Hydroxide Calculators

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Hydroxide (OH⁻) is the conjugate base of water and the fundamental anion in aqueous base chemistry. It forms when a base donates or attracts a proton, or when water autoionizes (H₂O ⇌ H⁺ + OH⁻). In water at 25°C, [OH⁻] = Kw/[H⁺] = 10⁻¹⁴/[H⁺]. Solutions with [OH⁻] > 10⁻⁷ M are basic (pH > 7). Hydroxide ions act as nucleophiles in organic chemistry, base catalysts in enzyme mechanisms, and define the alkalinity of biological fluids. Common hydroxide compounds include NaOH (lye), KOH (potassium hydroxide), Ca(OH)₂ (lime), and Mg(OH)₂ (milk of magnesia).

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Hydroxide in Aqueous Solutions

Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C

[OH⁻] = Kw / [H⁺] = 10⁻¹⁴ / 10^(−pH) = 10^(pH − 14)

pOH = −log[OH⁻]; pH + pOH = 14 (at 25°C)

Examples: pH 7.4 (blood): [OH⁻] = 10^(7.4−14) = 10^(−6.6) = 2.5 × 10⁻⁷ M. pH 13 (strong base): [OH⁻] = 10^(−1) = 0.1 M.

Strong vs. Weak Bases

  • Strong base: fully dissociates; [OH⁻] = base concentration. NaOH, KOH, Ca(OH)₂, Ba(OH)₂
  • Weak base: partial dissociation; [OH⁻] calculated from Kb. NH₃ + H₂O ⇌ NH₄⁺ + OH⁻; Kb(NH₃) = 1.8 × 10⁻⁵

Common Hydroxide Compounds

  • NaOH (sodium hydroxide): strong base; corrosive; used in saponification, drain cleaners, pH adjustment
  • KOH (potassium hydroxide): strong base; similar to NaOH; used in liquid soaps, batteries
  • Ca(OH)₂ (calcium hydroxide, slaked lime): slightly soluble; lime water; concrete; water treatment
  • Mg(OH)₂ (magnesium hydroxide): antacid (milk of magnesia); reacts with stomach HCl
  • Al(OH)₃: amphoteric; dissolves in both acid and base; used in antacids and water purification

Hydroxide in Biology

Enzyme active sites often use OH⁻ as a nucleophile (serine proteases, zinc metalloenzymes). Carbonic anhydrase converts CO₂ + OH⁻ → HCO₃⁻ in red blood cells. Alkaline phosphatase uses Zn²⁺-bound OH⁻ for phosphate ester hydrolysis.

Glossary

Hydroxide Ion (OH⁻)
The anion from water autoionization; [OH⁻] = Kw/[H⁺] = 10⁻¹⁴/[H⁺]; pOH = −log[OH⁻]; pH + pOH = 14 at 25°C; defines basic character of aqueous solutions.
Strong Base
A base that fully dissociates in water (NaOH, KOH, Ca(OH)₂); [OH⁻] = molar concentration of the base; produces high pH solutions at all concentrations.
pOH
−log₁₀[OH⁻]; related to pH by pH + pOH = 14 at 25°C; increases as solution becomes more acidic; pOH = 0 for [OH⁻] = 1 M (very strong base).

Frequently Asked Questions

The hydroxide ion (OH⁻) is the anion produced by water autoionization and defines basic character in aqueous solutions. [OH⁻] = Kw/[H⁺] = 10⁻¹⁴/[H⁺]. Alternatively: pOH = 14 − pH; [OH⁻] = 10^(−pOH). Examples: blood pH 7.4: pOH = 14 − 7.4 = 6.6; [OH⁻] = 10⁻⁶·⁶ = 2.5 × 10⁻⁷ M. Bleach pH ~12.5: [OH⁻] = 10^(12.5−14) = 10^(−1.5) = 0.032 M. Pure water at 25°C: [OH⁻] = [H⁺] = 10⁻⁷ M (neutral).

Strong bases fully dissociate in water: NaOH → Na⁺ + OH⁻ (100% dissociation). [OH⁻] directly equals the base concentration: 0.05 M NaOH → [OH⁻] = 0.05 M; pOH = 1.3; pH = 12.7. Weak bases partially dissociate: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻; Kb = [NH₄⁺][OH⁻]/[NH₃] = 1.8 × 10⁻⁵. For 0.1 M NH₃: [OH⁻] = √(Kb × C) = √(1.8×10⁻⁵ × 0.1) = 1.34 × 10⁻³ M; pH = 11.1 (much less basic than a strong base at the same concentration). Common strong bases: NaOH, KOH, Ca(OH)₂, Ba(OH)₂. Common weak bases: NH₃, amines, pyridine.

Magnesium hydroxide (Mg(OH)₂, milk of magnesia) is a weakly soluble hydroxide suspension used as an antacid and laxative. When ingested: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. It neutralizes excess stomach acid (HCl), raising gastric pH. Because it is only slightly soluble, it releases OH⁻ slowly and acts as a buffering antacid — less likely to cause rebound acid secretion than sodium bicarbonate. Also used as an osmotic laxative (Mg²⁺ draws water into the colon). Antacid doses provide ~0.6–1.8 g Mg(OH)₂; laxative doses: 2.4–4.8 g. Should not be used in renal impairment (accumulation of Mg²⁺).

OH⁻ is a powerful nucleophile (electron pair donor) in organic chemistry: Nucleophilic substitution (SN2): OH⁻ attacks a carbon bearing a leaving group → forms an alcohol. Example: CH₃Br + OH⁻ → CH₃OH + Br⁻. Elimination (E2): OH⁻ abstracts a proton adjacent to a leaving group → forms an alkene. Ester hydrolysis (saponification): RCOOR' + OH⁻ → RCOO⁻ + R'OH → soap from triglycerides + NaOH. Aldol reactions: OH⁻ deprotonates alpha-carbon → enolate nucleophile. In biochemistry: serine hydroxyl groups in enzyme active sites act as nucleophiles (serine proteases); zinc-bound OH⁻ in carbonic anhydrase and carboxypeptidase provides nucleophilic attack on substrates.