Common Operations and Wet Chemical Methods in Environmental Laboratories - Fundamentals of Environmental Sampling and Analysis - page 144

titration procedure can distinguish two types of alkalinity, that is, phenolphthalein
alkalinity and total alkalinity (methyl orange alkalinity). The phenolphthalein
alkalinity is the acid-neutralizing power of hydroxide and carbonate ions (OH and
CO 2 3 ) present in the water sample, while total alkalinity represents all the bases in it
(OH ,CO 2 3 and HCO 3 ).
The titration procedure for alkalinity is exactly the opposite of the procedure for
acidity. Acid (H 2 SO 4 ) rather than NaOH is used for titration and the two same
indicators are used but in the reverse order. The procedure utilizes the first indicator,
phenolphthalein, to signal the end points (pH 8.3) in titrating OH and CO 2 3 .
Further titration with H 2 SO 4 reaches the end point of the second indicator (methyl
orange) at around pH 4.5, which measures the total alkalinity. Note that the pH for
the color change is not at a fixed value, but rather a range, that is, pH 3.1-4.4 and pH
8.0-9.6 for methyl orange and phenolphthalein, respectively.
Like acidity, alkalinity is also commonly reported as mg CaCO 3 /L, hence
Eq. 6.5 can be used to calculate both phenolphthalein alkalinity and total alkalinity.
For alkalinity, the volume (V) and concentration (N) refer to H 2 SO 4 rather than
NaOH. To interpret the acidity and alkalinity results, one should be aware of their
physical meaning and their difference with commonly measured pH. Although water
with a high acidity (mineral acidity) is corrosive and undesirable, water with a high
alkalinity may not be totally unwanted. In some cases, an adequate level of alkalinity
should be maintained to retain the buffering capacity of the water. Acidity or
alkalinity are related to pH but they differ in that pH is an intensive parameter, which
is independent of the volume of water.

3. Hardness

Water hardness is caused by multivalent metallic cations, primarily the divalent
Ca 2þ and Mg 2þ . Anions and monovalent cations such as Na þ and K þ do not
contribute to hardness. Hard water adversely affects the suitability of water for uses
in domestic and industrial purposes. Hard waters produce scale in hot-water pipes,
heaters and boilers ðCa 2þ þ2HCO 3 ! CaCO 3 ðsÞþCO 2 ðgÞþH 2 OÞ. They also
require considerable amounts of soap to produce foam. If direct measuring
instruments (i.e., AA or ICP) are available, water hardness can be calculated from
the measured concentrations of Ca 2þ and Mg 2þ :

LasCaCO 3 Þ¼ X M 2þ ðmg

50
EW of M 2þ

Hardness ðmg = = LÞ ð6 : 6Þ

where the equivalent weight (EW) of Ca 2þ and Mg 2þ are 20.0 and 12.2, respectively.
In most laboratories, a simple titration procedure is used. In the presence of divalent
cations (M 2þ ), Eriochrome Black T (an indicator) forms a weak complex with M 2þ ,
turning from blue color to wine red.

M 2þ þEriochrome Black TðBlueÞ! M-Eriochrome Black TðRedÞ ð6 : 7Þ

When EDTA (ethlenediaminetetraacetic acid, or its sodium salt Na 2 EDTA) is added
to the water sample, EDTA forms a strong chelate with divalent cations. It disrupts