Q. Discuss the EDTA method of determining hardness of water.
What is hardness of Water? How it is determined by EDTA method? Enumerate units of hardness.
Give the chemical reactions involved in determination of hardness of water by EDTA titration.
Hardness of water can be determined by EDTA (complex metric) method:
Ans. EDTA (ethylene diamine tetra acetic acid) is an extraordinary strong complexion agent. As it is not very soluble in water and hence EDTA in the form of its disodium salt is normally used in all comlexometric titrations. This acts as a tetra/hexes dentate legend and attaches itself with metal ions (through O or N atoms) to give very stable, soluble and colourless complexes. Since hardness of water is due to Ca+2 and Mg+2 mainly, their presence can be conventionally determined by titrating a water sample against standard solution of Na2H2Y using a suitable ion indicator to note the point. When hard water is titrated against EDTA then EDTA or its sodium salts form stable complex ions with calcium (Ca+2) magnesium (Mg+2) ions in water.
Ca+2 + EDTA = [Ca+2 EDTA] complex
Mg+2 + EDTA = Mg+2 EDTA] complex
The titration is carried out in presence of indicator, Erichrome black-T. The indicator when added in small amount of hard water, buffered to a pH value of about 10, combines with a few of Ca+2, Mg+2 of ions to form a weak complex of wine-red colour which changes to blue when an excess drop of EDTA is added.
Ca+2 + Erichrome black-T = [Ca+2 Erichrome black-T]
Mg+2 + Erichrome black-T = [Mg+2 Erichrome black-T]
(Unstable complex) (Wine red)
= [Ca+2 + EDTA] complex + Erichrome black-T
[Mg+2 + EDTA] complex + Erichrome black-T
(Stable) (Blue) (Free indicator)
Thus, change of wine red colour to distinct blue, marks the end point of titrations. Various steps of involved in this method are:
1. Standardization of EDTA solution.
2. Determination of total hardness of water.
3. Determination of permanent hardness of water.
50 ml of standard hard water = V1 ml of EDTA
50* 1 mg of CaCO3 = V1 ml of EDTA
1 ml of EDTA = 50/V1 mg of CaCO3 eq.
Now 50 ml of given hard water = V2 mg of EDTA
V2*50/V1 mg of CaCO3 Eq.
1 L of given hard water = 1000V2 mg of CaCO3 eq. /V1
Total hardness = 1000V2 mg/l or ppm. /V1
Now 50 ml of boiled water = V3 ml of EDTA = V3*50 mg of CaCO3 eq. /V1
1000 ml of boiled water = 1000V3 mg of CaCO3 eq. /V1
Permanent hardness = 1000V3ppm/V1
Temporary hardness = (Total-Permanent) Hardness
Temporary hardness = 1000(V2-V3) ppm/V1
Units of hardness and their inter-relations
(1) Parts per million (ppm):
It is defined as the number of parts by weight of CaCO3 equivalent present in per million (106) parts by weight of water.
1 ppm = 1 part of CaCO3 equivalent hardness in (106) parts of water.
2. Milligrams per litre (mg/L):
It is defined as the number of milligrams of CaCO3 present in one equivalent hardness litre of water.
1mg/L = 1 mg of CaCO3 equivalent hardness present per little of water.
It can be easily proved that 1 mg/L = 1 ppm, for water
Weight of 1 litre of water = 1 Kg = 103 g = 106 mg =1000 * 1000g = 106 mg
1 mg/L = 1 mg of CaCO3 eq. Hardness per 106 mg
1 mg/L = 1 part of CaCO3 eq. Hardness per 106 parts of water
1 mg/L = 1 ppm
Degree Clarke (0Cl):
It is defined as the parts of CaCO3 equivalent hardness pert 70,000 part of water or it is number of grains (1/7000lb) of CaCO3 equivalent hardness per gallon (10 lb or 70,000 grains) of water.
10Cl = 1 part of CaCO3equivalent Hardness per 70000 parts of water.
Degree of French (0Fr):
It is defined as the parts of CaCO3 equivalent hardness per lace (105) parts of water.
1 0Fr = 1 part of CaCO3 eq. Hardness per 105 parts of water.
Mille equivalent per Litre (meq/L):
It is defined as the number of milli equivalents of hardness present per litre.
1meq. /L = 1meq. Of CaCO3 per L of water
= 10-3* 50 mg of CaCO3 eq. per litre of water
= 10-3*1000* 50 mg of CaCO3 eq. per litre of water
= 50 mg of CaCO3 eq. per litre of water
= 50 mg/L of CaCO3 eq. = 50 ppm
= 50 mg of CaCO3 eq. Hardness per 106 parts of water
= 1 mg of CaCO3 eq. Hardness per 106/50 mg of water
= 1 part of CaCO3 eq. Hardness per 20,000 parts of water