π§ͺ pH / pOH Converter
All four quantities from any one.
How to Use This Calculator
Pick which quantity you know and enter its value. All four values appear with an acidity classification.
The Relationships
pH + pOH = 14 | [H+][OH-] = 1.0e-14
These four equations are interlocked through the water autoionization constant. Knowing any one determines the other three at 25 C.
Worked Example
pH 3: [H+] = 1e-3 M, pOH = 11, [OH-] = 1e-11 M. Strongly acidic.
The pH Scale Is Logarithmic
Each pH unit represents a tenfold change in hydrogen ion concentration. Lemon juice at pH 2 has 100 times the [H+] of tomato juice at pH 4. This is why small pH shifts matter enormously in biology: human blood stays between 7.35 and 7.45, and a drop to 7.0 is life-threatening because it means nearly triple the hydrogen ions.
Buffers Resist pH Change
Your blood, and many lab solutions, contain buffers: weak acid/conjugate base pairs that absorb added H+ or OH-. The bicarbonate buffer system handles most of your blood's pH control, with the lungs and kidneys adjusting CO2 and bicarbonate levels. This calculator covers simple solutions; buffer pH needs the Henderson-Hasselbalch equation.
Acid Rain
Normal rain is pH 5.6 because CO2 dissolves to form carbonic acid. Acid rain below pH 5 comes from sulfur and nitrogen oxides. Because the scale is logarithmic, rain at pH 4.6 has ten times the acidity of normal rain, enough to damage stone buildings and acidify lakes over time.
Stomach Acid
Your stomach maintains pH around 1.5 to 2, acidic enough to denature proteins and kill most microbes. Antacids neutralize a fraction of this acid, raising pH slightly; because the scale is logarithmic, even small pH changes represent large concentration shifts. The stomach lining protects itself with a bicarbonate-rich mucus layer. Ulcers form when this defense fails and acid attacks the tissue directly.
Frequently Asked Questions
How do I convert pH to [H+]?
[H+] = 10^(-pH). A pH of 3 means 0.001 M hydrogen ions. Each pH unit is a tenfold change, so pH 3 is ten times more acidic than pH 4.
What is the relationship between pH and pOH?
pH + pOH = 14 at 25 C (from Kw = 1.0e-14). If pH is 9, pOH is 5. This calculator converts all four quantities at once.
Is pOH still used?
Less than pH, but it is essential for base problems. Strong base calculations naturally give [OH-] first, so converting through pOH is the cleanest path to pH.
Can pH be negative?
Yes for very concentrated acids. A 10 M strong acid has pH -1. The 0 to 14 scale is just the common range, not a law. This calculator handles any value.
Does pH + pOH = 14 at all temperatures?
No, only at 25 C. Kw rises with temperature (1.0e-14 at 25 C, about 5.5e-14 at 50 C), so neutral pH drops below 7 in hot water. The calculator uses 25 C standard.