Determination of Hardness of Water in Textile Industry
Apu Kumar Das
Dept. of Wet Processing
Textile Engineering College, Chitttagong
Hardness of Water:
The presence of Ca, Mg salt i.e bi-carbonates, sulphates, chloride in water is called causes of hardness of water. The water which contains these salts is called hard water. There are two types of hardness; Temporary hardness and permanent hardness. Soft water is relatively free of calcium and magnesium ions. It produces a rich foamy lather with soap. This is essential for the soap to be an effective emulsifying agent for oils and dirt. With hard water, the soluble sodium salt of soap reacts with the alkaline earth metal ions and precipitates as the useless and undesirable calcium or magnesium soap. The cleaning ability is lost. In this article I will discuss what is hardness, expression of hardness, estimation or determination of hardness of water.
Hardness is defined as the presence of soluble calcium and magnesium salts in the water. If these are present in the form of bicarbonates, the hardness is temporary. Heating hard water containing bicarbonates eliminates dissolved carbon dioxide and the causes precipitation of calcium carbonate. Magnesium carbonate is slightly soluble in water but heating will cause its hydrolysis into the much less soluble magnesium hydroxide. Simply boiling and filtering the water therefore eliminates temporary hardness. In regions where water has high temporary hardness, and is used directly without treatment, it is not uncommon to see hot water rinsing and washing baths with a generous crust of chalk (CaCO3) on the inner surfaces. This type of precipitation inside a boiler is also undesirable because the scale reduces the efficiency of heat transfer.
Mg(HCO3)2(aq) = MgCO3(s) + CO2(g) + H2O
MgCO3(aq) + H2O = Mg(OH)2(s) + CO2(g)
Permanent hardness arises when water contains soluble salts of calcium and magnesium such as chlorides and sulphates. It is unaffected by boiling the water. The total hardness of water is determined by the amount of dissolved calcium and magnesium, but expressed in the form of CaCO3. This is possible because calcium and magnesium carbonates have identical stoichiometry. Therefore, water that contains 1.0 mM calcium and 0.5 mM magnesium ions, has a total concentration of 1.5 mM. Since the molecular weight of CaCO3 is 100 g mol–1, 1.5 mM corresponds to 150 mg l–1 of CaCO3, or 150 ppm (parts per million), assuming that 1.0 l of the water has a mass of 1.0 kg. The permanent hardness is that remaining after the water has been boiled and filtered to remove the precipitated calcium and magnesium carbonates and magnesium hydroxide. The temporary hardness is the difference between the total and permanent hardness.
Occasionally, hardness due to magnesium alone is of interest, again expressed as the equivalent amount of CaCO3. For the water mentioned above, 0.5 mM of magnesium ions is equivalent to 50 mg l–1 or 50 ppm CaCO3.
Expression of Hardness:
Irrespective of the salts present, hardness is expressed in terms of CaCO3. Hardness may be expressed in degrees or in parts per million. Units of hardness vary from country to country, and some of them are listed in below Table. In British usage, water of 0°-4° is soft, 4°-7° is moderately hard, 7°-20° is hard, and >20° is very hard.
Table: Units of water hardness
|Quantity in parts per million
|1 grain CaCO3/US gallon
|17.1 as CaCO3
|1 grain CaCO3/UK gallon
|14.3 as CaCO3
|1 part CaCO3/105 parts water
|10 as CaCO3
|1 part CaO/105 parts water
|10 as CaO; 17.9 as CaCO3
|1 part Ca/106 parts water
|2.5 as CaCO3
Determination of Hardness of Water:
- Hardness buffer
- Calmagite Indicator
- HI 3812-0 EDTA Solution
For High Range – 0 to 300 mg/L CaCO3
- Remove the cap from the small plastic beaker. Rinse the plastic beaker with the water sample, fill to the 5 ml mark and replace the cap.
- Add 5 drops of Hardness Buffer through the cap port and mix carefully swirling the beaker in tight circles.
- Add 1 drop of Calmagite Indicator through the cap port and mix as described above. The solution becomes a red violet color.
- Take the titration syringe and push the plunger completely into the syringe. Insert tip into HI 3812-0 EDTA Solution and pull the plunger out until the lower edge of the seal is on the 0 ML mark of the syringe.
- Place the syringe tip into the cap port of the plastic beaker and slowly add the titration solution drop wise, swirling to mix after each drop.
- Continue adding the titration solution until the solution becomes purple, then mix for 15 seconds after each additional drop until the solution turns blue.
- Read off the milliliters of titration solution from the syringe scale and multiply by 300 to obtain mg/L (ppm) CaCO3.
For Low Range – 0.0 to 30.0 mg/L CaCO3
If result is lower than 30 mg/L, the precision of the test can be improved by following the procedure below.
- Remove the cap from the large plastic beaker. Rinse it with the water sample, fill to the 50 ml mark and replace the cap.
- Proceed with the titration as for the high range test.
- Read off the millilitres of titration solution from the syringe scale and
- Multiply by 30 to obtain mg/L (ppm) CaCO3.
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.