pOH Calculator
Calculate alkalinity (pOH), acidity (pH), and ion concentrations.
pOH Scale
The Science of Alkalinity
While pH gets all the fame, pOH is the engine room of basic solutions. It measures the power of Hydroxide ions (OH⁻), the chemical opposite of protons.
Calculating pOH
pOH allows you to handle the tiny concentrations of hydroxide ions using manageable numbers between 0 and 14.
The Mirror Effect
pOH is the mirror image of pH.
- AcidicHigh pOH (14) means there are almost no hydroxide ions. The solution is swarming with protons (pH 0).
- BasicLow pOH (0) means the solution is packed with hydroxide ions (1 mole per liter). The pH is 14.
pOH Scale Reference
| Solution Type | pOH | [OH⁻] Concentration | Equivalent pH |
|---|---|---|---|
| Pure Caustic Soda | 0 | 1.0 M | 14 |
| Ammonia | 3 | 0.001 M | 11 |
| Pure Water | 7 | 0.0000001 M | 7 |
| Vinegar | 11 | 10⁻¹¹ M | 3 |
| Stomach Acid | 13 | 10⁻¹³ M | 1 |
Soap Making
Soap is made by reacting fats with a very low pOH solution (Lye). The high concentration of OH- breaks the triglyceride bonds (Saponification).
Calculating Bases
To find the pH of 0.01M NaOH, you can't use -log[H+]. You MUST find pOH first (-log 0.01 = 2) and then subtract from 14 to get pH 12.
Alkali Burns
Low pOH solutions (Strong Bases) are dangerous because they dissolve proteins. Unlike acid burns that form a protective scab, alkali burns penetrate deeper into tissues.
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Frequently Asked Questions
What is pOH?
pOH stands for 'potential of Hydroxide'. It is a logarithmic measure of the hydroxide ion concentration [OH-] in a solution. It is the mirrored counterpart of pH.
What is the formulation for pOH?
The formula is pOH = -log[OH-]. To find concentration from pOH, use the inverse log: [OH-] = 10-pOH.
How does pOH relate to pH?
They are inversely related. For any aqueous solution at 25°C, pH + pOH = 14. If pH goes up (more basic), pOH goes down (more hydroxide ions).
What does a low pOH mean?
A low pOH (e.g., 1 or 2) indicates a high concentration of hydroxide ions, meaning the solution is strongly basic (alkaline). This is the opposite of pH, where low numbers mean acidic.
What is neutral pOH?
At standard temperature (25°C), a pOH of 7 is neutral. This corresponds to a pH of 7, where [H+] and [OH-] are exactly equal (10-7 M).
Why do we use pH more than pOH?
Purely by convention. Biology and industry standardized on monitoring Protons (H+). However, pOH is mathematically necessary for calculating the pH of base solutions like Sodium Hydroxide.
Can pOH be negative?
Yes. A solution with a hydroxide concentration greater than 1 M (e.g., 2 M NaOH) will have a negative pOH (e.g., -0.3). This is common in industrial caustic solutions.
What is a strong base?
A strong base (like NaOH or KOH) dissociates completely in water, releasing a massive amount of OH- ions. This results in a very high [OH-] and a very low pOH.
How do I calculate pH from pOH?
Simply subtract the pOH from 14. For example, if pOH is 3, then pH = 14 - 3 = 11.
Does temperature affect pOH?
Yes. As temperature rises, water dissociates more, increasing both [H+] and [OH-]. This means neutral pOH drops below 7 at high temperatures (e.g., neutral pOH is ~6.14 at 100°C).
What is the [OH-] of pure water?
1.0 × 10-7 M at 25°C. This is why pOH = -log(10-7) = 7.
How do I measure pOH?
You typically don't measure it directly. You measure pH with a probe and then mathematically convert it: pOH = 14 - pH.
What is the difference between pOH and Alkalinity?
pOH measures the concentration of OH- ions. Alkalinity is a measure of water's capacity to neutralize acids (buffering capacity), which includes carbonates and bicarbonates, not just hydroxide.
Is pOH logarithmic?
Yes. A change of 1 unit in pOH represents a 10-fold change in hydroxide concentration. pOH 3 has 10x more OH- ions than pOH 4.
What happens if pOH is 0?
If pOH = 0, then [OH-] = 100 = 1.0 M. The solution is very basic (pH 14).