ANALYSIS OF CALCIUM BY EDTA TITRATION TO ASSESS WATER HARDNESS

ANALYSIS OF CALCIUM BY EDTA TITRATION TO ASSESS WATER
HARDNESS
PRE-LAB QUERIES
1. What properties of a metal ion might be useful for the analysis of that ion?
2. We often speak of water samples as being “hard” or “soft”. What does this mean?
OBJECT
In this activity, you will perform a titrimetric analysis of Ca2+ in a standard solution and water
samples using the complexometric reaction of EDTA with metal ions.
INTRODUCTION
One measure of water quality is “hardness” which is defined by the amount of Mg2+ and Ca2+
(and sometimes Fe3+) ions in a given amount of water. The presence of calcium and magnesium
ions poses no health hazard but water hardness is of particular concern because the reaction of
these ions with “soap” (sodium salt of a large fatty acid) produces an insoluble product we know
as “soap scum”. This scum is abrasive and may weaken clothes fibers as they move against each
other when worn. It will also be deposited on hair and skin when soap is used as a cleaner in
hard water. Modern detergents do not produce the same degree of insoluble product and have
effectively replaced soap for most products including hand cleansers, bars, shampoos, and
laundry products.
The most common source for Mg2+ and Ca2+ in water is carbonate compounds in Earth materials.
As a result, hardness is often expressed as parts per million of CaCO3 (ppm) by mass. A
hardness of 100 ppm would be equivalent to 100 g of CaCO3 in 1 million grams of water (103 L)
or 0.1 g in 1.0 L or 100 mg in 1.0 L.
Further Explorations in the Chemical World 32 Prince George’s Community College
Titration procedures provide relatively inexpensive means for the analysis of different
substances. These titrations are based on chemical reactions with completion points that can be
monitored by some visible change in the reaction systems. As you are aware, the most common
system involves acids and bases with the use of an indicator that changes color as the system
moves from an acidic to basic composition. Calcium and magnesium ions can be measured
through reaction with a chelating agent EDTA (ethylenediaminetetraacetic acid). This molecule
has four carboxylic acid (~COOH) group sites and two nitrogens, all of which have lone pairs of
electrons. The EDTA molecule can form a complex with as many as six sites on a particular
cation like Ca2+. These EDTA complexes are generally very stable are always in 1:1
(metal:EDTA) molar ratios.