In-Situ TROLL 9500 Operators Manual - Manual (Page 104)

97
Multi-Parameter
Water Quality TROLL ®
0095110 rev. 007 01/09 TROLL 9500 Operator’s Manual
14 OXIDATION-REDUCTION POTENTIAL
Fluid ORP (mV)
Salt water aquarium ~ 350
(ARMFUL
Properly chlorinated
Typical ORP values
WHAT IS ORP?
Oxidation Reduction Potential (ORP) is a measure of a water system’s
capacity to either release or gain electrons in chemical reactions.
The process of oxidation involves losing electrons while reduction
involves gaining electrons. Oxidation and reduction (redox) reactions
control the behavior of many chemical constituents in drinking water,
wastewater, and aquatic environments. The reactivity and solubility
of critical elements in living systems is strongly dependent on redox
conditions. ORP values are used much like pH values to determine
water quality. While pH values characterize the relative state of a
system for receiving or donating hydrogen ions (acting as a base or
an acid), ORP values characterize the relative state of a system for
gaining or losing electrons. ORP values are affected by all oxidizing
and reducing agents, not just acids and bases.
WHY MEASURE ORP?
The effect that potable water has on plumbing is directly related to its
ORP value. Unfavorable values can cause excessive corrosion, lead-
ing to expensive repairs. ORP is one parameter that can be monitored
during the disinfecting process for drinking water, swimming pool
water, and spa water.
The life expectancy of bacteria in water is related to ORP. In fact,
studies have shown that the life span of bacteria in water is more
dependent on the ORP value than on the chlorine concentration. For
swimming pools at a normal pH value between 7.2 and 7.6, the ORP
value must be kept above 700 mV to kill unwanted organisms. Hypo-
chlorite or other oxidizing agents must be added when the ORP falls
below 700 mV. In contrast, natural waters need a much lower ORP
value in order to support life. Generally ORP values above 400 mV
are harmful to aquatic life. Ideally the ORP value in salt water aquari-
ums should be kept between 350 and 390 mV. ORP levels below 300
mV are to be avoided. An oxidizing environment is needed to convert
any ammonia (NH
3
) to nitrites (NO
2
– ) and nitrates (NO
3
– ). Ammonia
levels as low as 0.002 mg/l can be harmful to some fish species.
The determination of ORP is particularly worthwhile in water that
contains a relatively high concentration of a redox-active species, e.g.,
the salts of many metals (Fe
2
+ , Fe
3
+ ) and strong oxidizing (chlorine)
and reducing (sulfite ion) agents. Thus, ORP can sometimes be
utilized to track the metallic pollution of ground- or surface water, or
TO
THE pH/ORP SENSOR
The single-junction, three-electrode sensor uses a potentiometric
method to measure pH and ORP in a test solution. The pH electrode
consists of a pH-sensitive glass whose voltage is proportional to the
hydrogen ion concentration. The ORP electrode serves as an electron
donor or acceptor depending upon the test solution. The ref er ence
electrode supplies a constant stable output for comparison. Electrical
contact is made with the test solution using a saturated potassium
chloride (KCl) solution. The electrode behavior is described by the
Nernst equation:
E
m
= E
o
- (RT/nF) ln {[ox] / [red]}
where
E
m
is the potential from the ORP electrode,
E
o
is related to the potential of the reference electrode,
R is the Gas Law constant,