ALKALINITY
Definition:
The acid neutralizing capacity is known as alkalinity. It is due to OH-, CO32-, 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) CO32- only    iii) HCO3-.only  iv) OH- and  CO32-    v) CO32-and HCO3-.
The other combinations are not possible.  Because,
            OH- + HCO3-.   à   H2O  +  CO32-, it again form carbonate.
The possible reactions are
1.            OH- + H+   à   H2O
2.           CO32-  + 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 CO32- 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 CaCO3.
   We are multiplying by 1000 to convert it for gram to milligram)
b)Amount of CO32-
Volume of HCl (V1)               = 2(M-P ) ml
Strength of HCl (N1)             =  From step 1
Volume of water (V2)            = 20 ml
Strength of CO32- in water (N2)=  ?
As per volumetric principle, V1N1  = V2N2
                                                      N2  = V1N1  / V2
Amount of CO32-in water     =  N2  X  50  X  1000  ppm
( we are multiplying by 50 as it is eq.wt of CaCO3.
   We are multiplying by 1000 to convert it for gram to milligram)

OTHER TOPICS