"pH" is defined as the "negative log of the hydrogen ion concentration." This is actually a mathematical determination. Therefore, mathematically, this is expressed as pH = -log10 [H+]. You and I refer to it as the degree of acidity of a soil, solution, etc. The lower the pH value, below neutral 7, the greater of the acidity. The higher the value, above neutral 7, the more "basic" the solution.

0 --1 --2 --3 --4-- 5 --6-acidic----- (neutral "7")------basic--8 --9 --10--11 --12 --13-- 14

But what does this all REALLY MEAN? Well, I’m glad you asked!

Water is composed of two hydrogen atoms, and one oxygen atom. It is written as the well-known: H2 O

The water molecule, and most all other molecules never stay combined all the time this way. Every so often, water molecules "disassociate" to become one hydrogen atom (H+ ) and one hydroxyl atom (OH- ). This is often written as: H2 O <=> (H+) + (OH- ). This means that the two separate and come back together. Since we have defined acidity is the measurement of the hydrogen ion concentration, for something to be acidic, it must have MORE hydrogen ions (H+) than a neutral, or balanced state between the hydrogen ions (H+) and the hydroxyl ions (OH-).

There is a lot of confusion among operators, who have not had extensive chemistry courses, resulting from those who use the term "alkalinity" to describe values above the "neutral pH of 7." Most "real" chemists, (and my best friend, a GREAT teacher of chemistry), view alkalinity as a "buffering process" due to the actions of salts such as calcium carbonate, and not the determination of the relative occurrence of hydrogen or hydroxyl ions. Alkalinity, therefore, resists changes in pH by reacting with the addition of more hydrogen ions, by "neutralizing" them. Alkalinity is a very important subject in water and wastewater treatment, and will be discussed in depth, in future topics. To fully understand it, we have to first understand pH. So, back to the subject of pH we go.

pH is simply a set of numbers therefore, from 0 to 14, that quantifies the number of hydrogen ions (H+ ) in a solution. For the budding chemists in our group, in pure water, this number is 0.0000001 moles per liter. (A "mole" is the molecular weight expressed in grams.) I think we need to do a "math image" at this point. Lets make a mathematical reference table for pH values! The molecular weight of hydrogen is 1. Therefore, that will be reflected in our table(oh how I wish we had columns!):

0 pH is a mole wt of: 1.0 with a scientific notation of: 1 x 10
1 pH is a mole wt of: 0.1 with a scientific notation of: 1 x 10-1
2 pH is a mole wt of: 0.01 with a scientific notation of: 1 x 10-2
3 pH is a mole wt of: 0.001 with a scientific notation of: 1 x 10-3
4 pH is a mole wt of: 0.0001 with a scientific notation of: 1 x 10-4
5 pH is a mole wt of: 0.00001 with a scientific notation of: 1 x 10-5
6 pH is a mole wt of: 0.000001 with a scientific notation of: 1 x 10-6
7 pH is a mole wt of: 0.0000001 with a scientific notation of: 1 x 10-7
8 pH is a mole wt of: 0.00000001 with a scientific notation of: 1 x 10-8
9 pH is a mole wt of: 0.000000001 with a scientific notation of: 1 x 10-9
10 pH is a mole wt of: 0.0000000001 with a scientific notation of: 1 x 10-10
11 pH is a mole wt of: 0.00000000001 with a scientific notation of: 1 x 10-11
12 pH is a mole wt of: 0.000000000001 with a scientific notation of: 1 x 10-12
13 pH is a mole wt of: 0.0000000000001 with a scientific notation of: 1 x 10-13
14 pH is a mole wt of: 0.00000000000001 with a scientific notation of: 1 x 10-14

We see that as the pH increases, the number of hydrogen ions DECREASES; and accordingly, the scientific notation shows the log, base 10, goes negative.

Imagine having to determine the pH of a solution every time, "mathematically"! We’d first determine the number of moles per liter, say 0.00001 moles per liter in this sample on your desk. You then turn this into scientific notation, 1 x 10-5 . You then reverse the sign of the exponent, for a pH of 5!

Acids therefore supply hydrogen atoms, like our favorite acid example, hydrochloric acid:
HCl <=> H+ + Cl-

Bases supply the OPPOSITE of the hydrogen atoms, the hydroxyl OH- :
NaOH<=> Na+ + OH-

If we combine hydrochloric acid with sodium hydroxide, water and sodium chloride are produced:
HCl + NaOH =>H2O + NaCl
In this acid-base reaction above, notice how the hydrogen ion component of the acid wants to combine with the hydroxyl ion component of the base to create water, just as our examples in disassociation. This illustrates that when an acid reacts with a base, water and a "salt" is created.

It stands to reason that if we can find the pH by determining the hydrogen ion concentration, we could also do the same for the hydroxyl concentration, right? So if we use: pH = -log [H+] to find the acidity, we would use: pOH = -log [OH-] to find how basic the solution is.

Now, just some food for thought…. If we sum the pH and the pOH of each solution we test, what will we get for a value?? ANSWER: We will we get 14 every time!

You see, at a neutral pH of 7, there is a balance or equality between the number of hydrogen ions and the number of hydroxyl ions. They are equal in number. The pH is 7 and the pOH is 7, with the sum being 14.

Another example: If the pH of a solution is 5, the pOH for the solution would be 9, with the sum being 14.

Many operators summarize this by saying, "If the solution is acidic, there are a surplus of hydrogen, and if the solution is basic, there is a surplus of hydroxyl ions."

We will need to spend some time to understand this information, and then we will move on. "Alkalinity" is next!

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