Question

25 ml water sample required 11.5 ml of 0.05m EDTA
for end point, whereas 25 ml
boiled water
sample required 8.2 ml of
EDTA
son
end point calculate total,
permanent and temprory hardness of waters.
​

Answer 1

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).