calculate TDS concentrations from that empirically determined coefficient that the
user inputs into the instrument.
3.2 Chemical Sensors
Chemical sensors are used to measure chemical properties of water such as pH,
dissolved oxygen, and some pollutants such as nitrate, ammonium, and chloride.
3.2.1 pH Sensors
By definition, pH is a measure of hydrogen ion concentration and indicates level of
acidity or alkalinity in water. pH is measured on a log scale from 0 to 14; each
whole pH unit represents a tenfold change in the concentration of hydrogen (H
+
)
and hydroxide (OH
À ) ions. The pH of water has a direct impact on living resources
and can affect the toxicity and solubility of chemicals, heavy metals, and other
pollutants such as phosphorous and other nutrients [10]. In freshwater lakes,
changes in pH can increase nutrient solubility and plant uptake, resulting in
increased plant growth and ultimately eutrophic conditions. Fluctuations in pH
levels are often caused by anthropogenic sources of pollution, such as the combustion of fossil fuels, smelting and mining operations, agricultural runoff, and wastewater and industrial discharge. In many fish species, reproduction is impacted at pH
levels below 5.0, and death often occurs when levels drop below 4.0, while gill and
skin damage can occur at higher pH levels [11].
Historically, pH sensors have consisted of a glass sensing bulb filled with a
stable pH solution (usually 7) that experiences constant binding of H
+ ions, a
reference electrode, and a potentiometer. When the sensor is placed into the
water where the H
+ ions vary, the differential of H
+ ions creates an electrical
potential (mV) which is compared to the stable potential of the reference electrode.
The electrical potential is related to pH values through a form of the Nernst
equation, a formula that describes the potential of the electrochemical cell as a
function of the concentrations of the ions taking part in the reaction. Through the
continued miniaturization of electronics and microprocessors, the signal processing
electronics in today’s sensors have been placed in very close proximity to the
sensing element, resulting in decreased interference and increased sensitivity,
accuracy, and stability.
3.2.2 Dissolved Oxygen Sensors
Dissolved oxygen is one of the most critical water quality parameters. Natural water
systems produce and consume oxygen, and therefore, dissolved oxygen levels
exhibit diurnal variability as well as seasonal variability. Atmospheric exchange
and plant photosynthesis add oxygen to the system while respiration, decomposition,
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T. Younos and C.J. Heyer
user inputs into the instrument.
3.2 Chemical Sensors
Chemical sensors are used to measure chemical properties of water such as pH,
dissolved oxygen, and some pollutants such as nitrate, ammonium, and chloride.
3.2.1 pH Sensors
By definition, pH is a measure of hydrogen ion concentration and indicates level of
acidity or alkalinity in water. pH is measured on a log scale from 0 to 14; each
whole pH unit represents a tenfold change in the concentration of hydrogen (H
+
)
and hydroxide (OH
À ) ions. The pH of water has a direct impact on living resources
and can affect the toxicity and solubility of chemicals, heavy metals, and other
pollutants such as phosphorous and other nutrients [10]. In freshwater lakes,
changes in pH can increase nutrient solubility and plant uptake, resulting in
increased plant growth and ultimately eutrophic conditions. Fluctuations in pH
levels are often caused by anthropogenic sources of pollution, such as the combustion of fossil fuels, smelting and mining operations, agricultural runoff, and wastewater and industrial discharge. In many fish species, reproduction is impacted at pH
levels below 5.0, and death often occurs when levels drop below 4.0, while gill and
skin damage can occur at higher pH levels [11].
Historically, pH sensors have consisted of a glass sensing bulb filled with a
stable pH solution (usually 7) that experiences constant binding of H
+ ions, a
reference electrode, and a potentiometer. When the sensor is placed into the
water where the H
+ ions vary, the differential of H
+ ions creates an electrical
potential (mV) which is compared to the stable potential of the reference electrode.
The electrical potential is related to pH values through a form of the Nernst
equation, a formula that describes the potential of the electrochemical cell as a
function of the concentrations of the ions taking part in the reaction. Through the
continued miniaturization of electronics and microprocessors, the signal processing
electronics in today’s sensors have been placed in very close proximity to the
sensing element, resulting in decreased interference and increased sensitivity,
accuracy, and stability.
3.2.2 Dissolved Oxygen Sensors
Dissolved oxygen is one of the most critical water quality parameters. Natural water
systems produce and consume oxygen, and therefore, dissolved oxygen levels
exhibit diurnal variability as well as seasonal variability. Atmospheric exchange
and plant photosynthesis add oxygen to the system while respiration, decomposition,
186
T. Younos and C.J. Heyer