The pH scale is one of the most recognized concepts in all of chemistry, and for good reason. It tells you how acidic or basic a solution is, and it has practical implications everywhere from swimming pools to blood chemistry to soil management. But there is more to pH than just memorizing the scale from 0 to 14.
What pH Actually Measures
pH is a measure of the hydrogen ion concentration in a solution. Mathematically, pH = -log[H+], where [H+] is the concentration of hydrogen ions in moles per liter. A pH of 7 is neutral, meaning [H+] = [OH-] = 10^-7 M. Values below 7 are acidic (more H+ than OH-), and values above 7 are basic (more OH- than H+). The scale is logarithmic, which means each whole number change represents a tenfold change in hydrogen ion concentration. A solution with pH 4 is ten times more acidic than one with pH 5, and one hundred times more acidic than pH 6.
Strong vs Weak Acids and Bases
Strong acids like hydrochloric acid (HCl) and sulfuric acid (H2SO4) dissociate completely in water. If you dissolve 0.1 mol of HCl in enough water to make 1 L of solution, virtually every HCl molecule breaks apart into H+ and Cl-, giving [H+] = 0.1 M and pH = 1.0. Weak acids like acetic acid (CH3COOH, found in vinegar) only partially dissociate. A 0.1 M acetic acid solution has a pH of about 2.87, not 1.0, because only a small fraction of the acid molecules release H+.
Our pH Calculator lets you convert between pH, pOH, [H+], and [OH-] instantly.
Buffer Losungen
Buffers are solutions that resist changes in pH when small amounts of acid or base are added. They consist of a weak acid and its conjugate base (or a weak base and its conjugate acid). The Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]), is used to calculate the pH of a buffer solution. Buffers are critically important in biological systems: human blood is buffered to maintain a pH of about 7.4, and even small deviations can be life-threatening. The Buffer Calculator makes it easy to design buffer solutions for lab work.
Titration
A titration is a technique used to determine the concentration of an unknown acid or base by reacting it with a solution of known concentration. As you add titrant, the pH changes gradually at first, then undergoes a sharp change near the equivalence point. For strong acid-strong base titrations, the equivalence point is always at pH 7. For weak acid-strong base titrations, it is above 7. The Titration Calculator estimates the pH at different stages of the titration process.