Alkaline carbon dioxide behavior

Whether for a class demonstration, a practical joke, or perhaps a clandestine activity, disappearing ink is a fascinating substance. What is the secret to its action One formulation of disappearing ink contains a common acid-base indicator, that is, a substance that by its color shows the acid or basic nature of a solution. One acid-base indicator that shifts from a colorless hue under acidic conditions to a deep blue color in alkaline solutions is thymolphthalein. If the indicator starts off in a basic solution, perhaps containing sodium hydroxide, the typical blue color of an ink is perceived. How does the ink color disappear This behavior is dependent upon the contact of the ink with air. Over time, carbon dioxide in the air combines with the sodium hydroxide in the ink solution to form a less basic substance, sodium carbonate. The carbon dioxide also combines with water in the ink to form carbonic acid. The indicator solution responds to the production of acid and returns to its colorless acid form. A white residue (sodium carbonate) remains as the ink dries. 

Fresh concrete and cement mortars—which are identical from a chemical point of view—are relatively strongly alkaline (pH approximately 12.5). It later falls, however, due to the binding of carbon dioxide from the air. Depending on the special chemistry of the cement mortar, this process proceeds very slowly in the depth of the material. According to the composition of the cement mortar, this may last from a few months to many decades, until the pH value of such a mortar or concrete becomes neutral, even in the deepest layers.396"398 This chemical behavior explains the entire secret of the stability of reinforced concrete, which prevents the embedded steel from rusting further in the environment within the concrete, which remains alkaline for lengthy periods of time.399... 

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