WATER TECHNOLOGY---ALKALINITY---- CY2111 183101 –ENGINEERING CHEMISTRY –I Lecture Notes

ALKALINITY

Definition:

The acid neutralizing capacity is known as alkalinity. It is due to OH^-_, CO₃^2-, HCO₃^-.

1.Aim: 

To determine the type and amount of alkalinity of the water sample

2.Chemicals required: 

NaOH, HCl , Sample water, Phenolphthalein, Methyl orange

3.Principle:

There are five types of alkalinity.

i) OH^- only    ii) CO₃^2- only    iii) HCO₃^-.only  iv) OH^- and  CO₃^2-    v) CO₃^2-and HCO₃^-.

The other combinations are not possible.  Because,

            OH^- + HCO₃^-.   à   H₂O  +  CO₃^2-, it again form carbonate.

The possible reactions are

1.            OH^- + H⁺   à   H₂O

2.           CO₃^2-  + H⁺   à HCO₃^-    ( Half neutralization of carbonate)

3.           HCO₃^- + H⁺   à   H₂O + CO₂

Phenolphtalein can be used as indicator for 1 & 2.  But not for 3.

Methyl orange is used for all 1 , 2 & 3 reactions.

4. Formula table:

S.No	Case	OH-	CO32--.	HCO3-.
1	P=0	Nil	Nil	M
2	P=M	P=M	Nil	Nil
3	P = ½ M	Nil	2P	Nil
4	P > ½ M	2P – M	2(M – P)	Nil
5	P < ½ M	Nil	2P	(M – 2P)

5. Short Procedure

S.No	Content	Titration-I Standardisation of HCl	Titration-II Alkalinity estimation
1	Burette	HCl acid	Standard HCl
2	Pipette(20 ml)	Standard NaOH	Water sample
3	Indicator	Phenolphtahlein	i)Phenolphthalein ii) Methyl orange
4	Endpoint	Disappearance of pink colour	i)Disappearance of pink colour ii) Appearance of pink colour

6. Calculation:

Step-1  :   Standardisation of HCl

Volume of NaOH (V₁)           = 20ml

Strength of NaOH  (N₁)        =0.1N

Volume of HCl (V₂)               = x ml

Strength of HCl (N₂)             =  ?

 As per volumetric principle, V₁N₁  = V₂N₂

                                                      N₂  = V₁N₁  / V₂

Step -2  :  Estimation of alkalinity:

(let us consider P=8 and M = 10, as P>1/2 M , this comes under case (iv). We are having OH^- and CO₃^2- alkalinity)

a)Amount of OH^-

Volume of HCl (V₁)               = (2P – M ) ml

Strength of HCl (N₁)             =  From step 1

Volume of water (V₂)            = 20 ml

Strength of OH^- in water (N₂)=  ?

As per volumetric principle, V₁N₁  = V₂N₂

                                                     N₂  = V₁N₁  / V₂

Amount of OH^-  in water      =  N₂  X  50  X  1000  ppm

( we are multiplying by 50 as it is eq.wt of CaCO_3.

   We are multiplying by 1000 to convert it for gram to milligram)

b)Amount of CO₃^2-

Volume of HCl (V₁)               = 2(M-P ) ml

Strength of HCl (N₁)             =  From step 1

Volume of water (V₂)            = 20 ml

Strength of CO₃^2- in water (N₂)=  ?

As per volumetric principle, V₁N₁  = V₂N₂

                                                      N₂  = V₁N₁  / V₂

Amount of CO₃^2-in water     =  N₂  X  50  X  1000  ppm

( we are multiplying by 50 as it is eq.wt of CaCO_3.

   We are multiplying by 1000 to convert it for gram to milligram)

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