Short answers, pulled from the story.
Seawater contains chloride at a concentration of 19,400 milligrams per liter, making it the dominant ion in the ocean. Smaller but more concentrated chloride reserves occur in inland seas such as the Dead Sea in Israel and the Great Salt Lake in Utah.
Chloride is the most abundant anion outside cells and accounts for roughly one third of extracellular fluid's tonicity. It helps maintain acid-base balance, transmits nerve impulses through channels including the GABAA receptor, regulates fluid movement in and out of cells, and is required for the stomach's production of hydrochloric acid. The kidneys regulate its concentration in blood, a value called serum chloride.
Chloride is detected by adding silver nitrate to a solution; chloride ions react with silver ions to form a white silver chloride precipitate. A chloridometer can then measure the total chloride concentration by tracking silver ions once all chloride in the sample has precipitated.
Chloride significantly worsens pitting corrosion in most metals, including stainless steels and aluminum. In concrete, chloride-induced corrosion breaks down the protective oxide layer that alkaline concrete maintains around steel reinforcement, allowing localized corrosion to take hold.
High chloride concentrations can acidify streams, mobilize radioactive metals in soil through ion exchange, harm the reproduction and survival of aquatic plants and animals, and allow saltwater organisms to invade freshwater habitats. Sodium chloride can also alter microbial communities, slow denitrification, and inhibit nitrification even at relatively low concentrations.
The chlor-alkali process converts concentrated sodium chloride solutions into chlorine gas and sodium hydroxide through parallel electrochemical reactions. It is a major consumer of the world's energy budget and the outputs are used to manufacture a wide range of other chemicals and materials.