In-Situ TROLL 9500 Operators Manual - Manual (Page 73)

66
Multi-Parameter
Water Quality TROLL ®
0095110 rev. 007 01/09 TROLL 9500 Operator’s Manual
11 pH
WHAT IS pH?
The term pH is derived from “p” meaning power and “H” for the ele-
ment hydrogen and literally means “power of hydrogen.” pH is defined
as the negative logarithm of the hydrogen ion activity (or concentra-
tion in moles/liter):
pH = - log [ H + ] or [H + ] = 10 -pH
Water (H
2
O) dissociates into hydrogen ions (H + ) and hydroxide ions
(OH – –7 moles/L of
hydrogen ions and 1.0 x 10 –7 moles/L of hydroxide ions in pure water.
Thus the water is neutral (pH = 7) because there are equal amounts
of each ion. Addition of a substance with hydrogen or hydroxide ions
will shift the balance and cause the water to become either acidic or
basic.
The pH scale ranges from 0 (most acidic) to 14 (most basic or least
acidic). A change of 1 pH unit corresponds to a tenfold change in
hydrogen ion concentration.
WHY MEASURE pH?
A pH value indicates the amount of hydrogen ion that is present in an
aqueous environment. The hydrogen ion concentration gives an indi-
cation of the acidity of a substance. pH is an important measurement
in natural waters because most chemical and biochemical processes
are pH dependent. The physiological chemistry of most living organ-
isms can tolerate only small changes in pH and still provide the chemi-
cal reactions that sustain life. The solubility of many chemicals is pH
dependent. Thus, pH determines their availability to living organisms.
Natural waters usually have pH values in the range of 4 to 9. Most
natural waters are slightly basic (~ pH 8) because of the presence of
carbonates (CO
3
2– ) and bicarbonates (HCO
3
– ). Extremely fresh water
can even be slightly acidic (~ pH 6), depending on the concentration
of dissolved carbon dioxide (CO
2
). The carbon dioxide combines with
water to form a small amount of carbonic acid (H
2
CO
3
) and this pro-
cess lowers the pH. Nitrogen oxides (NO
x
) and sulfur dioxides (SO
2
)
from automobile exhaust and the burning of coal combine with water
in the atmosphere to form nitric (HNO
3
) and sulfuric acid (H
2
SO
4
).
This falls to the ground as acid rain and accumulates in surface water.
Runoff from mining spoils and the decomposition of plant materials
can also cause acidic surface water.
pH values below 5 in natural waters are generally considered to be
too acidic. Freshwater fish seem to do well in the pH range of 6 to 9.
Acidic drinking water is a concern because of its corrosive character-
istics to plumbing and appliances. pH also affects the ammonia/am-
monium (NH
3
/NH
4
+ ) equilibrium in water. Even a small amount of
ammonia is detrimental to fish while a moderate amount of ammo-
nium is tolerated. At a pH of 6.5 almost all ammonia is in the form of
ammonium. However, as the pH becomes slightly basic, ammonium is
changed into harmful ammonia. The lethal dose of ammonia for trout
is only 0.2 mg/L.
THE pH SENSOR
pH electrodes use a potentiometric method to measure the pH of
a solution. The pH sensor consists of a pH-sensitive glass whose
voltage is proportional to the hydrogen ion concentration. A second
sensor (electrode) serves as a reference, which supplies a constant
stable output. Electrical contact is made with the solution using a
saturated potassium chloride (KCl) solution. The electrode behavior is
described by the Nernst equation:
E
m
= E
o
+ (2.3 RT/nF) log [H + ]
where
E
m
is the potential from the pH electrode,
Fluid pH units
Acid rain < 5
Distilled water 5.6
Most natural waters 8
Safe for freshwater fish 6-9
Properly chlorinated
swimming pool
7.2 - 7.6
Typical pH values