Direct Energy, 2018a

9 BATTERIES AND FUEL CELLS 209
The concept of pH is fundamentally related to the ow of charges, a
concept which is very important to electrical engineers. Water is composed
of H 2 O molecules. In pure water, some of these molecules fall apart, ionizing
into H + ions (protons) and OH − ions. However, most of the molecules
remain intact. If some solutes are mixed with the water, more of the H 2 O
molecules will ionize than in pure water. For example, carbon dioxide will
bond with OH − ions forming carbonic acid HCO − 3 causing an increase in
H + ions. Since pH is the negative log of H + ion activity, increasing H + ion
concentration is equivalent to a pH decrease. If ammonia, NH 3 , is added to
water, NH + 4 is formed, and the number of OH − ions increases resulting in
an increase in pH. Since water is a liquid, both these positive and negative
ion charge carriers can move about relatively easily. If an external voltage
is applied across the liquid, ions will ow. Electrical conductivity will be
higher in a liquid with more ions present than in liquids with fewer ions
present.
As an example, consider what happens when neutral sodium atoms are
added to water. (For obvious reasons, don't try this at home [139].) It is
energetically favorable for the sodium atoms to ionize to Na + giving up an
electron. In the process, some more water molecules ionize, and some H +
ions become neutralH atoms.
Na + H 2 O → Na + + H + OH − (9.10)
By adding the solute sodium, the solution has fewer H + ions.
Consider what happens when neutralchlorine atoms are added to water.
It is energetically favorable for a chlorine atom to acquire an electron to a
form Cl − ion.
Cl + H 2 O → Cl − + H + + OH (9.11)
By adding the solute chlorine, the solution has more H + ions. While these
examples involve adding neutral atoms, the concept of pH applies to solutes
which are molecules too.
Solutions with pH less than 7 are called acidic. If a solution has a high
concentration of H + ions, it will have a low pH and be acidic. In strongly
acidic solutions, molecules of the solute rip apart many water molecules,
so lots of ions are present. Solutions with pH greater than 7 are called
alkaline or basic. If a solution has a low concentration of H + ions, and
hence a high concentration of OH − ions, it will have a high pH and be
alkaline. In strongly alkaline solutions, molecules of the solute rip apart
many water molecules, so again lots of ions are present. Neutral solutions
have a pH near 7, and some neutral solutions may be electrical insulators.
Solutions with a pH much below or much above 7 necessarily have many
ions present, and they are good electrical conductors.