CHEMISTRY 221 LABORATORY—QUANTITATIVE ANALYSIS

EXPERIMENT:  Determination of Calcium by Titration with a Chelating Ligand, Ethylenediamminetetraacetic Acid (EDTA)

Introduction:   Ethylenediamminetetraacetic acid, more commonly known as EDTA belongs to a class of synthetic compounds known as polyaminocarboxylic acids.  Acting as a ligand that shows multiple coordination sites, EDTA forms very strong 1:1 stoichiometric complexes with all +2 and higher charged metal ions in aqueous solution.  

EDTA contains six sites that can be protonated.  Since EDTA itself is quite insoluble in water, the disodium salt is normally used to make EDTA solutions.  To form the strongest complexes, EDTA solutions are usually buffered in a region that ensures that protonation reactions do not compete with the complexation reaction.  The important species in solution are generally H2Y2-   and HY3-.  Figure 15-4 in your text shows the ideal pH buffer ranges for the EDTA titration of various metal ions.

The analytical reaction in this experiment can be written as:
Ca2+  +  HY3-    CaY2-  +  H+
note that the reaction has a 1:1 stoichiometry.  Since EDTA forms very strong complexes with most metal ions with a charge greater than +2,  procedures designed to "mask" or complex impurity ions are often used.  In this experiment, however, relatively pure calcium carbonate unknowns are used so that masking reagents are not needed.  The overall procedure to be used involves the standardization of an EDTA solution by titration with a known amount of calcium followed by using the calibrated solution to determine an unknown amount of calcium.

Reagents: reagent calcium carbonate 6 M HCl
methyl red indicator 6 M NaOH
ethylenediamminetetracetic acid disodium salt dihydrate magnesium chloride hexahydrate

Summary Procedure: 

Preparation of EDTA: 

  1. Use an 800 mL beaker and add about 475 mL of distilled water from a graduated cylinder.
  2. Place the flask on a magnetic stirrer and  add 0.1 g of MgCl2•6 H2O or 20 mL of a 1% solution of magnesium chloride.
  3. Using the top loading balance weigh approximately 2 g of  disodium EDTA, Na2C10H14N2O8.
  4. Slowly add the disodium EDTA. Use a wash bottle filled with distilled water to rinse the last EDTA into the beaker.
  5. Use a magnetic stirrer to make sure that all of the disodium EDTA has dissolved..
  6. If necessary, rinse a clean 1 L plastic bottle with a little of the EDTA solution and then pour the solution into the bottle for storage.

Calculate the approximate concentration of your EDTA at this point:    

Preparation of primary standard calcium solution:

  1. The primary standard calcium carbonate should be dried at 150 degrees °C for at least 1 hour.  (Note the analytical purity on the bottle.)
  2. Prepare a calcium standard solution by accurately weighing 0.35 - 0.4 g of standard calcium carbonate and quantitatively transferring the solid to a 250 mL beaker.  (Be able to calculate the number of grams of pure calcium carbonate required to prepare a standard calcium solution that would require ~35 mL of 0.01 M EDTA for titration of a 10 mL aliquot.)
  3. Place the beaker in the hood and add 6M HCl dropwise until the sample completely dissolves.  Make the volume up to about 50 mL with distilled water.
  4. Gently boil the solution for a few minutes to expel excess CO2 being careful not to lose any of the solution.
  5. Allow the solution to cool and add two drops of methyl red indicator.
  6. Add 6 M NaOH a drop at a time until the methyl red indicator turns yellow (the yellow may be difficult to see but there is no trace of red.)
  7. Carefully rinse the beaker with distilled water into a 100 mL volumetric flask to quantitatively transfer all of the calcium solution.  Dilute to the mark in the volumetric flask.    Mix the solution carefully before continuing.

Calculate the calcium concentration of the standard at this point:    

Standardization of 0.01 M EDTA:

  1. Pipet a 10 mL aliquot of the calcium standard into a 250 mL Erlenmeyer flask and add 15 mL of the ammonium-ammonium chloride buffer.
  2. Add 10 drops of  the calmagite indicator.
  3. Add enough distilled water to have a total volume of  75-100 mL in the Erlenmeyer flask.
  4. Titrate from the initial "wine red" color to an endpoint of  "sky blue."  You should obtain three results that agree to ±1%.

Calculate the concentration of the EDTA solution at this point:    

Unknown calcium samples (treat in the same manner as the CaCO3 standard):

  1. Use the unknown calcium sample form the previous week's experiment and dry in in a weighing bottle in the oven at 150 °C for at least 30 minutes.
  2. Be sure to record the unknown number in your laboratory notebook and the laboratory sample log book
  3. Remove the unknown from the oven and allow it to cool before weighing.
  4. Estimate the number of grams of unknown required to use about 35 mL of your standardized EDTA solution.  
  5. Place the flask in the hood and add 6M HCl drop wise until the sample dissolves.  Some samples may have a trace of  very fine insoluble silica (sand) that appears transparent upon close observation.  Make the volume up to about 50 mL with distilled water.
  6. Gently boil the solution for a few minutes to expel excess CO2 being careful not to lose any of the solution.
  7. Allow the solution to cool and add two drops of methyl red indicator.
  8. Add drops of 6 M NaOH until the methyl red indicator turns yellow.
  9. Any flocculant precipitate that forms may indicate that too much NaOH has been added.  Add a drop or two of HCl to dissolve the precipitate.
  10. Carefully rinse the beaker with distilled water into a 100 mL volumetric flask to quantitatively transfer all of the calcium solution.  Dilute to the mark in the volumetric flask.    Mix the solution carefully before continuing.
  11. Pipet a 10 mL aliquot of the calcium unknown into a 250 mL Erlenmeyer flask and add 15 mL of the ammonium-ammonium chloride buffer.
  12. Add 10 drops of  the calmagite indicator.
  13. Add enough distilled water to have a total volume of  75-100 mL in the Erlenmeyer flask.
  14. Titrate from the initial "wine red" color to an endpoint of  "sky blue."
  15. Titrations should take between 30 and 40 mL of  EDTA titrant to minimize the buret volume error.
  16. Obtain at least three good reproducible titrations. Your results should agree to within ±1%.

Calculate the amount of calcium in the unknown at this point:    

Report the result in terms of the percentage of calcium oxide (%CaO) in your unknown.  The range on unknown values should be between 35% to 60%.
Check your result at this point:    

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