Ask A Scientist Chemistry Archive

name         Mickey
grade        9-12
location     KS

Question -   Is there a non-toxic solution (water based solution) that 
has a high pH (9 or above)that will drop its pH rapidly when exposed to air?


It is irrelevant what the solution is (as long as it is water-based). At 
the very heart of this is an acid-base reaction. Thus a base (regardless 
of what it is) will react with the acid that can be obtained from the air 
- this usually comes in the form of dissolved carbon dioxide reacting with 
water to form carbonic acid. So the controlling factors are: how well 
carbon dioxide dissolves in the solution which is controlled by the type 
of solvent (water) and the atmospheric pressure - both of which are 
constant for all types of base.

Greg (Roberto Gregorious)
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There are a number of things you do not specify that could affect the
answer. How toxic is "non-toxic"? How fast is "rapid" -- seconds, minutes,
hours? What is the temperature? The temperature is an important parameter.
I would start with dilute ammonia, but that is only a educated guess.

Vince Calder
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Maybe NaOH, Mickey, if you play some careful games.

A 1M solution of simple NaOH has pH=~14 .
Dilute it by ten: pH=~13.  100: pH12.
Or dilute it by 10,000, pH=~10.
If you put about 1 drop of 1m NaOH in about 2 gallons of D.I. water,
the dilution factor would be roughly 30,000,
and if nothing interferes the pH will be about 9.5.
Then atmospheric CO2 will infuse and neutralize it in minutes
if it is spread out in a broad, shallow container.
It changes the NaOH to NaHCO3, sodium bicarbonate.
A drop of this extremely dilute solution on paper
would be neutralized even quicker, because it is less than 0.5mm deep.

A 10,000:1 solution is so dilute that it is often impractical,
often dominated by some impurity.
You have to be careful that it cannot be neutralized pretty soon right 
inside your container.
Also, tap water will have dissolved CO2 or other weak acids in it
which must first be overcome by the amount of NaOH added,
then more NaOH can make the pH 9 or 10.
Even then, it will take more CO2 to neutralize than a perfectly pure solution.

If your solution was NH4OH instead of NaOH, maybe
you could start from a stronger solution  or higher pH  (~11) and still 
get fast action,
because until the pH goes below about 10, it will be evaporating out of 
the solution as NH3 gas.
Then CO2 would come along and neutralize the rest to NH4CO3.

There is 20% oxygen available in air, as opposed to <<1% CO2.
Unfortunately, using the O2 in air for a chemical reaction
usually consumes acid and makes low pH higher,
rather than consuming base and making high pH lower.
O2 + 4 H+ + 4(e-) --> 2 H2O
O2 + 2 H2O  + 4(e-) --> 4 OH-
O2 + 2 H2O + 4 Na  --> 4 NaOH
All go the wrong way for what you want.

Jim Swenson
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