Water & pH

A few more details…

Rain Water• Normal rain water is not pH 7 as you might

expect…

Rain Water…Instead, it is slightly acidic, usually around pH

5.6.

• Water vapor tends to combine with CO2 in the atmosphere, forming carbonic acid:

CO2 + H2O H2CO3

• But, carbonic acid is a weak acid, and partially dissociates:

CO2 + H2O H+ + HCO3-

Acid Rain

Acid Rain• Acid rain refers to rain water that has a

pH below 5.6!• Water vapor in the atmosphere combines

with pollutants like sulfur dioxide (SO2) to form Sulfurous Acid:

SO2 + H2O H2SO3

…Which can also dissociate:H2SO3 H+ + HSO3

-

Acid Rain

• Also, sulfur dioxide in pollution can be oxidized into sulfur trioxide:2SO2 + O2 2SO3

• Sulfur trioxide then reacts with water to form sulfuric acid:SO3 + H2O H2SO4

…which can donate lots of H+ to rain water!

Acid Rain

• The same sorts of reactions can occur with other pollutants, like nitrogen oxide (NO2):

2NO2 + H2O HNO2 + HNO3

…forming nitrous acid and nitric acid, both of which can further dissociate =

more H+

Acid Rain• The resulting low pH for rain water has

many effects:– Increased leaching of important nutrients OUT

of the soil.– Increased solubility of harmful metals (like

aluminum) in soils.– Disrupted photosynthesis due to poor

production of chlorophyll.– Killing of plants unable to secure nutrients.– Pollution of groundwater.

Isn’t there only 1 kind of water?

Tap Water

• “Government-treated water”.• Contains many minerals, like Calcium &

Magnesium.• Contains certain amounts of Chloride or

Fluoride (for “sanitation”).• Often contains dissolved organic

compounds. • Can contain metals.

Spring Water

• From a “Spring” – a point where groundwater meets the surface.

• Minerals become dissolved in the water as it moves through underground rock.

• Generally have high amounts of dissolved oxygen.

• Have impurities.

Distilled Water• Distilled water has gone through the

distillation process in a still: – Water is boiled, and the re-condensed steam

is collected as a liquid into a clean container• Most contaminants are left behind.

Deionized Water• Water is purified by going through an ion

exchange process, where the ionic impurities are removed.– Non-ionic impurities (like organic compounds)

remain.– Certain ions remain (OH- and H3O+).

What is this?What is this?

pH

H2O

H+ OH-

• What we pictured happening:

pH

• What actually happens:

• These Hydronium ions are VERY reactive, and THEY are technically what is measured by pH.

H2O + H2O H3O+ + OH-

Hydronium Ion

Water