QUESTION #424

# What is the pH of water?

Simply put the pH of pure water is 7. All acids have a pH that is lower than 7 and all bases have a pH that is higher than 7. But why is water the reference point for pH?

As you might know, water is not considered to be an acid or a base. It is neutral. This can be seen if we look at the chemical formula of water when compared with acids/bases.

Acids such as HCl, HNO3, and H2SO4 have a positively charged hydrogen atoms combined with a negatively charged ions:

H+ + Cl- = HCl

Bases such as NaOH and KOH usually have a positively charged ions combined with negatively charged hydroxide ions:

Na+ + (OH)- = NaOH

Water or H20 can also be described by the chemical formula HOH. Water can be viewed as a positively charged ion (hydrogen) combined with a negatively charged polyatomic ion (hydroxide).

H+ + (OH)- = HOH = water!

As you can see, water has the chemical formula of an acid and a base. Instead of acting as both, water displays the characteristics of a neutral substance, as acid and base cancel each other out.
Answered by: Daniel Caputo, High School Physics Student

The potential of electric for positive Hydrogen ions, or pH, can be important for a variety of reasons, especially for guessing electrolytic properties. It is determined from the H3O+ concentration produced from the disassociation of an acid or base in water. A constant and balanced ratio of H3O+ ion to OH- ions exist in water at any time such that:

[H3O+][OH-] = 1.0 x 10-14 mol/L (at 25' C)

When an acid or a base is added, it increases the value of the Hydrogen ion or the Hydroxide ion, respectively. This creates an imbalance in the pH, which makes the solution either acidic or basic. The following example shows a 1 L solution of 0.050M HNO3 (Nitric Acid).

HNO3 + H2O = H3O+ + OH-
0.050M*         0.050M*
* mol/L

Since water always contains a balanced amount of the two ions and the equation shown above is true for all water-based solutions, the following equation can be formed:

[0.050 + X][X] = 1.0 x 10-14 mol/L

Since X is obviously an extremely small value, the addition of X to 0.050M can be ignored in this case, yielding the equation:

0.050X = 1.0 x 10^-14 mol/L
2.0 x 10-13 mol (OH-)/L

In this particular case, the previous equations were not strictly required, but in basic solutions it may be a necessity. pH is actually determined by taking the negative logarithm of the concentration (Molarity) of H3O+. Returning to the previous example again, where a 1 L solution of 0.050M HNO3 (Nitric Acid) was used:

HNO3 + H2O = H3O+ + OH-
0.050M*       0.050M*
* mol/L

This shows that the H3O+ concentration of the solution is 0.050M. This concentration is then inserted into the following equation:

-log(H3O+ M)
-log(0.050)
1.3

The pH of a solution of 0.050M HNO3 (Nitric Acid) is about 1.3. Since water has a balanced amount of H3O+ ion and OH- ions, the H3O+ concentration is:

X2 = 1.0 x 10-14 mol/L
X = 1.0 x 10-7 mol/L

Since the concentration of the H3O+ ion is known, the pH can be determined by the equation.

-log(1.0 x 10-7)
7

So, in a round about way, I have shown you that water has a pH of 7; this makes it a perfect neutral solvent.
Answered by: Edward Cramp, High-school Student, OVHS, Oley, PA