The pH of pure water is 7 at 25 oC. The pH of rain water is around 5.6. This is due to gaseous substances dissolved in water. As temperature increases the pH of water decreases. As temperature increases the dissolution of CO2 in water decreases. Why does the pH decrease? What will happen to the pH of water if we remove all the gases and impurities from water?
Dear Li Shu
Normal rainwater has a pH of 5.6 (slightly acidic). This is because it is exposed to the carbon dioxide in the atmosphere. The carbon dioxide gets dissolved in the rainwater and forms carbonic acid (H{-2}CO{-3}).There are both natural and non-natural sources of materials that cause pH of rain water to change. Please go through the attached link for further datails
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Water/FreshWater/acidrain.html
http://www.nature.com/nature/journal/v295/n5851/abs/295683a0.html
http://www.fondriest.com/environmental-measurements/parameters/water-quality/ph/#p7
http://www.fondriest.com/environmental-measurements/parameters/
When rain water come in contact with carbon di oxide it forms carbonic acid. Carbonic acid is unstable and immediately convert to bicarbonate. However pH 5.6 is quite acidic. Furthermore, pH of rain water is not uniform , it varies place to place and depend on various factors. On line published work available for further reference.
I would agree manoj Kumar. That is why the acid rain occur.
Gas CO2 in the react with water to form a weak acid of carbonic acid
When you talk about rain water as pure water in equilibrium with a partial pressure of CO2 of pCO2=3.5, the pH is 5.6 at 25C (=pristine rain water). When you increase the temperature, less CO2 is dissolved and the equilibrium shifts also because the deprotonation constants for H2CO3 --> H+ + HCO3- and HCO3- -> H+ + CO32- change with temperature. At 50C the pH is 5.8 and at 70C the pH is 6.0.
In reality, the composition of rain water is much more complicated and also contains NH4+, CH3COOH, SO4, Cl, NO3 and has usually pH somewhat lower than that of pristine rain water (i.e. water in equilibrium with only atmospheric CO2). The composition can change significantly from one place to another. depending on industrial activities or volcanic activities resulting in different degree of atmospheric pollution.
Good textbooks dealing with this topic are:
Werner Stumm and James J. Morgan "Aquatic Chemistry - Chemical Equilibria and Rates in Natural Waters"
C.A.J. Appelo and D. Postma "Geochemistry, groundwater and pollution"
the pure water in equilibrium with a partial pressure of CO2 of pCO2=3.5, the pH is 5.6 at 25C (=pristine rain water). When you increase the temperature, less CO2 is dissolved and the equilibrium shifts also because the deprotonation constants for H2CO3 --> H+ + HCO3- and HCO3- -> H+ + CO32- change with temperature. At 50C the pH is 5.8 and at 70C the pH is 6.0
Pure water does not contain any ions and is neutral. However, once the pure water is exposed to air, its purity will drop rapidly. Even if only a few minutes, pH will be a significant decline. This is because the carbon dioxide in the air reacts with water to produce carbonic acid.
The hydrogen ion (H +) released by carbonic acid ionization decreases the pH of the water. The reaction is very rapid and the resistivity of pure water exposed to air drops from 18.2 MΩ · cm to 4 MΩ · cm in 60 minutes. Therefore, we advocate to use pure water as soon as possible, rather than storage.
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