Standardization of EDTA solution for use in complexometric titrations

0.01M EDTA standardization against metallic magnesium

EDTA can be standardized against many reagents, be it metallic magnesium, calcium carbonate, metallic bismuth and so on. For best results it is good to standardize EDTA solution against the same cation and using the same method as will be later used during sample analysis. Note, that EDTA solution can be prepared without a need for standardization, as EDTA itself can be obtained in form pure enough.

Reaction taking place during titration is

Mg²⁺ + EDTA^4- → MgEDTA^2-

Titration is done in pH 10 solution. At this pH about 25% if EDTA is in first protonated form (HEDTA^3-), but magnesium complex stability constant is high enough so that we don't have to worry about.

Procedure to follow:

Weight exactly about 0.24g of metallic Mg ribbon.
Dissolve metal in 25 mL of 1M HCl solution and dilute in volumetric flask to exactly 1L.
Transfer 25 mL of magnesium solution to Erlenmeyer flask.
Add 75 mL of distilled water.
Add 2 mL of pH 10 ammonia buffer solution.
Add a pinch of Eriochrome Black T ground with sodium chloride (100 mg of indicator plus 20 g of analytical grade NaCl).
Titrate with EDTA solution till the color changes to blue.

To calculate EDTA solution concentration use EBAS - stoichiometry calculator. Download EDTA standardization against metallic magnesium reaction file, open it with the free trial version of the stoichiometry calculator.

Enter Mg mass in the upper (input) frame in the mass edit field above magnesium formula (remember to multiply it by the volume fraction of the aliquot used against total volume of magnesium solution prepared). Click n=CV button below EDTA in the output frame, enter volume of the solution used, read solution concentration.

0.01M EDTA standardization against calcium carbonate

Reaction taking place during titration is

Ca²⁺ + EDTA^4- → CaEDTA^2-

Titration is done in about 0.1 sodium hydroxide solution (pH around 13). While this is not our intent when standardizing EDTA solution, at this high pH it is possible to titrate calcium in the presence of magnesium as the latter precipitates in the form of Mg(OH)₂.

Procedure to follow:

Weight exactly about 1g of dry calcium carbonate.
Dissolve solid in 25 mL of 1M HCl solution and dilute in volumetric flask to exactly 1L.
Transfer 25 mL of calcium solution to Erlenmeyer flask.
Add 75 mL of distilled water.
Add 10 mL of 1M sodium hydroxide solution.
Add a pinch of murexide ground with sodium chloride (100 mg of indicator plus 20 g of analytical grade NaCl).
Titrate with EDTA solution till the color changes to violet.

To make end point detection easier it is worth to prepare a comparison solution, identical to the titrated one; this way first color change is easier to spot.

To calculate EDTA solution concentration use EBAS - stoichiometry calculator. Download EDTA standardization against calcium carbonate reaction file, open it with the free trial version of the stoichiometry calculator.

Note, that to be consistent with the use of calcium carbonate and its molar mass, reaction equation is not the one shown above, but

CaCO₃ + EDTA^4- -> CaEDTA^2- + CO₃^2-

These are equivalent. Enter CaCO₃ mass in the upper (input) frame in the mass edit field above calcium carbonate formula (remember to multiply it by the volume fraction of the aliquot used against total volume of magnesium solution prepared). Click n=CV button below EDTA^4- in the output frame, enter volume of the solution used, read solution concentration.