Hardness Determination for Water Samples*
Purpose
To introduce the concept of complex formation and stability and to illustrate the analytical application of this concept to the measurement of hardness in water.
To determine the total hardness for water and test for Ca and Mg solutions and unknown samples.

Introduction

Hardness of water is a property caused by the presence of polyvalent metal cations, primarily Ca2+ and Mg2+ in natural waters. Hardness is undesirable in a water supply because it results in scale formation and in soap wastage (Sample Lab, Fiu). If one ever has gone to a place where ground water is abundant and is used for drinking and cooking, one can see that the pots have a white mark done to the long exposure to the metals that these water contains, or the shampoo doesn’t produce bubbles.
Chelation process is usually the one used most frequently to measure water hardness. The metal ions in this water may have electrons that can be shared in pairs with a donor to form a coordination bond. If a molecule or ion has more than one "free" electron pair which can be shared with a metal ion or similar species, it is called a chelating agent. The complex is termed a chelate.
The chelating reagent that we will be using in this lab is EDTA (ethylene-diamine-tetra-acetic acid ). This reagent is the most commonly used for this purpose. As the name inplies, EDTA is a an acid, a tetraprotic one.
In the completely deprotonated form, EDTA can form coordination bonds at six sites, and the resulting metal complexes are very stable. If the deprotonation of EDTA is not complete, these chelates are not as stable. In the determination of hardness with EDTA, several competing equilibria are involved. The sample solution is buffered at a pH 10 as a compromise between chelate stability, as EDTA chelate stability increases with increasing pH, and the need to prevent precipitation of metal ions being analyzed (e.g., as CaCO3(s) or Mg(OH)2(s)). The ammonia buffer used in the test helps to prevent precipitation of metal ions since ammonia can form weak complexes with them.
Since EDTA and its hardness complexes are not colored, an additional chelating agent, Eriochrome Black T (EBT), is used to facilitate endpoint detection.
EBT exists as a blue-colored anion at pH 10. A small amount of EBT is added to the test solution prior to titration with EDTA and a red-colored complex is formed with Mg2+ present in solution (i.e., the titration starts with red color). The buffer is frequently "spiked" with a trace of EDTAMg 2+ to facilitate endpoint detection in the unlikely event that the sample does not contain Mg2+.
As EDTA is added to the solution, it combines first with Ca2+, and then with Mg2+ because the EDTACa2+ complex is more stable than the EDTAMg2+ complex. The metals are readily removed from their ammonia complexes because the EDTA-metal ion chelate is much more stable. After EDTA has complexed all of the free Mg2+, it will remove Mg2+ from the red EBTMg 2+ complex causing a color change from red to blue.

Procedure

1. Calculate the amount of solid needed
1. Measure 50 mL sample into a 250 mL beaker.
2. Add 1-2 mL buffer solution. The pH should be 10.0 ± 0.1.
3. Add 1-2 drops EBT indicator.
4. Titrate to a blue color. The duration of the titration should not exceed 5 minutes.
5. Perform titration on each of samples provided.
6. Titrate 50 mL of distilled water to which buffer has been added to obtain a blank.

Samples
1. 100 mg/L Mg (prepared in earlier laboratory exercise)
2. 100 mg/L Ca (prepared in earlier laboratory exercise)
3. 50 mg/L each of Ca and Mg (by mixing equal volumes of samples 1 and 2)
4. Tap water and any other water sample you may wish to analyze for total hardness.

Reagents
_ Standard EDTA solution, 0.01 M (1 mL = 1 mg hardness as CaCO3): Dissolve 3.723 g disodium-ethylenediamine- tetraacetate-dihydrate in distilled water and dilute to 1 liter. (Approximately 200 mL per student group). _ Buffer solution: Dissolve 16.9 g ammonium chloride (NH4Cl) in 143 mL concentrated ammonium hydroxide; add 1.25 g of the di-magnesium salt of EDTA and dilute to 250 mL with distilled water. Keep in a tightly stoppered plastic or resistant glass container. (Approximately 50 mL per student group).