Common Operations and Wet Chemical Methods in Environmental Laboratories - Fundamentals of Environmental Sampling and Analysis - page 143

Al(H 2 O) 3 6 @ Al(H 2 O) 5 OH 2þ þH þ ). In polluted water, acidity may be caused by

free mineral acid (H 2 SO 4 , HCl) from metallurgical industry, acid mine drainage,
acid rain, and organic acid waste.
To measure acidity, an alkaline solution (NaOH) and two indictors are used. The
first indicator (methyl orange) changes color from red to yellow at approximately pH
3.7 (Eq. 6.3). This corresponds to mineral acidity or methyl orange acidity. If NaOH
is continued to titrate, the second indicator phenolphthalein changes color from
colorless to pink at pH 8.3 (Eq. 6.4). Titration to the phenolphthalein end point of pH
8.3 measures acidity from both mineral acids and weak acids. This total acidity is
also termed phenolphthalein acidity.

- H +

3 S

N N N+ H

3 S

N N N

+ H +

Red Yellow

Methyl Orange

ð6 : 3Þ

HO OH O

- 2H +

C C

O

+ 2H +

CO 2

ð6:4Þ

O

Phenolphthalein

Colorless

Pink

To calculate mineral acidity or total acidity from the titration data, the amount of
NaOH must be converted to the equivalent amount of CaCO 3 (in mg) per liter of
water. Since the molecular weight (MW) of CaCO 3 is 100 g/mol or 110 5 mg/mol,
its equivalent weight is ½MWor 510 4 . This is used as the conversion factor so that
the acidity can be reported as mg CaCO 3 /L:

N V 5 10 4
V S

Acidity ¼ ð6 : 5Þ

where N is the normality of base used in titration, and V s the volume of water sample
(mL), V the volume of base used in titration (mL). For mineral acidity, V¼mL
NaOH needed to the first end point (pH 3.7) and for total acidity, V¼mL NaOH
needed to the second end point (pH 8.3).

2. Alkalinity
Alkalinity is the capacity of water to neutralize an acid (H þ ). In natural water,
alkalinity is due to H 2 CO 3 (CO 2 dissolved in water). Alkalinity from other materials
is insignificant and may be ignored. In polluted waters, minor amounts of NH 3 and
salts of weak acids such as borate, silicate, and phosphate (i.e., the conjugate bases
of HBO 3 ,H 4 SiO 4 ,H 3 PO 4 ), and organic acids will contribute to alkalinity. The