pH ⇄ pOH Converter
Instantly convert between Acidity (pH) and Alkalinity (pOH) scales. ($pH + pOH = 14$)
Standard range: 0 - 14
Chemical Balance
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The Dual Scale of Acidity
While the world speaks in pH, chemistry relies on the perfect balance between two opposing forces: Protons (H+) and Hydroxide ions (OH-).
The Connection
The relationship between pH and pOH is derived from the properties of pure water. Their sum is constant because water is in a fixed state of equilibrium.
The See-Saw Effect
Think of pH and pOH as children on a see-saw.
- pH ↑As the solution becomes more Basic, pH goes up (e.g., to 12), so pOH must go down (to 2).
- pOH ↑As the solution becomes more Acidic, pOH goes up (e.g., to 13), so pH must go down (to 1).
Comparison of Scales
| Nature | pH | pOH | Dominant Ion |
|---|---|---|---|
| Strong Acid | 0 | 14 | H+ >> OH- |
| Acidic Rain | 5 | 9 | H+ > OH- |
| Neutral Water | 7 | 7 | H+ = OH- |
| Sea Water | 8 | 6 | OH- > H+ |
| Bleach | 13 | 1 | OH- >> H+ |
Temperature
Be careful! At 100°C (boiling water), pure water is still neutral, but its pH is about 6.14 and pOH is 6.14. They add up to 12.28, not 14, because heat helps water break apart into ions more easily.
Pool Chemistry
Pool managers focus on pH (7.2-7.6) to keep chlorine effective. However, pOH is the hidden variable that determines scale formation. In high pOH water (low pH), calcium stays dissolved; in low pOH water (high pH), it scales.
Hydroxide Poisoning
While we fear strong acids, pure alkalis (pOH 0-1) are often more dangerous. They saponify the fats in your skin, turning your own tissue into soap, and cause deep, painless burns that are hard to treat.
Frequently Asked Questions
What is the relationship between pH and pOH?
They are inversely related. As pH increases (becomes more basic), pOH decreases. For aqueous solutions at 25°C, their sum is always equal to 14.
Why do pH and pOH add up to 14?
This comes from the Ion Product Constant of Water (Kw), which is 1.0 × 10-14 at 25°C. Since pH = -log[H+] and pOH = -log[OH-], their sum equals -log(10-14), which is 14.
Does this formula work at all temperatures?
No. The neutral point and the sum (pKw) change with temperature. For example, at 100°C, the Kw is higher, so the sum pH + pOH is about 12.28, not 14. However, 14 is the standard value used for almost all room-temperature chemistry.
Can I have a negative pOH?
Yes, just like negative pH. If a solution is extremely basic (e.g., 2 M NaOH), the [OH-] is greater than 1, so the pOH will be negative (approx -0.3).
How do I convert pH to pOH?
It is a simple subtraction: pOH = 14 - pH. For example, if pH is 3, pOH is 11.
How do I convert pOH to pH?
Similarly, subtract pOH from 14: pH = 14 - pOH. For example, if pOH is 8, pH is 6.
What if pH and pOH are equal?
If pH = pOH, then both must be 7 (at 25°C). This indicates a perfectly neutral solution where [H+] = [OH-], such as pure water.
Why is water considered neutral?
Pure water naturally dissociates into equal parts H+ and OH-. Since the concentrations are balanced, the pH and pOH are identical, making it neutral.
What is pKw?
pKw is the negative log of Kw. At 25°C, Kw = 10-14, so pKw = 14. This is the origin of the number 14 in the equation.
Is pOH used in real life?
Rarely. pH is the standard for almost all industries (pools, food, medicine). pOH is mostly used in academic chemistry to calculate the concentration of hydroxide ions in base solutions.
Does salt affect pH/pOH?
It depends. Neutral salts (like NaCl) don't change the pH. But salts formed from weak acids/bases (like Sodium Acetate) will undergo hydrolysis and shift the pH/pOH balance.
How does dilution affect pOH?
Adding water pushes the solution towards neutral. If you have a basic solution (pOH 2, pH 12), diluting it increases the pOH (lowers the pH) towards 7.
Can pOH be zero?
Yes. Use the formula: pOH = -log[OH-]. If the concentration of Hydroxide is 1.0 M, then log(1) = 0, so pOH = 0. This corresponds to a pH of 14.
What is the difference between Alkaline and Basic?
They are often used interchangeably, but technically, an Alkali is a base that dissolves in water to release OH- ions. All alkalis are bases, but not all bases are alkalis.
What is the hydronium ion?
H3O+. When H+ dissociates in water, it attaches to a water molecule. So pH is actually -log[H3O+], but we write [H+] for simplicity.