Titrimetric WATER HARDNESS DETERMINATION

 Background

The total hardness of water (i.e. the concentration of dissolved cations such as Ca²⁺, Mg²⁺, Fe³⁺, etc.) is commonly determined by the titration of a water sample with a standardized solution of the disodium salt of ethylenediaminetetraacetic acid (EDTA) using eriochrome black T or calmagite as an indicator. Calmagite has the advantage of forming indefinitely stable solutions and will be substituted without any other modification to the procedure.

The reaction that takes place can be written:

  Y^4- + Ca²⁺ ó CaY^2- at pH 10,

 Y^4- is the EDTA anion and CaY^2- is the complex ion. The CaY^2- complex is extremely stable a pH 10 and the titration proceeds essentially to completion.

 Endpoint detection is achieved by use of the visual indicator, calmagite. Calmagite is a complexing agent as is EDTA, but EDTA forms more stable complexes than calmagite. The endpoint detection is as follows:

1. At pH 10, HIn^2- and Mg²⁺ form a red complex. (To ensure the presence of Mg²⁺ a small amount of MgCl₂ is added to the titrant.) 

Mg²⁺ + Hin^2- (blue) ó MgIn^- (red) + H⁺

2. EDTA forms a weaker complex with Mg²⁺ than Ca²⁺. Ca²⁺ reacts with Y^4- first, leaving the red MgIn^- solution

Ca²⁺ + MgIn^- (red) + Y^-4 ó CaY^-2 + MgIn^- (red)

3. When all the Ca²⁺ is titrated by Y^4-, MgIn^- reacts with Y^4- 

MgIn^- (red) + Y^4- ó MgY^2- + In^3- (colorless)

4. In^-3 hydrolyzes.

In^3- (colorless) + H₂O ó Hin^2- (blue) + OH^-

If one knows the concentration of the titrant (often expressed in terms of the titer value, mg CaCO₃/mL titrant) the hardness of the water sample can be found. Water hardness is usually reported as parts per million (ppm) of CaCO₃ even though CaCO₃ is not the species in solution. Remember that ppm is equivalent to mg/Kg or mg/liter for dilute solutions.

Preliminary Calculations

1. Determine the amount of the disodium salt of EDTA to weigh out to make one liter of 0.01 F Na₂ EDTA.
2. Determine the amount of CaCO₃ to weigh out to make 500.0 mL of a solution that contains the equivalent of 0.800 mg CaCO₃/mL.
3. Determine how many mL of the CaCO₃ solution in problem 2 should be titrated with the Na₂EDTA solution to require 40.0 mL of titrant.
4. Find the titer value of an EDTA solution in mg CaCO₃/mL if 50.00 mL of the CaCO₃ standard with 0.800 mg CaCO₃/mL requires 45.00 mL of the EDTA titrant to reach a Calmagite endpoint.
5. If 100.00 mL of a water sample was found to require 14.90 mL of the EDTA titrant of problem 4 to reach the calmagite endpoint, find the hardness of the water in ppm CaCO₃.

Procedure

Preparation and Standardization of the Na₂EDTA Titrant

NOTE: Save the EDTA solution for the Ion Exchange experiment

1. Weigh out enough Na₂EDTA into a clean 400 mL beaker to prepare 1.0 L of 0.01 F EDTA (see preliminary calculation 1). Add about 0.1 g MgCl₂^.6H₃2O. Dissolve the solids in distilled water and transfer to 1 liter bottle. If some of the solids will not dissolve, add small amounts of 0.1 N NaOH until all solids dissolve. Dilute to 1 L and mix thoroughly.
2. Use the standard calcium solution to make 250 mL of a standard that will be ~0.8 mg/mL. Obtain not more than ~25 mL of this standard from the dispenser and record in your notebook the concentration of this standard. Dilute volumetrically and record in your notebook the concentration of your standard solution.
3. Pipet a 25.00 mL portion of the standard into a 250 mL Erlenmeyer flask and add 25 mL of distilled water and 5 mL of an ammonia-ammonium chloride buffer (reagent shelf). Then add 5 drops of calmagite solution to the solution to be titrated.
4. Titrate carefully with the EDTA solution to the point where the color changes from wine red to pure blue (no trace of red left). Repeat the titration with two other portions. Calculate the calcium carbonate titer value of the EDTA solution.

Determination of Total Hardness of Water

1. Turn in to the laboratory instructor a clean, 250 mL, volumetric flask clearly marked with your name and section number.
2. Dilute the sample given to you to mark.
3. Transfer 50.00 mL of the sample of a 250 mL Erlenmeyer and titrate in the same manner as in the standardization. Remember to add 5 mL of buffer! Repeat the titration two more times. Calculate the total hardness of the water as ppm CaCO₃. Remember that the volumes of standard and unknown are different.

Questions

1. What effect on the total hardness would be observed if the water sample contained: (a) 50 ppm Na? b. 100 ppm NH₃?
2. What effect on the total hardness would be observed if the soluble cationic species were primarily Mg²⁺ instead of Ca²⁺?
3. Propose a titration scheme by which one could differentiate between Mg²⁺ hardness and Ca²⁺ hardness.
4. Express the hardness of a water sample in terms of grains CaCO₃ per gallon if it has a hardness of 100 ppm CaCO₃. (There are 700 grains per pound).

Results

Turn in unknown hardness as ppm CaCO₃. This number is 100% of your score.

Reference

D. C. Harris Quantitative Chemical Analysis 4th Ed., W. H. Freeman and Company, New York 1995 Chapters 5 and 1

Tuesday, August 03, 2004