Ethylenediaminetetraacetic acid or EDTA forms a 1:1 complex with calcium and magnesium cations at pH — Complexometric Titration Chemistry Question
Problem Context
Ethylenediaminetetraacetic acid or EDTA forms a 1:1 complex with calcium and magnesium cations at pH 10. The indicator Eriochrome Black T forms a red colored complex with metal ions and is blue/black in their absence. Using the provided materials, devise and carry out an experiment to determine the total combined concentration of calcium and magnesium in milk.
Give a brief description of your experimental plan.
Model Answer
Indication that titration will be used to solve the problem
• Students may plan to test titration (observation of the endpoint) with the magnesium chloride solution, indicator, buffer, and EDTA.
• Excellent plan indicates a strategy for determining volume added
• Experimental details on titration
o One possible approach – mix buffer, indicator, and milk. Titrate with EDTA solution until indicator changes from red to blue.
o Another possible approach – mix buffer, indicator, milk, and set volume of EDTA – solution must be blue/black in color. Back titrate with magnesium chloride solution until color changes from blue to red.
• Detailed equipment to use for the titrations.
• Replicate measurements
Record your data/observations.
Model Answer
Clear data table indicating starting volumes of each solution used (i.e. milk, magnesium chloride solution, EDTA, buffer, indicator – specific solutions depends on method of titration) and the volume of titrant (EDTA or MgCl2) used – minimum of duplicate trials
• Observations – color changes observed in the titrations
• Clear indication of how number of drops are converted to volume or how the volume of solutions used was determined
Show all calculations.
Model Answer
The calculations will depend upon the method of titration selected by the student.
a) Titration of milk directly with EDTA:
• Calculate moles of EDTA added to the solution
• Calculate based on a 1:1 ratio the moles of Mg2+ or Ca2+ in the milk
• Determine the total concentration (mol/L) of Mg2+ and Ca2+ in milk by dividing moles of divalent cations by the volume of milk used
b) Back-titration with magnesium chloride solution
• Calculate moles of EDTA added to the milk
• Calculate the moles of Mg2+ added to reach the endpoint = moles of excess EDTA
• Calculate the moles of divalent cation complexed with the EDTA = moles of divalent cations in milk
• Determine the total concentration (mol/L) of Mg2+ and Ca2+ in milk by dividing moles of divalent cations by the volume of milk used
• Average value – from duplicate trials
The total combined concentration in mol/L of calcium and magnesium in milk is _____________________.
Model Answer
USDA reports the following values for Skim milk
Calcium 122 - 143 mg/ 100 g
Magnesium 11 - 16 mg/100 g
The density of milk is reported to be between 1.03-1.04 g/mL
The total combined concentration in mol/L of calcium and magnesium in milk ranges from 0.036 – 0.044 mol/L.
Accuracy of value
0.03-0.045 M full credit
0.02-0.03 or 0.045-0.06 M 3 points
0.015-0.02 or 0.06 – 0.07 M 2 points