Determination of Water Hardness By Complexometric Titration Class Notes


Hard Water

Hard water is due to metal ions(minerals) that are dissolved in the ground water. These minerals include Ca2+, Mg2+, Fe3+, SO42-, and HCO3-. Our hard water in the southern Indiana area is due to rain moving through the vast amount of limestone, CaCO3, that occurs in our area to the aquifer. This is why we measure hardness in terms of CaCO3; The concentration of the Ca2+ ions is greater than the concentration of any other metal ion in our water.

Water hardness map from the USGS U.S. Geological Survey, Department of Interior/USGS


Why Be Concerned About Hard Water?

The determination of water hardness is a useful test that provides a measure of quality of water for households and industrial uses. Originally, water hardness was defined as the measure of the capacity of the water to precipitate soap. Hard water is not a health hazard. People regularly take calcium supplements. Drinking hard water contributes a small amount of calcium and magnesium toward the total human dietary needs of calcium and magnesium. The National Academy of Science states that consuming extremely hard water could be a major contributor of calcium and magnesium to the diet.

Hard water does cause soap scum, clog pipes and clog boilers.

Soap scum is formed when the calcium ion binds with the soap. This causes an insoluble compound that precipitates to form the scum you see. Soap actually softens hard water by removing the Ca2+ ions from the water.

When hard water is heated, CaCO3 precipitates out, which then clogs pipes and industrial boilers. This leads to malfunction or damage and is expensive to remove.


Water Softeners

If you have hard water you may use a water softener to remove the hardness. Salt is mixed with water. The Na+ ion from the salt replaces the Ca2+ ion, but this causes the water to be too salty for drinking. Water that has been softened should be used only for laundry and bathing.


Types of Hardness

There are two types of water hardness, temporary and permanent.

Temporary Hardness is due to the bicarbonate ion, HCO3-, being present in the water. This type of hardness can be removed by boiling the water to expel the CO2, as indicated by the following equation:

Bicarbonate hardness is classified as temporary hardness.


Permanent hardness is due to the presence of the ions Ca2+, Mg+2, Fe3+ and SO4-. This type of hardness cannot be eliminated by boiling. The water with this type of hardness is said to be permanently hard.

How Hard Is The Water?

The degree of hardness of the water is classified in terms of its calcium carbonate concentration as follows:


Hardness rating

Concentration of Calcium Carbonate (mg/L)

Concentration of Calcium Carbonate ppm

Soft

0 to 60

0 to 60

Medium hard

61 to 120

61 to 120

Hard

121 to 180

121 to 180

Very hard

181 and greater

181 and greater

Information for the table above from USGS


Complexometric Titration

Permanent hardness is usually determined by titrating it with a standard solution of ethylenediamminetetraacetic acid, EDTA. The EDTA is a complexing, or chelating agent used to capture the metal ions. This causes the water to become softened, but the metal ions are not removed from the water. EDTA simply binds the metal ions to it very tightly.


EDTA

EDTA is a versatile chelating agent. A chelating agent is a substance whose molecules can form several bonds to a single metal ion. Chelating agents are multi-dentate ligands. A ligand is a substance that binds with a metal ion to form a complex ion. Multidentate ligands are many clawed, holding onto the metal ion to form a very stable complex. EDTA can form four or six bonds with a metal ion.

It is frequently used in soaps and detergents because it forms complexes with calcium and magnesium ions. These ions which are in hard water are bound to the EDTA and cannot interfere with the cleaning action of the soap or detergent.

EDTA is also used in foods. Certain enzymes are responsible for food spoilage. EDTA is used to remove metal ions from these enzymes. It is used to promote color retention in dried bananas, beans, chick peas, canned clams, pecan pie filling, frozen potatoes and canned shrimp. It is used to improve flavor retention in canned carbonated beverages, beer, salad dressings, mayonnaise, margarine, and sauces. It inhibits rancidity in salad dressings, mayonnaise, sauces and salad spreads.


Why Do We Heat the Water?

We heat the water to make sure all the calcium ions and other metal ions are dissolved in the water. If they are not dissolved in the water, the indicator cannot form a complex with them and your end point will not be the true end point of the titration.


Total Permanent Hardness

In this lab you will be asked to determine the total permanent hardness. EDTA grabs all the metal ions in the water, not just the Ca2+ ions. This gives us a value that is not truly the concentration of Ca2+ ions. This causes an experimental error of about 1% which is acceptable due to the "fuzzy" endpoints in this type of titration.


Erio - T Indicator

EDTA ENDPOINT COLOR CHANGE

Erio - T indicator or Eriochrome Black-T indicator is used in this titration. When it is chealted or acidifies, it produces a PINK RED solution. When it is not chelated and under basic conditions it is BLUE.

The picture on the left shows the color of the indicator before titration. The three pictures to the right show the end point in this titration. There is a 1 drop difference of 0.01 M EDTA between the first and second pictures and between the second and third pictures. Two or three seconds were allowed for colors in the second and third pictures to develop after adding the additional drop. In each case the solution was thoroughly mixed. This color change from wine red to violet to blue is due to the compact nature of the complex. The statement "the compact nature of the complex" means when the indicator is added to the hard water, the indicator Erio-T forms a complex with the Ca+2 ions that is pink in color. As EDTA is added to the solution, the EDTA forms a complex with the Ca+2 leaving the indicator Erio-T uncomplexed, which is blue in color.


Sample Problem

You are using EDTA with a molarity of .0080 for the titration. You titrate 50.00 mL of water sample using 10.68 mL of EDTA.

What is the CONCENTRATION of Ca2+ ion?



What is the hardness?



This material may be copied for personal use only.
If it is reproduced, written permission must be granted by the author: D. Spurlock


HOME INDIANA UNIVERSITY SOUTHEAST

Send comments to: dspurloc @ ius.edu

Unless othrwise credited, all pages by D. Spurlock
Unless otherwise noted, all pages Copyright © 1999-2014 D. Spurlock



last updated: March 31, 2014