About pH Calculator
Calculate pH, pOH, hydrogen ion concentration, and hydroxide ion concentration. Convert between all four values instantly with common substance presets.
How to use
- Select an input mode: enter a pH value, pOH value, hydrogen ion concentration [H+], or hydroxide ion concentration [OH-]. The calculator accepts any of the four and computes the other three.
- Type your value into the input field. For ion concentrations, use scientific notation (e.g., 1e-7 for pure water). For pH or pOH, enter a number typically between 0 and 14.
- Alternatively, select a common substance from the preset dropdown (battery acid, lemon juice, blood, bleach, etc.) to auto-fill a known pH value and see all corresponding values.
- Review the results showing pH, pOH, [H+], and [OH-] along with a visual indicator showing where the solution falls on the acid-base spectrum. The classification (strongly acidic, weakly basic, neutral, etc.) is displayed automatically.
Frequently asked questions
What is pH and what does the pH scale measure?
pH stands for 'potential of hydrogen' and measures the hydrogen ion activity in a solution. The scale runs from 0 (most acidic) to 14 (most basic/alkaline), with 7 being neutral. The scale is logarithmic: each unit represents a tenfold change in H+ concentration. pH 4 is 10 times more acidic than pH 5 and 100 times more acidic than pH 6. The formula is pH = -log10([H+]). Most natural water systems have pH between 6.5 and 8.5, human blood is tightly regulated at 7.35-7.45, and stomach acid ranges from pH 1.5 to 3.5.
How do you convert between pH, pOH, [H+], and [OH-]?
All four values are mathematically linked at 25 degrees C. From pH: pOH = 14 - pH, [H+] = 10^(-pH), [OH-] = 10^(-pOH). From [H+]: pH = -log10([H+]), then derive pOH and [OH-]. From [OH-]: pOH = -log10([OH-]), pH = 14 - pOH, [H+] = 10^(-pH). The key relationship is Kw = [H+] x [OH-] = 1 x 10^-14 at 25 degrees C. This calculator performs all conversions instantly from any starting value.
Why is pH important in everyday life?
pH affects water quality (drinking water should be pH 6.5-8.5), pool maintenance (target pH 7.2-7.6 to prevent eye irritation and equipment corrosion), gardening (most plants prefer soil pH 6.0-7.0; blueberries need 4.5-5.5), cooking (bread dough acidity affects yeast activity), skincare (healthy skin is pH 4.5-5.5), and aquariums (freshwater fish need pH 6.5-7.5). Industrial processes like brewing, dyeing, and pharmaceutical manufacturing require precise pH control.
What is the difference between a strong acid and a weak acid?
Strong acids (hydrochloric acid, sulfuric acid, nitric acid) fully dissociate in water: every molecule releases its H+ ions. Their pH can be calculated directly from concentration. Weak acids (acetic acid, citric acid, carbonic acid) only partially dissociate. A 0.1 M solution of HCl has pH = 1, but a 0.1 M solution of acetic acid has pH = 2.87 because only about 1.3% of its molecules release H+ ions. The dissociation constant Ka quantifies this: strong acids have very large Ka values, weak acids have small ones.
Can pH go below 0 or above 14?
Yes, though it is uncommon. Concentrated strong acids can have negative pH values: 10 M hydrochloric acid has a theoretical pH of -1. Concentrated strong bases can exceed pH 14: a 10 M sodium hydroxide solution has a theoretical pOH of -1 (pH = 15). The 0-14 range applies to dilute aqueous solutions at 25 degrees C where concentrations stay at or below 1 M. At other temperatures, the neutral pH also changes because Kw varies with temperature.
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