Hardness of Water for 1st Year B Tech Students

Dr. Vivek Pandey

Rajasthan Technical University Kota

1
 WATER
• Water is an colorless, tasteless, odorless and transparent chemical substance, H2O, which is main constituent of Earth’s
hydrosphere. It exist in solid, liquid and gaseous forms as ice, water and vapor and very useful for the mankind.
H2O(Solid) ⇌ H2O(Liquid) ⇌ H2O (Gas)
• Our earth is blue planet as water covers around 71% of its surface. 97 % of available water is an ocean/sea water which is
almost useless cannot be used for mankind. Only about 2.5 percent of the Earth's water is freshwater and most of that water
(98.8 percent) is in form of ice and groundwater.
• Water is vital for the survival of the all forms of life, and also used for agriculture purpose.
• Water is very use full engineering material it is used as steam for power generation and to run various process industries. It
is also being used as universal solvent in chemical processing units.
• Rain water is supposed to be pure source of water this involves evaporation of water from the earth’s surface during
summers followed by condensation and precipitation as rain.
• Rain water or melted snow flows down the slope over the earth’s surface and due to hydrolysis or hydration reactions water
gets polluted and impurities are added in to the water as dissolved salts and makes the water a hard water.

H2O + CaCO3 → No Reaction

H2O + CaCO3 + CO2 → Ca(HCO3)2
 HARDNESS OF WATER

Hard water is water that has high content of dissolved minerals and is formed when water percolates through deposits
of limestone, chalk, gypsum and other minerals. Hardness in water is imparted by the presence of soluble salts of Ca,
Mg, Fe, Al and Mn in the form of chlorides, sulfates and bicarbonates. However the major contribution of hardness is
by Ca and Mg while Fe, Al and Mn contributes the hardness at a trace level.

Hardness of water may also be defined as the soap consuming capacity of water in giving the lather when water is
mixed with soap. Higher the consumption of soap higher will be the hardness of water. Soap is sodium/potassium salt
of higher fatty acid and when hard water is mixed then it forms insoluble calcium/ magnesium stearate which is useless
white scum having no detergent value.

2 C17H35COONa + CaCl2 → (C17H35COO)2Ca + 2 NaCl

(White Scum)

Hardness of water may have moderate health benefits, but can pose critical problems in its industrial uses where water
hardness may cause costly breakdowns in boilers, cooling towers and other equipment that handles water. Hard water
buildup scale inside water supply pipes, restricting water flow.
 TYPES OF HARDNESS
Hardness of water is categorized as temporary hardness and permanent hardness depending upon the nature salt is dissolved in
water.

TEMPORARY HARDNESS: Hardness of water is temporary when bicarbonates of Ca/Mg/Fe/Al/Mn etc. are soluble in
water and also known as carbonate hardness. This temporary hardness is simply removed by boiling the hard water. When
water boils the soluble bicarbonate decomposes to give the insoluble precipitate of calcium carbonate or magnesium
hydroxide:

Ca(HCO3)2 → CaCO3↓ + CO2↑ + H2O

Mg(HCO3)2 → Mg(OH)2↓ + 2CO2↑

PERMANENT HARDNESS: When chlorides or sulfates of Ca/Mg/Fe/Al/Mn etc. are soluble in water, hardness is called
permanent hardness and also known as non-carbonate hardness. Permanent hardness of water can be removed by using
following water softening methods:
1. Lime Soda softening method
2. Zeolite Softening method
3. Deionization softening method
 UNITS TO EXPRESS HARDNESS OF WATER

The hardness of water is due to the presence of bicarbonates, chlorides and sulfates of calcium and magnesium
and other ions. The hardness of water is expressed in terms of calcium carbonate, CaCO3. The calcium carbonate
is preferably most stable compound among all the hardness causing ions (Ca/Mg/Fe/Al/Mn etc.). Therefore, to
express the hardness we have to convert the concentration of hardness causing salts, equivalent in terms of
CaCO3. Following formula is being used to covert the concentration any hardness causing salt equivalent to
CaCO3:
Concentation of Hardness causing salt
Equivalent as CaCO3 = x 100(M W of CaCO3)
Molecular weight (M W) of hardness causing salt
UNITS FOR HARDNESS:
1. Milligram Per Liter(Mg/Liter): It is the number of mille gram of hardness causing salts, equivalent in
terms of calcium carbonate, present in one liter of water.
2. Parts Per Million(PPM): It is the number of parts of hardness causing salts, equivalent in terms of calcium
carbonate, present in million (106) parts of water.
3. Degree French(oFr): It is the number of parts of hardness causing salts, equivalent in terms of calcium
carbonate, present in 105 parts of water.
4. Degree Clark(oCl): It is the number of grains of hardness causing salts, equivalent in terms of calcium
carbonate, present in one gallon(70000 grains) of water.
1 Mg/Liter = 1 PPM = 0.1 oFr = 0.07 oCl
 DETERMINATION OF HARDNESS OF WATER
Complexometric EDTA Method
The complexometric method is supposed to be a most accurate method of water hardness determination. In this method
ethylene diamine tetra acetic acid (EDTA) and sodium-1-(1- hydroxyl-2-naphthylazo)-6-nitro-2-naphthol-4-sulphonate
also called Eriochrome Black T (EBT) is used. EDTA a strong chelating agent which forms strong stable complexes
with hardness causing Ca2+ and Mg2+ ions present in the water while EBT acts as indicator and forms the weak
complexes with Ca2+ and Mg2+ ions.

Structure of EDTA

Structure of EBT Indicator

The stable complex of EDTA and weak complexes of EBT with hardness causing Ca2+ and Mg2+ ions present in the
water is only formed when pH of the medium is alkaline. Ammonia buffer is therefore used, in this complexometric
titration method, to maintain the pH of the medium at around 10.

Ca2+ (Mg2+) + EDTA → [Ca2+ (Mg2+) EDTA](Stable Complex)

Ca2+ (Mg2+) + EBT → [Ca2+ (Mg2+) EBT] (Weak Complex)

PRINCIPLE: In complexometric determination of hardness, ammonia buffer is first added to a known quantity
of hard water sample to which EBT indicator is added. The weak complex formation takes place as a result wine
red color is developed.
Ca2+ (Mg2+) + EBT → [Ca2+ (Mg2+)EBT] (Weak Complex)
Blue Color Wine Red Color

On further addition of standard EDTA solution from burette the weak complex of hardness causing Ca2+ and
Mg2+ ions and EBT indicator is first braked and a strong complexes between Ca2+ and Mg2+ ions and EDTA is
formed. At the end point all the weak complexes of EBT are converted in to strong complexes of EDTA and
color of the medium changes to blue the color of free EBT indicator.

[Ca2+ (Mg2+)EBT] (Weak Complex) + EDTA → [Ca2+ (Mg2+)EDTA](Stable Complex) + EBT

Wine Red Color Blue Color
REQUIREMENTS: Hard water sample, Standard EDTA solution, Eriochrome Black T indicator, ammonia
buffer, distilled water, burette, pipette, 250 mL conical flask and measuring flask.
PROCEDURE:
• Fill the burette with standard EDTA solution and take known volume of hard water as V1 ml sample into 250 mL
conical flask.
• Add about 5 mL of ammonia buffer and few drops of EBT indicator. Color of the medium becomes wine red because
of the formation of weak complex between EBT and Ca2+ and Mg2+ ions.
• Start adding standard EDTA solution from the burette drop wise in the conical flask till blue color appears as end
point is attained as result of the formation of strong complex of EDTA with Ca 2+ and Mg2+ ions by breaking the weak
complex of EBT.
• Volume of Standard EDTA is noted as V mL.
• Take three readings and finally volume V2 of Standard EDTA is considered as concordant volume for the calculation
purpose.
OBSERVATION TABLE:
Sr Volume of Hard water Volume of standard EDTA use (ml) Actual Volume of
No. taken (ml) Initial Reading Final Reading EDTA used (ml)

1 V1 0.0 V
2 V1 0.0 V V2

3 V1 0.0 V
CALCULATION:
From observation table we know that hardness of V1 volume of hard water is neutralized with V2 ml of standard EDTA
solution having known normality N2.
We know that, N1 × V1 = N2 × V2
Strength of hardness in hard water, N1 = (N2 × V2) / V1
Hardness of hard water in gm/Liter = {(N2 × V2) / V1} × Equivalent Wt. of CaCO3
Hardness as CaCO3 in ppm = {(N2 × V2) / V1} × 50× 1000 ppm or mg/Liter

RESULT:
Hardness of given hard water sample as CaCO3 is …………….. ppm or mg/Liter.
On the basis of the analysis result of the hardness of water sample, water is generally categorized as:

Water Ratings as per the Concentration as Calcium Carbonate

Hardness (mg/L)
Soft 0 to <75
Medium hard 75 to <150
Hard 150 to <300
Very hard 300 and greater