Jeff Rigby wrote:
Not that weak. If you take a distilled water and left it open pH goes
down from 7 to about 5.7 - just because of the presence of dissolved
carbon dioxide.
PH changes from 7, in other words from 5-9 in distilled water take very
little acid or base, in other words it's a very weak acid or base.
Remember that this change takes place in the presence of only about 380
ppmv of CO2 in the air. Increase amount of CO2 in the atmosphere and
the change will be higher. That doesn't mean carbonic acid is not a
weak one, you just don't need a strong acid to change pH when you have
solution close to pH 7.
That's like saying water is a solvent and it dissolves most compounds
because the hydrogen and oxygen molecule's geometry lends itself to tearing
apart most compounds. We should be concerned with water too. In fact it's
not the carbon dioxide that creates the acid, it's water. Without water
carbon dioxide is not an acid.
Water dissociates creating the same amounts of H+ and OH- ions. First
are connected with solution acidity, the latter with solution basicity.
Thus water is acidic and basic - to the same extent - at the same time.
That's why pure water is neutral. Almost every substance you can
dissolve will change pH of the solution, just sometimes this change is
unmeasurable. CO2 is so called acid anhdyride (just like SO3, P2O5 or
N2O5 oxides) - it reacts with water to create carbonic acid, which in
turns dissociate and acidifies the solution. Other anhydrides listed
create stable acids (known as strong mineral acids - sulfuris, nitric
and phosphoric). Trick is, carbonic acid is very unstable, thus it
concentration in water is very low, much lower than the concentration
of CO2. In fact there are papers that suggest that carbonic acid is
much stronger than it is commonly believed, just - as its concentration
is very low - measurable effect (ie pH change) is low. Trick is, if you
want to calculate pH of CO2 saturated solution you have to deal with at
least two reactions - first being carbonic acid creation, second its
dissociation. You know only how much CO2 is present and what is
solution pH - these are controllable/measurable. But the "internal
workings" - ie carbonic acid creation/carbonic acid dissociation - are
seen as one step with one equilibrium constant which can not be split
without additional data.
But that's completely OT here
I'm not disagreeing with your comments, I find them VERY enlightening.
Following is typical household chemicals and their Ph ranking:
Note: pH, not Ph.
1 Stomach Fluids
2 Lemon Juice
3 Vinegar
4 Tomatoes
5 Coffee
6 Milk
7 Pure Water
8 Blood
9 Baking Soda
10 Borax
11 Rolaids, Tums
12 Household Ammonia
13 Bleach
14 Lye
Yup. Classical list shown whenever pH scale is discussed:
http://www.chembuddy.com/?left=pH-ca...right=pH-scale
http://www.ph-meter.info/pH-scale
also wiki and many *.edu sites.
and so on
I would just change blood pH to 7.4, as 8 is misleading.
Other solutions have pH that can vary, blood pH doesn't change much
(see my other post about panting somewhere in this thread).
Borek
--
http://www.chembuddy.com
http://www.ph-meter.info