# The Reef's pH A reef research station where cabbage indicator, a titration, a buffer and a CO2\-bubbled reef tank teach acids, bases and pH By Numa Topic: Physics & Chemistry [Open the interactive Jig](https://ask.jig.com/j/hU4hd8TxeI3C) ## Transcript ### 1. Chemistry on a coral reef This research station watches one number in the water: pH\. It shows how acidic or basic a solution is, set by its hydrogen ions, and usually runs from 0 to 14\. Pure water is 7, neutral\. The tank's seawater is 8\.1, slightly basic\. Tap a coloured jar to read its pH\. Below 7 is acidic and above 7 is basic\. A base that dissolves in water is called an alkali\. ### 2. Acids and bases An acid gives hydrogen ions, which are bare protons, to water\. A base takes them up, often by releasing hydroxide ions\. A strong acid gives up every proton\. A weak acid, like vinegar, gives up only a few\. Choose one to see it in water\. Arrhenius: acids make hydrogen ions in water, bases make hydroxide ions\. Brønsted–Lowry: acids give protons, bases take them\. Real vinegar ionises far fewer than shown\. ### 3. The pH scale counts in tens The red dropper holds acid\. One drop takes this jar of water from pH 7 to pH 4\. Ten drops reach pH 3, and a hundred drops reach pH 2\. Each pH unit is a factor of ten, so pH 3 has ten times more hydrogen ions than pH 4\. ### 4. Indicators: colour reads the pH An indicator is a dye that changes colour with pH\. Red cabbage juice turns red in acid, purple near neutral, and blue, green, then yellow in strong base\. Watch each everyday liquid go in and see where it lands on the scale\. Litmus is a simpler indicator: red in acid, blue in base\. Universal indicator shows a colour for every pH\. ### 5. Neutralisation and titration When an acid meets a base, hydrogen ions and hydroxide ions make water, leaving a salt\. This is neutralisation\. A titration adds base bit by bit to find exactly how much neutralises the acid\. That exact point is the equivalence point\. The flask holds 25 millilitres of hydrochloric acid and an indicator\. Drip in sodium hydroxide of the same strength\. At 25 millilitres it turns pink\. Phenolphthalein, the indicator here, turns pink above pH 8\.2\. Near the equivalence point, one drop swings the pH from about 4 to 10\. ### 6. Buffers resist change The left jar holds pure water and the right jar holds seawater\. Both contain cabbage indicator\. The acid dropper gives each jar the same three drops\. Seawater is a buffer, a solution that resists changes in pH, because its carbonate ions catch hydrogen ions\. Your blood is buffered too, and stays between pH 7\.35 and 7\.45\. ### 7. Ocean acidification The ocean has absorbed about a quarter of the carbon dioxide people have released\. Here it bubbles into the tank\. It forms carbonic acid, so the pH falls\. The extra hydrogen ions use up the carbonate that shells are built from, and the shell starts to dissolve\. This tank goes much further and faster than a real ocean\. Real seawater went from about 8\.17 in 1850 to 8\.08 today\. ### 8. A small number, a big change Slide through the years to watch the reef's pH\. Since 1850 it has fallen by about a tenth of a unit\. That means roughly a quarter more hydrogen ions\. On a high emissions path it could reach about 7\.7 by 2100\. With low emissions it levels off near 8\.0\. Lemon 2, vinegar 3, rain 5\.6, water 7, seawater 8\.1, baking soda 9, bleach 12\.