
"The art of chemical equilibrium," Rei smiled.
Toma prepared a new sample. "Let's try with hydrochloric acid next."
"It's a strong acid, so the curve shape will be different," Rei predicted. "No buffer region."
They started titration. This time, pH changed sharply from the start.
"It's true!" Kana was surprised. "Completely different from weak acid."
"Strong acids are fully dissociated, so additional base immediately raises pH."
Toma overlaid the graphs. "Strong acid and weak acid, different shapes."
"But they eventually reach the same place," Rei pointed out. "Once fully neutralized, pH is the same."
Kana suddenly thought. "Stomach acid is hydrochloric acid, right?"
"Yes. pH is 1 to 2. Very strong acid."
"But it gets neutralized when food enters?"
"Partially. Especially proteins and minerals have buffering action."
Toma became interested. "So titration happens in the stomach too?"
"In a sense. But the stomach quickly replenishes acid. To keep pH low."
Rei supplemented. "Acidic environment is needed for digestion. But it changes to alkaline in the small intestine."
"How?" Kana asked.
"Bicarbonate ion is secreted from the pancreas. It neutralizes stomach acid."
Toma laughed. "The body is full of titrations."
"Exactly," Rei acknowledged. "Life is a battle of pH control."
Kana stared at the flask. Pink liquid. Evidence of a small battle.
"One drop changes the balance."
"Yes. Delicate, precise, and beautiful," Rei said.
Toma started washing the burette. "Let's battle again tomorrow."
"The small battle of neutralization," Kana murmured.
Rei nodded too. Acid and base. While opposing, they create balance. That's chemistry. That's life.