To measure the hardness of a given sample of water in laboratory.

Ø Experiment 2

    

 To measure the hardness of a given sample of water in laboratory.

Ø Principle

                       

EDTA Tritrimetric Method.

Ø Theory

§  Hard water

This refers to the presence of dissolved ions, mainly of calcium Ca²⁺ and magnesium Mg²⁺ which are acquired through contact with rocks and sediments in the environment.

Or

Hardness is a measurement of the concentration of divalent metal ions such as calcium, magnesium, iron, and zinc etc, usually acquired as rainwater percolates through rock. In most water it consist mainly of calcium and magnesium salts, with trace amounts of other metals

§  Origin of hardness

Carbon dioxide reacts with water to form carbonic acid which at ordinary environmental pH exists mostly as bicarbonate ion. Microscopic marine organisms take this up as carbonate to form calcite skeletons which, over millions of years, have built up extensive limestone deposits. Ground water, made slightly acidic by CO₂ (both that absorbed from the air and from the respiration of soil bacteria) dissolve the limestone thereby acquiring calcium and bicarbonate ions and becoming "hard". If the HCO₃^– concentration is sufficiently great, it causes calcium carbonate to precipitate out on surfaces such as the insides of pipes. (Calcium bicarbonate itself does not form a solid, but always precipitates as CaCO₃.)

§  Types of hardness of water

 There are two types of hardness

1. permanent hardness
2. Temporary Hardness

 The sum of both types of hardness is called the “general or total hardness”.

1. Temporary Hardness

 It refers to the hardness derived mainly from carbonate and bicarbonate ions and directly reflects the buffering capacity of the water. This form of hardness is also called carbonate hardness or temporary hardness because it can be precipitated and removed by boiling the water.

H₂O(l) + CO₂(g) + CaCO₃(s) Ca(HCO₃)₂(aq)

            H₂O(l) + CO₂(g) + MgCO₃(s) Mg(HCO₃)₂(aq)

Temporary hardness of water can be removed by boiling or by the addition of lime (calcium hydroxide). It is caused by dissolved calcium bicarbonate in the water. Calcium carbonate is less soluble in hot water than in cold water; therefore, boiling (which promotes the formation of carbonate) precipitates calcium carbonate out of solution, leaving water that is less hard on cooling.

2. Permanent hardness

It measures the ions such as nitrates, sulphate, and chlorides etc, that are not removed by boiling. Most of these are not involved with buffering but can affect pH values.

aq. + CaSO₄(s)Ca²⁺(aq) + SO₄^2-(aq)

            aq. + MgSO₄(s)Mg²⁺(aq) + SO₄^2-(aq)

§  Effects of hard water

1.      Laundering

Clothes washed in hard water often look dull, and feel harsh and scratchy. The minerals combine with some soils to form insoluble salts, making them difficult to remove. Soil on clothes can even introduce more hardness minerals into the wash water. Continuous laundering in hard water can damage fibers, shortening the life of clothes by up to 40%. In hard water, detergent performance is also reduced, and more detergent is required.

2.      Bathing

Bathing with soap in hard water leaves a film of sticky soap scum on the skin. This film may prevent removal of bacteria and soil. Soap scum interferes with the return of skin to its normal, slightly acid condition, and may lead to irritation. It may also make hair dull, lifeless, and difficult to manage.

3.      Problems in Water Boiler Systems and Pipe work

Hard water also leads to inefficient and costly operation of water-using appliances. Heated hard water forms a scale of calcium and magnesium minerals (lime scale deposits) that can contribute to the inefficient operation or failure of such appliances. Pipes can become blocked with scale that reduces water flow, ultimately leading to pipe replacement. 

§  Identifying Hard Water

The best way to determine whether or not your water is hard is to have it tested. However, you can usually detect hard water by the evidence in your home, including:

• Increased water heating costs due to scale buildup and mineral deposits, and more frequent replacement of hot water heating elements
• Soap scum on bathtubs, shower tiles, and basins
• Film left on the body resulting in dry skin and dull, limp hair
• Decreased sudsing and cleaning capabilities of soaps and detergents, resulting in dingy laundry and reduced life of
• Increased buildup of scale on plumbing fixtures and cooking utensils such as a tea kettle, coffee maker, pasta pot, and dish
• Clogged pipes or appliances resulting in reduced water flow and increased repairs

§  Treatment

The most common method to treat hard water is through ion exchange water softening. Ion exchange water softening is a process in which the hardness ions, magnesium and calcium, are exchanged with either sodium or occasionally, potassium ions. This is accomplished by directing the flow of hard water over a bed of plastic resin beads. Each bead has a slight electric charge, which holds the sodium on the bead. As the water flows over the beads, the hardness minerals (ions) are attracted to the beads. When the hardness minerals attach themselves to the beads, the sodium ions are displaced. Hence, the hardness ions are replaced by the sodium ions.

As some point the plastic resin beads will be covered with hardness ions and will no longer be able to remove hardness from the water. In order to remove the hardness ions from the beads, brine or salt (sodium chloride) solution is added to the resin bed. This solution contains a high concentration of sodium ions, which remove the hardness ions from the beads. Next the solution and the hardness ions are flushed out of the resin bed with fresh water, and once again the beads can remove hardness from the water. This process is called regeneration.

Ø Utilization

This test  is used for the following processes as under,

§  In the design of boilers .

§  For checking the sutibility of water for industrial and domestic use.

§  In textile dying

§  In horticultural use.

Ø Apparatus

                            

                             Beakers, Hardness Measuring Kit.

Ø Procedure

§  Remove the cap of the small plastic vessel. Rinse it with the sample water. Fill it up to the 5ml mark and replace the cap.

§   Add 5 drops of hardness buffer through the cap port and mix it carefully swirling the vessel in tight circles.

§  Add one drop of Camigate Indicator through the cap port and mix. The solution becomes of red-violet color.

§  Take a titration syringe and push the plunger completely. Insert the tip into HI3812-0 EDTA solution and pull the plunger until the lower edge of the seal is on 0 mark.

§  Place the syringe tip on the cap port of the plastic vessel and slowly drop the titration solution in it drop wise swirling to mix after each drop.

§  Continue adding the solution until the color becomes purple. Then mix for 15 Sec after each drop until the solution turns blue

§  Read off the millimeters of the titration solution from the syringe scale and calculate the harness.

Ø Precautions

§  Handle the indicator and other chemical of the kit carefully.

§  Always wear gloves during experiment.

§  Never add more than one drop of the solution in water during titration.

Ø Observations & calculations

                   

                        Syringe reading (ml) x 300 =  hardness (mg/l)

                                    

                                                    0.76 ml  x 300 =  228 mg/l

Ø Remarks

§  Due to personal error in reading the syringe scale their was a confusion in the reading.

Steps of water hardening

Hardness expressed as mg/l of CaCO3.
mg/l or ppm a	Water hardness
0-75	Soft
75-150	Moderately hard
150-300	Hard
Over 300	Very Hard
a When expressed as grains of hardness, 1 grain = 17.1 mg/l (ppm).