Hardness Analysis of water

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 CaCo3 mg/l
Concentration of CaCO3 (grains/US Gallon)
Soft 0 to < 75 0 to < 5.2
Medium Hard 75 to 150 5.2 < 10.5
Hard 150 < 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 REDsolution. 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.

The Hardness Test
The titration method for determining water hardness is vastly superior to the old soap test that is slow, tedious, and often may give misleading results. The procedure is based on the fact that when a sample of water is titrated with  a  solution  of  EDTA  (sodium  ethylene,  diamine  tetra-acetate), calcium and magnesium react with the EDTA to form  soluble  compounds  in  which  calcium  and magnesium are tied up so firmly that they cannot react with other materials. Standard EDTA solution is added toa water sample and the end point is detected by an indicator that is red in the presence of calcium and magnesium ions and blue in their absence.
A total hardness test set (EDTA) is shown in figure
REAGENTS —the following reagents are used in testing for water hardness:

  • EDTA solution
  • Hardness indicator powder
  • Hardness buffer
  • Hardness reagent

PROCEDURE—In determining water hardness, here is the procedure to follow.
1. Place a 50-ml sample in a 250-ml Erlenmeyerflask. (water sample to be tested)

2. Add 1 dipper of hardness indicator powder (Erichrome Black T).

3.   Add 0.50 ml of hardness buffer to hold the pH at around 10. The color of the mixture will be red if any hardness is present. (liquor Ammonia)
4.   Add the hardness reagent from a burette until the red color just disappears, giving way to a pure blue. (EDTA solution).

CALCULATIONS
The burette reading in ml is multiplied by 20 to give the total hardness.
ppm total hardness as calcium carbonate= ml burette reading × 20

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