CVEN 3454/5404 Water Chemistry

Lab 4: Hardness Measurement by EDTA Titration

Purpose

The purpose of this lab is to understand the role of metal complexation by a ligand in the measurement of calcium and magnesium ions in standard solutions and water samples from the Lefthand Creek watershed and to assess the hardness of the waters coming from two different mines, the Burlington Mine drainage, which flows through Balarat Gulch into Little James Creek, and the Big Five Tunnel drainage, which flows into the Lefthand Creek.  The Burlington Mine was a fluorospar, or fluorite (CaF₂), mine.

Principle

In this method, the sodium salt of ethylenediametetraacetic acid (EDTA) is used to titrate solutions and samples containing calcium and magnesium, which together make up "hardness."  As the EDTA is added and calcium and magnesium complexes with EDTA are formed, an indicator (the dye Eriochrome Black T) changes color from wine red to blue.  Appearance of the blue color indicates the endpoint of the titration.  This method is adopted from Method 309 B, EDTA Titrimetic Method (American Public Health Association, 1976, Standard Methods for the Examination of Water and Wastewater, 14th Ed.).

Materials

• buffer solution (16.9 g NH₄Cl, 143 mL concentrated ammonium hydroxide, 1.25 g MgEDTA, diluted to 250 mL with water)
• indicator solution (0.5 g Eriochrome Black T mixed in 100 g triethanolamine)
• EDTA titrant solution, 0.001 M approximately (0.372 g disodium EDTA dihydrate in 1 L water)
• calcium standard solution (about 0.0025 M calcium chloride dihydrate solution; exact concentration will be provided)
• stir plate, magnetic stir bar, burette, dropper and bulbs, 100 mL beaker, 50 mL graduated cylinder
• samples from the Burlington Mine pond, Little James Creek, the Big Five tunnel drainage, and Lefthand Creek
  1.  Big Five Tunnel drainage, +250 m from tunnel, at confluence with Lefthand Creek
  2.  Lefthand Creek, +40 m downstream from Big Five Tunnel drainage
  3.  Burlington Mine Pond drainage, +5 m from subsidence pond
  4.  Little James Creek, + 10 m downstream of Balarat Gulch
   

Procedure

1. Calibrate the pH meter with pH 7.00 and 10.00 standards.
   
   
2. Dilute 10.0 mL of the calcium standard solution to 50.0 mL with deionized water in a 150-200 mL beaker.  You can measure the volumes by pipette or by using the balance.
   
    
3. Add 1 mL of the buffer solution to the diluted standard solution to raise the pH to 10.0.  Check the pH of the solution -- it should be within 0.2 pH units of pH 10.0 (if not, new buffer solution is needed).
   
    
4. Add 1-2 drops of the indicator solution to produce the reddish color.  Add a stir bar and stir the solution by placing the beaker on a stir plate.
   
    
5. Calculate the expected titration volume using the relationship presented in the Lab Technique Note below before starting and check your result with the TA.
   
    
6. Fill the burette with EDTA titrant solution and add EDTA titrant to the beaker.  Add the titrant slowly (a drop every 3-5 sec) over the last few milliliters based on your expected titration volume.
   
    
7. Observe the change in the color of the solution from red to blue (look for an absence of any reddish tinge) and record the volume at which the color change occurs.  Record your impression of color versus volume for the last few milliliters in case you add too much titrant.
   
   
8. Repeat the measure of the calcium standard solution two more times (steps 2-7) to determine the precision of the EDTA titration.
   
   
9. For each sample (sequentially), add 50.0 mL of the sample and 1-2 mL of the buffer solution to a 150-200 mL beaker.  Check the pH (again, should be pH 10).
   
    
10. Add 1-2 drops of the indicator solution to produce the reddish color, and titrate with the EDTA solution as described in steps 6 and 7 above.
     

Lab Technique Note:

Expect to add about 1 mL of EDTA titrant for every 1 mg of CaCO₃ hardness in the sample (e.g., if the hardness of the sample is 100 mg CaCO₃ L^-1, and the sample size is 25 mL, then 2.5 mg CaCO₃ hardness will be present in the beaker, and 2.5 mL of the EDTA titrant will be required).

The standard method for hardness includes addition of an "inhibitor" to reduce interference in the measurement of hardness by EDTA titration.  Ideally, EDTA would form complexes only with calcium and magnesium ions to measure hardness, but EDTA will also form complexes with other cations (e.g., barium, cadmium, cobalt, copper, iron, lead, manganese, nickel, strontium, and zinc).  To reduce interferences by some of these ions, various inhibitor solution contains ligands like cyanide and sulfide.  The ligands will bind some of the metals more strongly than EDTA and prevent them from being "counted" as calcium and magnesium.

Questions to Address in Lab Report:

(Results) Determine the exact concentration of the EDTA titrant solution using the endpoint measured for the calcium standard solution.  Express the concentration in units of M as a mean of the triplicate analyses.

(Results) What was the precision of the EDTA titration based on the triplicate analyses of the calcium standard solution? Express the precision of this method as a coefficient of variance (%).

(Results) What were the hardness concentrations (in units of molar and mg CaCO₃ L^-1) in the Little James and Lefthand Creek watershed samples?  Present these results in a table with the endpoint of each sample included as a table entry.

(Discussion) Explain the differences in hardness between the Burlington Mine pond (+5 m sample) and Big Five Tunnel drainages.  Describe the effect of the two different mine drainages on the Little James and Lefthand Creeks.

(Discussion) What are the zinc and lead "table value standards" in units of ug L^-1 for Little James and Lefthand Creeks based on the hardness of the Little James and Lefthand Creeks?  Comment on the difference. 

Last updated on July 31, 2007 at 07:19 AM by Joe Ryan