pOH Calculator
Your expert tool for calculating pOH, pH, and ion concentrations in chemistry.
pOH is a unitless value, typically between 0 and 14.
7.00
1.00e-7 mol/L
1.00e-7 mol/L
Neutral
pH and pOH Relationship
Visual representation of pH + pOH = 14.
What is a pOH Calculator?
A pOH calculator is a scientific tool used to determine the alkalinity of an aqueous solution. pOH stands for the “potential of hydroxide” and is a logarithmic measure of the hydroxide ion (OH⁻) concentration. In chemistry, the pOH scale is a convenient way to express the basicity of a solution, running inversely to the more commonly known pH scale. This calculator helps students, chemists, and researchers quickly convert between pOH, pH, hydroxide ion concentration [OH⁻], and hydronium ion concentration [H⁺], providing a comprehensive view of a solution’s acid-base properties.
Understanding pOH is critical for anyone working in fields like environmental science, medicine, and chemical engineering, where the basicity of a solution can significantly impact chemical reactions, biological processes, and product quality.
pOH Formula and Explanation
The primary relationship this pOH calculator uses is the definition of pOH itself, which is the negative base-10 logarithm of the molar concentration of hydroxide ions ([OH⁻]).
pOH = -log10([OH⁻])
Furthermore, in any aqueous solution at 25°C (77°F), the relationship between pH and pOH is constant, governed by the autoionization of water (Kw). This simple and powerful formula allows for easy conversion between the two scales:
pH + pOH = 14
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| pOH | The negative logarithm of the hydroxide ion concentration. A measure of alkalinity. | Unitless | 0 to 14 |
| pH | The negative logarithm of the hydronium ion concentration. A measure of acidity. | Unitless | 0 to 14 |
| [OH⁻] | Molar concentration of hydroxide ions. | mol/L (M) | 10-14 M to 1 M |
| [H⁺] | Molar concentration of hydronium ions. | mol/L (M) | 1 M to 10-14 M |
Practical Examples
Example 1: Calculating pOH from Concentration
Suppose you have a 0.05 M solution of Sodium Hydroxide (NaOH), a strong base. Since NaOH dissociates completely, the [OH⁻] is 0.05 mol/L.
- Input: Hydroxide [OH⁻] Concentration = 0.05 M
- Calculation: pOH = -log10(0.05) ≈ 1.30
- Derived pH: pH = 14 – 1.30 = 12.70
- Result: The solution has a pOH of 1.30 and is strongly basic.
Example 2: Calculating pOH from pH
A sample of lemon juice is found to have a pH of 2.4. What is its pOH?
- Input: pH = 2.4
- Calculation: pOH = 14 – 2.4 = 11.6
- Derived [OH⁻]: [OH⁻] = 10-11.6 ≈ 2.51 x 10-12 M
- Result: The lemon juice has a pOH of 11.6, confirming it is acidic.
How to Use This pOH Calculator
Using this calculator is straightforward and intuitive. Follow these steps to get a complete analysis of your solution:
- Select Your Known Value: Use the dropdown menu to choose what you are starting with: pOH, pH, Hydroxide [OH⁻] Concentration, or Hydronium [H⁺] Concentration.
- Enter the Value: Input your known value into the text field. The label will update to reflect your selection. For concentrations, use scientific notation if needed (e.g., 1.5e-5 for 1.5 x 10-5).
- View Real-Time Results: The calculator updates automatically as you type. All four key values (pOH, pH, [OH⁻], [H⁺]) and the solution type (acidic, basic, or neutral) are displayed instantly.
- Interpret the Results: The primary result (pOH) is highlighted, with other values listed below. The bar chart provides a quick visual comparison of the pH and pOH levels.
- Reset or Copy: Use the “Reset” button to return the calculator to its default neutral state. Use the “Copy Results” button to copy a summary to your clipboard.
Key Factors That Affect pOH
Several factors can influence the pOH of a solution. Understanding them is key to accurate measurements and predictions.
- Temperature: The relationship pH + pOH = 14 is standard at 25°C. The autoionization constant of water (Kw) is temperature-dependent. At higher temperatures, Kw increases, and the sum of pH and pOH decreases, meaning the neutral point shifts below 7.
- Concentration: The most direct factor. The higher the concentration of a base (or the lower the concentration of an acid), the lower the pOH will be.
- Strength of the Acid/Base: Strong bases (like KOH) dissociate completely, directly releasing OH⁻ ions. Weak bases (like NH₃) only partially react with water, resulting in a less significant change in pOH for the same molar concentration. See our Molarity Calculator for more on concentration.
- The Common Ion Effect: If a solution already contains an ion that is a product of a weak base’s dissociation (e.g., adding NH₄Cl to an NH₃ solution), the equilibrium will shift, suppressing the dissociation of the weak base and increasing the pOH.
- Solvent: While this calculator assumes an aqueous (water) solution, the concepts of acidity and basicity exist in other solvents, though the scales and reference points would change.
- Ionic Strength: In highly concentrated solutions, the interactions between ions can affect their activity (effective concentration), causing a slight deviation from the pOH calculated from simple molarity. Our Dilution Calculator can help manage concentrations.
Frequently Asked Questions (FAQ)
- 1. What is the difference between pH and pOH?
- pH measures the concentration of hydronium (H⁺) ions and indicates acidity, while pOH measures the concentration of hydroxide (OH⁻) ions and indicates alkalinity or basicity. They are inversely related: when one goes up, the other goes down.
- 2. Can pOH be negative or greater than 14?
- Yes. For very concentrated solutions of strong bases (e.g., 2 M NaOH), the [OH⁻] is greater than 1 M, so the pOH = -log(2) is a negative number (-0.30). Correspondingly, for very strong acids, the pOH can be greater than 14.
- 3. Why is the scale based on 14?
- The scale is based on the ion-product constant for water (Kw) at 25°C, which is 1.0 x 10-14. Taking the negative logarithm gives pKw = 14, which leads to the relationship pH + pOH = 14.
- 4. Is a solution with a pOH of 6 acidic or basic?
- A pOH of 6 is basic. If the pOH is 6, the pH is 14 – 6 = 8. Since the pH is greater than 7, the solution is basic. A pOH less than 7 indicates a basic solution.
- 5. How does this pOH calculator handle units?
- The calculator assumes all concentration inputs ([OH⁻] or [H⁺]) are in moles per liter (mol/L), which is the standard unit for these calculations. The pH and pOH values are dimensionless.
- 6. Do I need to press a “calculate” button?
- No. For your convenience, this pOH calculator updates all results in real-time as you type your input value, providing instant feedback.
- 7. What is a neutral solution in terms of pOH?
- A neutral solution has a pOH of 7 (at 25°C). This corresponds to a pH of 7 and means the concentrations of [H⁺] and [OH⁻] are equal (1.0 x 10-7 M).
- 8. How do I interpret the concentrations in scientific notation (e.g., 1.00e-7)?
- The “e” stands for “x 10^”. So, 1.00e-7 is the computer-friendly way of writing 1.00 x 10-7, which is 0.0000001.