WATER QUALITY
http://h2o.enr.state.nc.us/
What is Water Quality?
• EPA defn:”Water Quality defines the goals for a waterbody by
designating its uses, setting criteria to protect those uses, and establishing
provisions to protect the water from pollutants.”
• Requirement for compliance with Clean Water Act (1977)
• Two main parts:
(1) designated uses of the water body (e.g., recreation, water supply,
aquatic life, agriculture)
(2) water quality criteria to protect designated uses (numeric pollutant
concentrations and narrative requirements)
• Website with 50 States WQ Standards:
http://www.epa.gov/waterscience/standards/states/
How Measure Water Quality?
•
Did you ever stop to wonder how we get our information on the condition of our Nation's
streams, lakes, estuaries, and coastal waters? On whether these waters are safe enough to
swim in, fish from, or use for drinking or irrigation purposes? Monitoring provides this basic
information.
There are many ways to monitor water conditions. To monitor the constituents in water, sediments, and fish
tissue -- such as levels of dissolved oxygen, suspended sediments, nutrients, metals, oils, and
pesticides -- monitoring specialists perform chemical measurements.
Physical measurements of general conditions such as temperature, flow, water color, and the condition of
stream banks and lake shores are also important.
Biological measurements of the abundance and variety of aquatic plant and animal life and the ability of
test organisms to survive in sample water are also widely used to monitor water conditions.
INDICATORS
•
Monitoring can be conducted for many purposes. Five major purposes
are to:
1. characterize waters and identify changes or trends in water quality over time;
2. identify specific existing or emerging water quality problems;
3. gather information to design specific pollution prevention or remediation programs
4. determine whether program goals -- such as compliance with pollution regulations or implementation of
effective pollution control actions -- are being met; and
5. respond to emergencies, such as spills and floods.
Desirable Properties of an
Indicator
• EPA definition:” An indicator is a sign or signal that relays a complex
message, potentially from numerous sources, in a simplified and useful manner.
An ecological indicator is defined here as a measure, an index of measures, or a
model that characterizes an ecosystem or one of its critical components.”
http://www.epa.gov/bioindicators/
• Based on scientific principles related to ecosystem response to
stressors
• Capable of indicating presence of stressors prior to significant loss
of habitat
• Diagnostic of identity and magnitude of stressors
• Useful for making management decisions
• Useful as one of a suite of indicators
Monthly vs continuous
Upper
monitoring
Middle
Lower
Temperature & pH
Lecture 1
Temperature Scales
• Fahrenheit: 32 – 212 F (180 units)
• Celsius: 0 – 100 C (100 units)
• Kelvin: 273.15 – 373.15 K, where absolute
zero = -273.15 C. (SI unit)
• http://en.wikipedia.org/wiki/Fahrenheit
• http://en.wikipedia.org/wiki/Celsius
• http://en.wikipedia.org/wiki/Kelvin
1 F = 5/9 C
1 C = 9/5 F
Thermometer
• http://en.wikipedia.org/wiki/Thermometer
• thermo (Gk) = heat, meter (Gk) to msr
• Secondary thermometers – are calibrated
at a number of fixed temperatures (e.g.
NIST traceable).
• Mercury-, alcohol-filled, digital, Infra-red.
• Expansion of “liquid”: mercury (Hg).
Health issues – clean up requires special caution,
Disposal concerns, Hg becoming increasingly prevalent in
environment.
http://www.epa.gov/region1/eco/mercury/spillstherm.html
• Bi-metal strip:
• Thermistor: a type of resistor used to measure
temperature changes,
ΔR = kΔT, where
ΔR = change in resistance
ΔT = change in temperature
k = first-order temperature coefficient of resistance
http://en.wikipedia.org/wiki/Thermistor
Digital IR thermometer
• An infrared thermometer is a
non-contact temperature
measurement device. Infrared
Thermometers detect the
infrared energy emitted by all
materials -- at temperatures
above absolute zero,
(0°Kelvin)-- and converts the
energy factor into a
temperature reading.
pH
H+ ion concentration
pH
pH is defined as minus the decimal logarithm of the hydrogen ion activity in an
aqueous solution. By virtue of its logarithmic nature, pH is a dimensionless quantity.
pH is a measure of the acidity or basicity of a solution. It is defined as
the cologarithm of the activity of dissolved hydrogen ions (H+). Hydrogen ion activity
coefficients cannot be measured experimentally, so they are based on theoretical
calculations. The pH scale is not an absolute scale; it is relative to a set of standard
solutions whose pH is established by international agreement.
Pure water is said to be neutral. The pH for pure water at 25 °C (77 °F) is
close to 7.0. Solutions with a pH less than 7 are said to be acidic and
solutions with a pH greater than 7 are said to be basic or alkaline.
In natural waters pH will vary between 4-10, with marine waters generally between
7.5-9. Because marine waters are well “buffered”, you will calibrate using pH 7 and
pH 10 standards.
pH is commonly measured by means of a combined glass electrode, which measures
the potential difference, or electromotive force, E, between an electrode sensitive to
the hydrogen ion activity and a reference electrode, such as a calomel electrode or
a silver chloride electrode.
Each pH step,
there is 10x change
in H+ concentration.
Therefore entire
scale there is a 1014
change (100 trillion)
Marine
waters
http://en.wikipedia.org/wiki/PH
Why is the pH well buffered?
Dissolved Carbon Dioxide
Dissolved carbon dioxide in seawater occurs mainly in three
inorganic forms (see Figure next slide): free aqueous carbon
dioxide (CO2(aq)), bicarbonate (HCO3-), and carbonate ions
(CO32-).
A minor form is true carbonic acid (H2CO3) whose
concentration is less than 0.3% of [CO2(aq)]. The sum of
[CO2(aq)] and [H2CO3] is denoted as [CO2]. The majority of
dissolved inorganic carbon in the ocean is in the form of
HCO3- (>85%).
At typical surface seawater pH of 8.2, the speciation between
[CO2], [HCO3-], and [CO32-] hence is 0.5%, 89%, and
10.5%, showing that most of the dissolved CO2 is in the
form of HCO3http://www.eoearth.org/article/Marine_carbonate_chemistry
bicarbonate
H2CO3
This world map of ocean acidity shows that ocean pH varies from about 7.90 to 8.20
but along the coast one may find much larger variations from 7.3 inside deep estuaries
to 8.6 in productive coastal plankton blooms and 9.5 in tide pools. The map shows that
pH is lowest in the most productive regions where upwellings occur. It is thought that
the average acidity of the oceans decreased from 8.25 to 8.14 since the advent of
fossil fuel (Jacobson M Z, 2005).
http://www.seafriends.org.nz/oceano/seawater.htm
Anthropogenic CO2
Since the beginning of the industrialization, the oceans
have taken up about 50% of the anthropogenic CO2
produced by fossil fuel burning and cementmanufacturing (cf. greenhouse gases). The carbon
dioxide dissolves in seawater, produces hydrogen ions
and neutralizes carbonate ions (CO32-):
CO2 + H2O + CO32- → 2 HCO3-
Buffering by the carbonate – bicarbonate system
maintains seawater in near equilibrium wrt pH at this time.
pH electrodes
• The most common type of pH electrodes are the
"glass" electrodes. They consist of a special
glass membrane that is sensitive to variations in
pH, as pH variation also changes the electrical
potential across the glass.
• In order to be able to measure this potential, a
second electrode, the "reference" electrode, is
required. Both electrodes can be present in a
"combined" pH electrode, or two physicallyseparate electrodes can be used.
pH electrode function
•
•
•
•
2 electrodes – glass and reference
The glass electrode consists of a glass shaft on
which a bulb of a special glass is mounted. The
inner is usually filled with 3 Mol/Litre aqueous
KCl and sealed. Electrical contact is provided
by a silver wire immersed in the KCl.
The reference electrode, which traditionally
used silver chloride (AgCl) has been
superseded by the kalomel (mercurous
chloride, HgCl2) electrode which uses mercuric
chloride (HgCl) in a potassium chloride (KCl)
solution
A pH meter measures essentially the electrochemical potential between a known liquid
inside the glass electrode (membrane) and an
unknown liquid outside. Because the thin glass
bulb allows mainly the agile and small
hydrogen ions to interact with the glass, the
glass electrode measures the electro-chemical
potential of hydrogen ions or the potential of
hydrogen. To complete the electrical circuit,
also a reference electrode is needed. Note that
the instrument does not measure a current but
only an electrical voltage.
glass
reference
electrodes
Simple test of pH probe
• The voltage produced by one pH unit (say from pH=7.00
to 8.00) is typically about 60 mV (milli Volt)
• In contact with different pH solutions a typical glass
electrode gives, when compared to the reference
electrode, a voltage of about 0 mV at pH 7, increasing
by 60 mV per pH unit above 7, or decreasing by 60 mV
per pH unit below 7. Both the slope, and the intercept of
the curve between pH and generated potential, are
temperature dependent.
• Test of pH probe accuracy is to see if mV reading differs
approx by 180mV between pH 7 and pH 4 or 10
calibration solutions (3 x 60mV steps).
Calibration
• Insert pH probe in pH 7 calibration solution, should read pH 7
and 0mV
• Rinse with next solution, typically pH 10, and calibrate to 10,
mV reading should be about 180mV
• People assume pH measurements are accurate, however many
potential errors exist. There can be errors caused by the pHsensitive glass, reference electrode, electrical components, as
well as externally generated errors, see:
http://stason.org/TULARC/science-engineering/chemistry/26-2How-do-pH-electrodes-work.html
• A pH meter should not be used in moving liquids of low
conductivity (thus measuring inside small containers is
preferable).
• Remember also that the calibration solutions consist of
chemical buffers that 'try' to keep pH levels constant, so
contamination of your test vial with a buffer is really serious
Calibration Solutions
• Today, most pH meters are minicomputers that measure
the voltage and translate them into pH values at the
correct temperature under the most adverse conditions.
These meters with glass electrodes are now calibrated
with buffer solutions (resistant to changes in pH) that can
be traced to standards at the NIST.
• Typical standard buffers include:
• potassium hydrogen phthalate (pH = 4.01)
• disodium hydrogen phosphate + potassium dihydrogen
phosphate (pH = 7.00)
• borax (pH = 10.01)
• The pH values are at 25°C and accurate to ±0.002.
http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=pHTheory.htm&ID=560
Calibration & Care
•
Frequent calibration and adjustment of pH meters are necessary. To check the pH
meter, at least two standard buffer solutions are used to cover the range of
interest. The pH meter should be on for at least 30 minutes prior to calibration to
ensure that all components are at thermal equilibrium, and calibration solutions
should be immersed for at least a minute to ensure equilibrium.
•
First use the buffer at pH 7, and adjust the zero (or the intercept).
Then, after thorough rinsing with water, use the other buffer to adjust the slope.
This cycle in repeated at least once, or until no further adjustments are necessary.
Many modern pH meters have an automatic calibration feature,which requires
each buffer only once.
•
But electrodes do not have eternal life (usually 2 years) and need to be replaced
when they drift unacceptably or take unusually long to settle.
•
Caring for a pH meter depends on the types of electrode in use. Study the
manufacturer's recommendations. When used frequently, it is better to keep the
electrode moist. However for prolonged periods, it is recommended to moist it with
a solution of potassium chloride at pH=4 or in the pH=4.01 acidic calibration buffer.
pH meters do not like to be left in distilled water.
Lab
• Check temperature on 2 Sondes in same
bucket.
• Read temperature on a number of
thermometers in same bucket, compare
results – what is the error between
“instruments”
• Calibrate a pH meter, measure pH of 4
water samples (0, 10, 20, 30 ppt salinity)