• Dissolved oxygen
Each of these is described by standard method
published by the EPA or ASTM
National Environmental Methods Index
Standard Operating Procedures: Continuous Water
Quality Monitoring Network (TCEQ)
Aquatic Sensor Workgroup
Surface Water Quality Monitoring
• ORP – oxidation/reduction potential
• Turbidity – cloudiness or haziness. Measured with a nephelometer.
• Nitrate (NO3-)
• Ammonium (NH4+) / ammonia (NH3-)
• Chloride (Cl-)
• Rhodamine – for monitoring dyes
• Reference: http://www.ysi.com/parameters.php
A measurement of the average kinetic energy of particles
in a substance.
Thermistor – a semiconductor material that has a change in
resistance in proportion to the change in temperature
Calibration standards are based on a constant room temperature
of 25ºC (77ºF)
• In chemistry, a measurement of the acidity or alkalinity of a solution
which is determined by the number of H+ or OH- ions in solution.
• First introduced by a Danish chemist in 1909 and revised to the modern
version in 1924.
• Origins of ‘pH’: The subscript ‘h’ referenced in first papers. Could have
been ‘power’ or ‘power of hydrogen’ (from German or French).
• Based on a logarithmic scale (10x) and is from the range of 0 to 14 where 7
is defined as neutral
• Streams, creeks, rivers, lakes, groundwater
• Affected by: bedrock and soil composition, organic growth in the water,
dumping of chemicals, acid rain, drainage waste (from mine tailings for
• References: http://www.all-about-ph.com/ ,
● 10x for pH
The pH probe is made up of a few essential parts.
The glass bulb, KCL referencegel, Silver Chloride coated wire,
and the reference junction.
The glass bulb is filled with a stable pH solution, usually pH 7,
that experiences constant binding of Hydrogen ions.
The outside of the bulb is exposed to the water, where the
Hydrogen ions vary.
The difference in Hydrogen ions creates a potential that is read
versus the stable potential of the referenceelectrode.
This signal is sent via the Silver Chloride coated wire to the
instrument, where it is measured as a voltage and then
converted to pH units.
Based on the Nernst Equation which is
Mathematical description of an ideal pH electrode
Behavior. Introduced by the German chemist
Hermann Nernst in 1889.
For a (3) point calibration, buffers 4, 7, 10 used.
One can do a 1, 2, or 3 point calibration.
The figure to right shows the effecton the electrode
signal at various temperatures. The isothermal point
of a pH electrode is defined as the intersection
point of different temperature lines, see the figure.
The isopotential point, or the zero point, is then
point where electrode output is 0.0 mV.
For an ideal pH electrode, this would represent an
isothermal intersectionat the zero point
(0 mV at pH 7) for different temperatures.
• Be sure you always store your pH probe in a moist environment, at the least, to prevent it
from drying out.
• Water-saturated air is the minimum you would need to store your probe in a short-term
situation. Short-term is three weeks or less.
• The best way to store your probe when not in use is submerged in pH 4 buffer or pH storage
solution. This is also recommended for long-term storage, which is longer than three weeks.
• Do develop a routine maintenance program that includes reconditioning the pH probe prior
to the field season.
• Do maintain good calibration records to track your probes performance.
• Never store pH Probe submerged in DI water. It will dilute the internal electrolyte and
shorten the probe’s life.
• Never allow your probe to sit in dry air for extended periods of time. If the probe has been
improperly stored or allowed to dry out, it is very likely the probe will need to be replaced.
• Be sure to prevent probes from being subjected to below freezing temperatures or exposed to
• When cleaning, avoid using a brush or any other material that could scratch the surface of the
glass bulb. Scratches will affect your probe’s stability.
Conductivity is a measure of the ability of water to pass an electrical current.
Conductivity in water is affected by the presence of inorganic dissolved solids
such as chloride, nitrate, sulfate, and phosphate anions (ions that carry a
negative charge) or sodium, magnesium, calcium, iron, and aluminum cations
(ions that carry a positive charge).
Organic compounds like oil, phenol, alcohol, and sugar do not conduct electrical current very well
and therefore have a low conductivity when in water.
Conductivity is also affected by temperature: the warmer the water, the higher the conductivity.
For this reason, conductivity is reported as conductivity at 25 degrees Celsius (25 C).
● Units: milli-Siemens/cm, micro-Siemens/cm
Fig 2: Most conductivity meters have a
two-electrode cell (see illustration)
available in either dip or flow-through
styles. The electrode surface is usually
platinum, titanium, gold-plated nickel, or
● Fig 1: The basic unit of conductance is the
siemen (S), formerly called the mho. Since
cell geometry affects conductivity values,
standardized measurements are
expressed in specific conductivity units
(S/cm) to compensate for variations in
electrode dimensions. Specific
conductivity (C) is simply the product of
measured conductivity (G) and the
electrode cell constant (L/A), where L is
the length of the column of liquid between
the electrodes and A is the area of the
electrodes (see illustration). C = G x (L/A)
Varies slightly from manufacturer, but typically a (1) point
calibration. TCEQ uses 1413 uS.
The procedure is to rinse the probe in DI water, and shake
and wipe excess water. Place the probe into the calibration
solution and make sure to dunk the entire probe into the
Once the reading is settled, set the calibration point
(depends upon meter).
Conductivity solution maybe a potassium chloride,
sodium chloride, or TDS 442 (mix of mixture of sodium
sulfate (40%), sodium bicarbonate (40%) and sodium
chloride (20%). The 442 mixture is designed to mimic the
ions often present in natural fresh water systems.
Each is a measure of salts dissolved in sample.
● Conductivity – how well the water conducts
electrical current which is proportional to the
dissolved ions in a sample
● Salinity - is a special measure of dissolved solids
based upon concentration of ocean salts. Technically,
the measurement of salinity requires comparing a
sample's TDS (Total Dissolved Solids), conductivity,
or other physio-chemical property with that of a
standard sea water.
● Salinity is the usual measure of salts in sea water
and in brackish water derived from mixing of fresh
and sea water.
The amount of oxygen (O2) dissolved in water.
Also referred to sometimes as oxygen saturation.
Cold water can hold higher levels than warm water.
Correction factors for solubility and salinity
Typical units: millilitres O2 per liter (ml/L), millimoles O2 per liter (mmol/L),
milligrams O2 per liter (mg/L) and moles O2 per cubic meter (mol/m3),
Percent saturation - the amount of oxygen in a liter of water relative to the total
amount of oxygen that the water can hold at that temperature
Dissolved Oxygen Values above 100% (Read)
Note: mg/L is ppm
The electrochemical method of measuring DO requires a cathode,
anode, electrolyte solution and a gas permeable membrane.
The material of the membrane is specially selected to permit
oxygen to pass through. Oxygen is consumed by the cathode which
will create a partial pressure across the membrane. Oxygen will
then diffuses into the electrolyte solution.
Thus, a DO meter actually measures the pressure of oxygen in water.
It can be used to measure DO in any medium.
Polarographicor Clark Cell Method
Dr. Clark first discovered the cell to measure oxygen in 1956. This is basically an
amperometric (polarographic) cell that is polarized around 800 mV. This cell, named after
Dr. Clark, is built around the popular Ag/AgCl half-cell and a noble metal such as gold,
platinum or palladium. Reduction of oxygen is achieved between 400 to 1200 mV, hence a
need for a voltage of around 800 mV. This is provided externally by a battery source.
Galvanic Method using to dissimilar metals and an electrolyte, so it requires no warm up
time or external voltage source
Lifetime and intensity optical measurement methods detect dissolved oxygen
based on the well documented principle that dissolved oxygen quenches
both the lifetime and intensity of the luminescence associated with carefully chosen
chemical dyes. When there is no oxygen present, the lifetime and
intensity of the signal are at their maximum. As oxygen is introduced
to the sensing element, both the lifetime and intensity of the luminescence
become shorter. Therefore, the lifetime and intensity of the luminescence
are inversely proportional to the amount of oxygen present. The relationship
between the oxygen pressure outside the sensor and the lifetime or intensity
of the luminescence in the dye layer of the sensing element can be generally
quantified by the Stern-Volmer equation (Io/I = 1 + kqt0 * O2). However, the Stern-
Volmer equation implies an inversely linear relationship which is not strictly
true especially at higher oxygen concentrations; therefore, YSI employs the
use of a 3rd order polynomial to correct for this non-linearity and to obtain
the desired range of dissolved oxygen readings.
Reference: The Dissolved Oxygen Handbook
According to YSI, the sensors should
be calibrated in the following order:
pH mid-range (7.0),
pH low (4.0) or high range (10),
Compare with values at FM 70 and past values.
“Petronila Creek, Segment 2204, is a 44-mile freshwater stream spanning
Kleberg and Nueces counties. Located southwest of Corpus Christi,
PetronilaCreek is part of the Baffin Bay watershed.
In 2000, water quality testing found elevated levels of chloride, sulfate, and
TDS in the creek. High concentrations of these salts are indicators of general
water quality that are not tied to a specific use, but may affect several uses.
Water temperature and conductivity collected in real time at the creek: