Gastric acid
The highest concentration that gastric acid reaches in the stomach is 160 mM in the canaliculi. This is about 3 million times that of arterial blood, but almost exactly isotonic with other bodily fluids. The lowest pH of the secreted acid is 0.8, but the acid is diluted in the stomach lumen to a pH between 1 and 3.
Hypolimnetic aeration
For the pH of drinking water, the threshold value is 6.5 - 9.5. pH values outside the neutral range (pH 6.5 - 7.5) are principally critical, since they provide an indication of the corrosion behaviour of water. Slightly acidic water (pH 4 - 6.5) usually corrodes galvanized iron pipes, but also copper and asbestos cement pipes. This process is known as acid corrosion. Practical experience has shown that the use of unprotected steel pipes is only possible at neutral pH values. By lower pH values, the removal of the pure zinc layer is promoted. As a result of the dissolved salts and gases, natural cold waters usually show a slightly alkaline reaction. These properties are created by setting the equilibrium concentrations of dissolved carbon dioxide in the form of bicarbonate ions and carbonate ions. Higher alkaline pH values (pH 9 - 14) in the presence of oxygen as an oxidant lead to so-called oxygen corrosion. To avoid the described acid or oxygen corrosion, buffer solutions are added to the raw water for drinking water production. By the pH-stabilizing effect of hypolimnetic aeration, the application of these buffer solutions can be reduced, thus reducing operating costs.