| 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.
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:
HCO3 ¯ == H2O + CO2
(THIS IS TEMPORARY HARDNESS DUE TO BICARBONATES)
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 (grains/US gallon) |
Soft |
0 to <75 |
0 to <5.2 |
Medium hard |
75 to <150 |
5.2 to <10.5 |
Hard |
150 to <300 |
10.5 to <21 |
Very hard |
300 and greater |
21 and greater |
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.
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.
Eri-chrome Black - T Indicator
EDTA ENDPOINT COLOR CHANGE
Erio - T indicator or Eriochrome Black-T indicator is used in this titration. When it is chelated or acidifies, it produces a PINK RED solution. When it is not chelated and
under basic conditions it is BLUE.
The three pictures 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.
Sample Analysis
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?
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