Experiments in Analytical Chemistry -EDTA determination of Ca and Mg in water

EDTA (Ethylenediaminetetraacetic acid) -a representative 1:1 metal-chelating agent

11-1. Metal-Chelate Complexes Chelating ligand Chelation Chelate effect

11-2 EDTA -the most widely used complexing agent in analytical chemistry EDTA is 1)Acid: K 1 ~ K 6 2)Forms a 1:1 complex with most metal ions 3)Formation constants (K f ) are usually very large 4)H 6 Y 2+ (fully protonated) ~ Y 4- (fully deprotonated) 5)Metal complex form: MY n-4 M n+ + Y 4- ↔ MY n-4

D= [H + ] 6 + [H + ] 5 K 1 + [H + ] 4 K 1 K 2 + [H + ] 3 K 1 K 2 K 3 + [H + ] 2 K 1 K 2 K 3 K 4 + [H + ] K 1 K 2 K 3 K 4 K 5 + K 1 K 2 K 3 K 4 K 5 K 6

Conditional Formation Constant [EDTA] : Total concentration of all EDTA species (7 forms) not bound to metal ion. [Y 4- ] = α Y4- [EDTA]

11-3 EDTA Titration Curve

EDTA titration

 Calculation of metal ion concentration in EDTA titration  Derivation of a titration curve Vary [M] and get V L to construct a titration curve

Auxiliary complexing agents -for preventing metal hydroxide precipitation

The fraction of metal ion in the uncomplexed state, M

Three formation constants -K f, K f ’, K f ’’

Examples of Zn 2+ titration in the presence of ammonia

Metal Ion Indicator Indicators also form complex with metal ions Metal-free and complexed forms have different colors K f for an M-In should be less than K f for an M-EDTA  When EDTA is added, M-In releases metal ion

Determination of water hardness  Calcium and magnesium salts dissolved in water cause water hardness.  Water hardness can be measured using a titration with ethylenediaminetetraacetic acid (EDTA). EDTA dissolved in water forms a colourless solution.  At pH 10, calcium and magnesium ions form colourless, water soluble, complexes with EDTA: calcium ion complexed by EDTA : CaEDTA 2- magnesium ion complexed by EDTA : MgEDTA 2-  An indicator, known as a metal ion indicator, is required for the titration. This indicator can be used to detect the presence or absence of free EDTA ions in solution.  Endpoint of the titration is when all the Ca2+ and Mg2+ ions have been complexed by the EDTA. Before the endpoint, Ca2+ and Mg2+ are in excess, there is no free EDTA in solution. Immediately after the endpoint, there is an excess of EDTA.  It is essential that deionised water (water in which ions have been removed) is used in preparing all solutions, and in rinsing the conical flask. Ca 2+ and Mg 2+ ions must NOT be present in the water used to prepare the solutions, or in rinsing.

Preparing the Standard EDTA solution 1.Dry some EDTA at 80 o C to remove any moisture. EDTA have the formula Na 2 H 2 C 10 H 12 O 8 N 2.2H 2 O Molar mass of this EDTA = g mol -1 2.Accurately weigh out the required amount of EDTA onto a watchglass. For example, to prepare 250 mL of a mol L -1 solution of EDTA: Determine the moles of EDTA required. 3.Carefully transfer the EDTA to the volumetric flask Place a clean, dry, glass funnel in the neck of the volumetric flask. Use a clean, dry, spatula to transfer some of the EDTA from the watchglass to the funnel. Use a small amount of water from a wash bottle filled with deionized water to wash the EDTA out of the funnel and into the flask. 4.Fill the volumetric flask to the mark with deionised water. Deionized water can be poured into the flask using the funnel until the volume of solution in the flask is a bit below the mark on the flask.