The average pH is 8.2 but this figure varies up and down by 0.3 pH units due to local, regional and seasonal variations. The surface waters of the world's oceans are slightly alkaline. The rapid changes already being observed in ocean chemistry are likely to affect organisms that rely on carbonate ions to build external calcium carbonate shells and skeletons because a reduction in pH reduces the amount of raw materials available for them to do so. As this demonstration shows, increasing the amount of carbon dioxide dissolved in water decreases pH. The majority of scientists now agree that climate change as a result of increased concentrations of carbon dioxide and other greenhouse gas from the burning of fossil fuels is a reality.Ītmospheric concentrations of carbon dioxide have risen from 280 parts per million in the pre-industrial era to about 380 parts per million today.Īnother major concern that has only been realised in the last decade or so is the potential impact of the increased concentrations of carbon dioxide on seawater. The Catalyst ' Ocean Acidification' story summarises the evidence for ocean acidification and its potential consequences. It is unlikely that your students will immediately link this demonstration with the potential problems of rising atmospheric carbon dioxide concentrations and the impact on marine organisms and ecosystems. ![]() Once you have established that exhaled air contains much more carbon dioxide than fresh air, the class may deduce that it is this gas that causes the pH to change. They should successfully deduce that the solution has become acidic.ĭiscuss what could be causing the solution's pH to change and, if necessary, guide them by asking them to recall the difference between exhaled air and fresh air. The solution will slowly but visibly change colour from blue/green to yellow/green.Īsk the class to observe what has happened to the pH of the solution. The solution will turn blue indicating a pH of 7.6 or more (note: the pH of baking soda in water is 8.2 but this is outside the range of the pH indicator solution).Ĥ. Now sprinkle a few grains of citric acid into the same beaker and the solution will change colour to yellow again.īefore proceeding, discuss the reaction of the pH indicator to acids and bases, that is, it turns yellow when the solution becomes acidic and blue when it becomes basic.ĥ. Ask a student volunteer to blow into the second beaker of water for thirty seconds. The solution will turn yellow indicating a pH 6.0 or less.ģ. Next, sprinkle a few grains of baking soda to the same beaker to demonstrate the effect of a base on the colour of the pH indicator solution. The water will appear green/blue indicating a pH of roughly 7.2 to 7.6 (use the colour chart provided with the indicator solution).Ģ. Add a few drops of white vinegar (acetic acid) to one of the beakers to demonstrate the effect of an acid on the colour of the pH indicator solution. Instructionsġ. Add several drops of aquarium pH indicator to each of two separate beaker or glass of water. ![]() ![]() The pH scale as shown above is called sometimes "concentration pH scale" as opposed to the "thermodynamic pH scale".This simple demonstration for teachers explains the process of ocean acidification. It is the effective concentration of H + and OH – that determines the pH and pOH. At very high concentrations (10 M hydrochloric acid or sodium hydroxide, for example,) a significant fraction of the ions will be associated into neutral pairs such as H +Cl –, thus reducing the concentration of “available” ions to a smaller value which we will call the effective concentration. This is a reasonably accurate definition at low concentrations (the dilute limit) of H +. When measuring pH, is in units of moles of H + per liter of solution. ![]() The historical definition of pH is correct for those solutions that are so dilute and so pure the H + ions are not influenced by anything but the solvent molecules (usually water). The identity of these solutions vary from one authority to another, but all give the same values of pH to ± 0.005 pH unit. The activity of the H + ion is determined as accurately as possible for the standard solutions used.
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