MadSci Network: Chemistry

Re: How can you change the pH of water?

Date: Thu May 24 22:38:14 2001
Posted By: Michel Ouellet, Grad student in Microbiology / Immunology
Area of science: Chemistry
ID: 986775843.Ch

Hi Amanda,

It all comes down to what is the strongest acid and what is the strongest 
base.  The stronger an acid is, the lower is its pKa value.  The strongest 
acid has a pKa of -9 and is sulfuric acid (H2SO4).

Conversely, the stronger a base is, the higher is its pKa value.  The 
strongest base has a pKa of 49 and is a methyl ion (CH3-)

Bearing in mind that...

pKa = -log Ka

Ka = Concentration of anion multiplied by concentration of hydronium ions 
divided by concentration of the acid

For example, for a dilute solution of acetic acid (CH3COOH)
Ka = [CH3COO-]x[H3O+]/[CH3COOH] = 1.75x10-5
pKa thus equals -log 1.75x10-5 = 4.76

Hope this helps!


Dan Berger adds:

1. If you're putting it in water, you don't need to worry as long as it's a 
strong acid or base in water. Sulfuric, hydrochloric, nitric and phosphoric 
acids are all "strong acids" in water, which means that they lose 100% of their 
protons and lower the pH accordingly. The same goes for bases. Most hydroxide 
bases (except so-called "ammonium hydroxide" which is a solution of ammonia in 
water--ammonia is a weak base!) are strong enough; you don't need to go to 
exotic and dangerous bases like methyl anion.

2. Given (1), the major thing for the answer to your question is to get the 
biggest bang per gram (or per molecule, not necessarily the same thing). For 
that you need the most H+ or OH- per gram or per molecule; and for THAT you need 
to know the structure of your acid or base. Pitfalls: phosphoric acid (H3PO4) 
only loses TWO (2) H+ per mole; the third H is a weak acid. Aluminum hydroxide, 
Al(OH)3 is strongly-enough bonded that it doesn't lose hydroxide easily at all; 
in fact, it's just as likely to lose H+!


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