What is pOH?-Calculation, And Applications

Definition of pOH

pOH stands for “potential (or negative log) of hydroxide ion concentration”. It is a measure of the hydroxide ion (OH^–) concentration in an aqueous solution, similar to how pH measures the hydrogen ion (H⁺) concentration.

Calculation of pOH

pOH is calculated using this formula:

pOH = -log[OH^–]

Where:

pOH = The negative log of the hydroxide ion concentration

[OH^–] = Molar concentration of hydroxide ions in mol/L

log = Base 10 logarithm

For example, if a solution has a hydroxide ion concentration of 1 x 10^-3 mol/L:

[OH^–] = 1 x 10^-3 mol/L pOH = -log(1 x 10^-3) = 3

So this solution would have a pOH of 3.

The pOH scale runs from 0 to 14, similar to the pH scale. A low pOH indicates a high hydroxide concentration, while a high pOH indicates low hydroxide concentration.

The pH and pOH of a aqueous solution at 25°C will always sum to 14, because the product of [H+] and [OH-] equals the ion product constant for water (Kw = 1 x 10^-14).

What is the pOH scale?

The pOH scale is a logarithmic measurement scale used to express the alkalinity (basicity) of a solution in a manner similar to how the pH scale measures acidity. While the pH scale measures the concentration of hydrogen ions (H⁺) in a solution, the pOH scale quantifies the concentration of hydroxide ions (OH⁻). Just as in the pH scale, the pOH scale ranges from 0 to 14, with 7 as the midpoint or neutral point.

Here’s how the pOH scale works:

1. Basic Solutions (pOH < 7): Solutions with a pOH lower than 7 are considered basic or alkaline. The lower the pOH value, the more basic the solution. For example, substances with a pOH of 1 or 2 are highly alkaline.
2. Neutral Solution (pOH = 7): A pOH of 7 represents neutrality, just like a pH of 7. In this case, the concentration of hydroxide ions (OH⁻) is equal to the concentration of hydrogen ions (H⁺), making the solution neutral.
3. Acidic Solutions (pOH > 7): Solutions with a pOH greater than 7 are considered acidic. The higher the pOH value, the more acidic the solution. However, acidic solutions are not as commonly described using pOH, and the pH scale is more commonly used for this purpose.

Relation between pH and pOH

The pH and pOH scales are directly related to each other in aqueous solutions at 25°C. This relationship arises because the product of the hydrogen ion (H+) and hydroxide ion (OH-) concentrations must equal the ion product constant for water (Kw).

The key relationship between pH and pOH is:

pH + pOH = 14

Where:

pH = -log[H⁺]

pOH = -log[OH^–]

At 25°C, Kw = [H+][OH-] = 1 x 10^-14

Some examples:

1. If a solution has [H⁺] = 1 x 10^-8 M, then: pH = -log(1 x 10^-8) = 8 pOH = 14 – pH = 14 – 8 = 6
2. A solution with [OH^–] = 1 x 10^-11 M has: pOH = -log(1 x 10^-11) = 11 pH = 14 – pOH = 14 – 11 = 3
3. Pure neutral water has [H⁺] = [OH^–] = 1 x 10^-7 M pH = 7 pOH = 7

So if you know either the pH or the pOH, you can easily calculate the other. This provides two complementary ways to characterize aqueous acid/base solutions. Strong acids have low pH and high pOH, while strong bases have high pH and low pOH. This math relationship strengthens the utility of the pH/pOH scales.

pOH of Difference Substance

Here is a table showing the pOH values for 20 different substances:

Substance	pOH Range	pH Range	Description
Sodium hydroxide	0-1	13-14	Strong alkaline, corrosive
Ammonia	1-2	12-13	Weakly alkaline, pungent
Bleach	1-2	12-13	Strong alkaline, disinfectant
Oven cleaner	1-2	12-13	Strong alkaline, caustic
Baking soda	4-5	9-10	Mildly alkaline, leavening
Milk of magnesia	2-3	11-12	Mildly alkaline, antacid
Seawater	6	7.5-8.5	Slightly alkaline, saline
Milk	7-8	6.5-7.5	Slightly acidic, dairy
Blood	6.55-6.65	7.35-7.45	Slightly acidic, vital
Pure water	7	7	Neutral, H2O
Rainwater	8-9	1-2	Slightly acidic, collected
Beer	8-9	3-4	Mildly acidic, alcoholic
Coffee	8-9	4.5-5.5	Mildly acidic, caffeinated
Soda	8-10	4-6	Acidic, carbonated
Vinegar	10-12	2-4	Acidic, acetic acid
Lemon juice	11-12	2-3	Highly acidic, citrus
Tomatoes	12-13	1.5-3.5	Highly acidic, fruits
Stomach acid	13-14	0-1	Strongly acidic, digestive
Battery acid	13-14	0-1	Strongly acidic, corrosive
Limewater	1-2	12-13	Strong alkaline, lime water

Applications of the pOH scale

Here are some key applications of the pOH scale:

• Measuring alkaline solutions – The pOH scale is useful for measuring the strength of bases and alkaline solutions. A low pOH indicates a high hydroxide concentration.
• Environmental monitoring – Testing the pOH can detect alkaline pollution in water systems from industrial effluents or agricultural runoff. This can harm aquatic ecosystems.
• Bleach and cleaning products – Bleach and many cleaners derive their cleaning power from hydroxide ions. The pOH measures the alkaline strength of these products.
• Water treatment – Adjusting the pOH is an important part of water purification, reducing hardness and controlling solubility of metals. Lime or soda ash may be added.
• Food processing – Some food preparation techniques rely on soaking in an alkaline solution to alter texture or initiate fermentation reactions. The pOH monitors processing.
• Soil science – The pOH helps characterize the alkalinity of soils. Plant nutrients can become unavailable if the soil pOH is too high. Additives can lower pOH.
• Titration analysis – Acid-base titrations can be monitored using either pH or pOH indicators. pOH works well for titrating strong bases.
• Buffer systems – Alkaline buffer solutions are most effective in their high pOH range. The pOH verifies the buffer has adequate hydroxide levels.
• Non-aqueous titrations – The pOH scale can be applied to non-aqueous titrations, as hydroxide concentrations in solvents like ammonia can be measured.
• Weak base calculations – The pOH equation allows calculation of hydroxide concentrations for weak base solutions, from which the pH can also be determined.

So while pH gets more use, pOH fills an important niche for quantifying alkaline solutions in both chemical research and industrial processes. The two scales complement each other.