Prepare your standard hard water sample by dissolving x gm of CaCO3 in 1000 litre .
Important Note
where x ={Last two digits of your roll no
Example If Roll no is
T20EJICS002 then x = 2
T20EJICS025 then x=25
T20EJICS060 then x= 60
50 ml sample water consumed 12 ml of EDTA.
The sample water is boiled and filtered, 20 ml of
this boiled water sample consumed 10 ml of EDTA
50 ml of standard hard water consumed 25 ml of EDTA.
Answer these question based on above data
What is the theory behind this estimation?
Explain the process in brief?
Calculate the total temporary and permanent hardness of water.
If the amount of EDTA consumed with sample hard water and boiled water is same then what inference can be obtained?
Answer:
Alkalinity is a measure of the ability of a water sample to neutralize acids. It is an aggregate
property that is derived from the sum of the neutralising capabilities of all bases present in a
water sample. Alkalinity is measured by volumetric analysis using a standardized acid titrant.
The endpoint is signalled by a colour change of a pH indicator, such as phenolphthalein or
methyl orange or by using a pH meter. Alkalinity is a general water chemistry parameter and can
be used to predict photosynthetic productivity and the buffering capacity of a lake against acid
deposition. Note that alkalinity is not the same as pH (or if you like pOH). Alkalinity is a
measure of a capacity factor, whereas the pOH (- log [OH-
]) is an intensity factor. The common
ions that contribute to alkalinity in natural waters are hydroxide (OH-
), carbonate (CO3
2-) and
bicarbonate (HCO3
–
, aka hydrogen carbonate). The bicarbonate ion is usually the dominant
anion and the largest contributor to the alkalinity.
Neutralization Reactions
hydroxide OH-
+ H+ Æ H2O {1}
carbonate CO3
2- + 2 H+ Æ H2CO3 {2}
bicarbonate HCO3
–
+ H+ Æ H2CO3 {3}
calcium carbonate CaCO3 + 2 H+ Æ Ca2+ + H2CO3 {4}
In some waters/wastewaters, other species such as ammonia, borates, phosphates and silicates
may contribute to the alkalinity. Alkalinity can be measured as either the ‘phenolphthalein’
alkalinity (neutralization to a pH ~ 8.3) or as the ‘total’ alkalinity (pH ~ 4.2). The
phenolphthalein alkalinity, written as [alk]P when expressed as the number of moles of H+
neutralized per litre, is equal to the sum of the molar concentrations of OH-
and CO3
2- (see
equation below). Since the [OH-
] can be readily determined from the sample pH, the [CO3
2-] can
be calculated from the phenolphthalein alkalinity. If the initial pH of the sample is less than 8.3
to begin with, the phenolphthalein alkalinity is zero and [OH-
] and [CO3
2-] ~ 0. The total
alkalinity, [alk]T, is a measure of all contributing ions (see equation below).
Chemical Definitions of Phenolphthalein and Total Alkalinity
[alk]P = [OH-
] + [CO3
2-] {5}
[alk]T = [HCO3
–
] + [OH-
] + 2[CO3
2-] {6}
Note that the carbonate ion concentration is multiplied by a factor of two since each CO3
2- ion
will neutralize two protons when titrated to pH 4.2. To determine the concentration of the
hydroxide, bicarbonate and carbonate ion, one needs to know the initial pH of the original
sample and either the phenolphthalein alkalinity or the total alkalinity. Although, alkalinity is
measured and used in stoichiometric calculations expressed as mol/L H+
, it is normally reported
as mg/L CaCO3 (i.e., as having the same neutralizing ability as a certain concentration of
CaCO3). For example, a water sample that has a total alkalinity reported as Talk = 45 mg/L
CaCO3 has the same neutralising capacity as a solution which contains 45 mg/L of CaCO3 (i.e.,
9.0 x 10-4 mol H+
/L), even though the sample may not contain any CaCO3 (see calculation
below).