Complexometric Titration Calculator

Water hardness is primarily caused by dissolved Ca²⁺ and Mg²⁺ ions. Total hardness is measured by EDTA titration at pH 10 using Eriochrome Black T indicator (blue → wine red endpoint). Calcium hardness alone is determined at pH 12-13 using murexide indicator (pink → purple), where Mg²⁺ precipitates as Mg(OH)₂. Results are reported as mg/L CaCO₃ equivalent.

EDTA has four carboxylic acid groups (pKa values 2.0, 2.7, 6.2, 10.3) and two amine groups. At low pH, EDTA is heavily protonated and cannot effectively bind metals. The conditional formation constant increases with pH. Most divalent metal titrations require pH 10. However, too high a pH can cause metal hydroxide precipitation. pH buffers (usually NH₃/NH₄Cl) maintain optimal conditions.

Metallochromic indicators are organic dyes that change color when they bind to metal ions versus when they are free. Eriochrome Black T is blue when free and wine-red when complexed with Ca²⁺ or Mg²⁺. At the endpoint, EDTA strips the metal from the indicator (because EDTA complexes are more stable), causing the color change from wine-red to blue.

Masking agents are reagents that selectively bind interfering metal ions, preventing them from reacting with EDTA. Common examples: KCN masks Cu²⁺, Ni²⁺, Co²⁺, Zn²⁺; triethanolamine masks Fe³⁺, Al³⁺; fluoride masks Al³⁺, Fe³⁺; ascorbic acid reduces Fe³⁺ to Fe²⁺ (lower stability constant). This allows selective determination of target metals in complex mixtures.

EDTA directly titrates only metal cations. However, anions can be determined indirectly: add excess known metal ion that precipitates with the anion (e.g., Ba²⁺ for SO₄²⁻, Pb²⁺ for CrO₄²⁻), filter off the precipitate, and back-titrate the excess metal with EDTA. The difference gives the anion concentration.

Direct titration: EDTA is added directly to the metal solution until the endpoint. Back titration: excess EDTA is added to the sample, and the unreacted EDTA is titrated with a standard metal solution (usually Mg²⁺ or Zn²⁺). Back titration is used when the metal-EDTA reaction is slow, when the metal precipitates at the required pH, or when no suitable indicator exists for the direct titration.

EDTA solution is typically standardized against a primary standard calcium carbonate (CaCO₃, dried at 110°C) dissolved in dilute HCl, or against analytical-grade zinc metal dissolved in acid. Alternatively, commercially available EDTA disodium salt dihydrate (Na₂H₂Y·2H₂O) can be dried at 80°C and weighed directly as a primary standard with 0.3% uncertainty.

Stability generally increases with ionic charge and decreases with ionic radius. Typical log K_f values: Mg²⁺ (8.7), Ca²⁺ (10.7), Mn²⁺ (13.9), Fe²⁺ (14.3), Co²⁺ (16.3), Ni²⁺ (18.6), Cu²⁺ (18.8), Zn²⁺ (16.5), Fe³⁺ (25.1). Trivalent ions generally form much more stable complexes than divalent ions, which is why pH and masking agents are essential for selective analysis.

Eriochrome Black T (EBT) has a sharper endpoint with Mg²⁺ than with Ca²⁺ because the Mg-EBT complex has an ideal stability constant for visual detection. A small amount of Mg-EDTA complex is often added to the EDTA solution or the buffer. The Mg²⁺ released when EDTA is added to the sample is the last to be titrated, producing the sharp blue endpoint characteristic of EBT.