Acetic acid

Acetic acid is the acid in vinegar, which is mostly water with about 4 to 8% acetic acid. The pure chemical is a clear, sharp-smelling liquid that freezes at about 17 °C, so on a cool day it forms ice-like crystals; that's why it's called glacial acetic acid. Its formula is CH₃COOH: two carbon atoms, four hydrogen atoms and two oxygen atoms.

Vinegar makers let bacteria turn the alcohol in wine or cider into acetic acid. Industry is faster. Most acetic acid is now made by methanol carbonylation: methanol, itself made from natural gas or coal, reacts with carbon monoxide over a rhodium or iridium catalyst, and the two molecules join into acetic acid. Monsanto developed the process around 1970, and BP improved it in the 1990s as the Cativa process.

In the TV build it has two jobs. Joined with ethylene and oxygen, it becomes vinyl acetate, the start of the PVA film in a polarizer. Reacted with cellulose, usually after first being turned into acetic anhydride, it adds the acetyl groups that make cellulose triacetate, the clear protective film on each side of the polarizer.