Next: Lab-3 Field Procedure
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Each group will take a sample of water and analyze it for a number of
parameters using several meters or probes, as explained below. Some
of the meters must be calibrated before use.
Lab-3 Parameters to be Measured
- measured using pH
meter or dissolved oxygen
meter, or thermometer
- ``acidity'' of the water, measured using pH
meter, or pH paper (yields low
- the clarity of the water, measured using
portable turbidimeter (Hach 2100P)
- we will use swimming pool test strips to
measure hardness, Cl, pH, and alkalinity
- Dissolved Oxygen content. Measured using a
hand-held dissolved oxygen meter
- see ``Other Parameters''. Measured using the
Record both the first reading (units are ``'') which is
the raw electrical conductivity and the second, which is the
temperature compensated conductivity or specific conductance
(temperature flashes when meter is in this mode).
- calculated by the D.O. meter based on temperature
and conductivity measurements. Or use Pocket TDS meter.
Portable turbidity meter (see
Cover opens for insertion of sample container.
In general electronic instruments are used to measure chemical
properties of water. As such, they are indirect methods of
measurement, and therefore must be calibrated. This is
essentially the same as synchronizing watches; each watch may tell
time differences accurately, but the true time requires
resetting or calibrating each watch to an agreed-upon standard. To
calibrate chemical probes/instruments, two or more samples of known
concentration are measured, and then a calibration factor is entered
into the device (or applied to the final result) to adjust the output
to agree with standard values. To save time, we will calibrate only
the pH meter while out in the field.
Using the Turbidimeter
The turbidimeter measures the light transmittance of a sample in NTU's
(Nephelometric Turbidity Units, a standard measure). It needs no
field calibration. Handle the sample vials only by their ends
(preferably the lid) so as not to affect the transmittance; wipe any
fingerprints, spots, etc. from the outside of the vial; and be sure to
close the vial-compartment lid when taking a measurement. The
turbidimeter should display ``AUTO RNG'' (for auto-range selection)
and ``SIG AVG'' (for take an average reading) when ready for use
Turbidimeter controls and
display. Press ``READ'' when ready to take a measurement. After Hach
Model 2100P Turbidimeter Instruction Manual, 1997, p. 24.
Use the following procedure when measuring the turbidity of your
- Fill turbidity vial (has white line around top of glass with
downward arrow) to the line (about 15 mL) with
unfiltered water. Take care to handle the sample cell by
the top. Cap the cell.
- Wipe the cell with a soft, lint-free cloth to remove water spots and
- Press I/O - the instrument will turn on. Place the instrument on a
flat surface. Do not hold the instrument while making readings.
- Put the sample vial in the instrument cell compartment so its
diamond mark aligns with the raised orientation mark in
front of the cell compartment. Close the cover.
- Set automatic range by pressing the RANGE key. The display will
show ``AUTO RNG''.
- Select signal averaging (reports average of 10 measurements)
by pressing ``Signal Average'' key, display should show ``SIG AVG''
- Press READ - the display will show ``--NTU'' and the light
bulb icon will flash 10 times (once for each reading). The final
average will be displayed as numbers in NTU after the lamp symbol
Using the pH Meter
This meter is used to measure the acidity of the water by comparing
readings from a reference electrode and a sample electrode. To
determine pH the output of these electrodes must be
temperature-compensated, so most pH meters also measure temperature.
On the Hach EC10 meter (Fig. 4.8), pH is displayed in the
center right of the LCD screen, temperature (
displayed in the lower center. pH meters generally require frequent
calibration in the field, and calibration is part of this lab.
ph Meter display and controls.
The procedure for pH meter calibration is:
- Be sure some electrolyte is present at the tip of the probe
(ask a TA to check, or blow off any liquid from the probe tip and
depress dispenser button). If needed, dispense some electrolyte
by depressing the dispenser button on the top of the electrode
until a click is heard. Natural water pH measurements outside the
range of 7.0-8.9 probably mean electrolyte wasn't present, and
are invalid readings.
- Turn on device (press I/O) key
- Be sure meter is in pH mode (triangle at base of display is
above the ``pH'' label''). If not, press MODE key until mode is correct
- Press ``CAL'' button to enter calibrate mode, display will flash
- Rinse electrode and place into the 4.0 buffer.
- Press CAL key. ``CALIBRATE'' will be displayed above the
main field. After the calibration slope is displayed, P1 will be
displayed in the lower field. P1 indicates that the meter is ready to
accept the first buffer point.
- When the electrode is stable, the meter will beep and
READY will be displayed along with the temperature corrected
value for the buffer (flashing). Press YES key to accept this point.
- The display will remain fixed momentarily, then P2 will be
displayed in the lower field. The meter is now ready for the second
- Rinse the electrode and place it in the second buffer (7.0). Wait
for a stable pH reading (the meter will beep) and for READY to be
displayed. Press the YES key to accept the second point.
- The meter will automatically enter measure mode and MEASURE
appears above the main display.
- The meter is now ready to take pH measurements. The EC10
retains this calibration until a new calibration is entered or until
power is disconnected from the instrument.
The procedure for making a pH-temperature
- rinse the electrode tip in deionized water
- depress the dispenser button on the top of the electrode until
a click is heard (releases reference electrolyte at tip of
- wait until the readings become steady (``READY'' indicator
shows, and meter beeps once)
- record results (including temperature)
- if readings become erratic, dispense more electrolyte
Using the Spectrophotometer
The spectrophotometer uses indicator compounds to determine the
concentration of the desired species. In this case we'll be measuring
nitrate concentration, one of the principal contaminants in urban
surface waters. Use of the Hach Test kit involves mixing a
pre-measured amount (a ``pillow'') of the indicator compound with a
measured amount of sample water, then measuring the absorption of
light of a particular wavelength. After mixing and a delay for
reaction, a color will appear (for nitrogen it is amber color), the
darkness of which is measured in the spectrophotometer. The sample is
introduced to the spectrophotometer via a
pour-through-cell apparatus (Fig. 4.9), to minimize
optical interferences from sample handling.
Pour-through cell arrangement for spectrophotometer
measurements. After Hach DR/2010 Spectrophotometer Instrument Manual,
p. 10, 1997.
The procedure for making a nitrate analysis via spectrophotometry is:
- Obtain at least 50 ml of filtered sample
- set up a filter and funnel in the funnel holder (fold the
round filter paper in half, then half again, place in funnel and
spread to make open cone)
- place clear 250 ml sample jar (these have light blue lids)
under funnel to catch filtrate
- pour sample into filter until water level is close to the top
- wait for sample to gravity drain into clear jar
- repeat, after second filter-full you should have enough
sample to analyze for nitrate
- Prepare spectrophotometer
- make sure drain hose from pour-through apparatus leads to a
large-volume waste container
- rinse out pour-through-cell pour 25-50 ml of de-ionized water
into the pour-through funnel and allow it to drain
- Enter the stored program number for high range nitrate
nitrogen (NO3-N). Press 355 ENTER. The display will show Dial nm
- If needed, rotate the wavelength dial until the small display
shows: 500 nm . When the correct wavelength is dialed in, the
display will quickly show : ``Zero Sample'', then `` mg/L NO3-N HR''
(indicating measurement of nitrogen ``N'' reported as nitrate
``NO'' for expected high range ``HR'' of concentration)
- Prepare sample for measurement
- Fill a mixing bottle (clear with white lid, has fill-lines on it
for 20 and 25 mL) with 25 mL of filtered sample (top line)
- Put on rubber gloves, move DOWNWIND from the class
- Tear open one NitraVer 5 Nitrate Reagent packet, push the
sides together to form a square opening along the top
- Add entire contents of packet to the mixing bottle, cap
- Follow measurement program:
- Press SHIFT (release) then TIMER, display will show 1
- Shake the cell vigorously until the timer beeps
- Press SHIFT then TIMER again. Spectrophotometer will begin five
minute reaction period countdown
- while waiting, fill the pour-through funnel with filtered
sample, this will be the blank against which the color
of the reacted sample will be compared. The spectrophotometer
reports the difference between these two as the final composition.
- When the timer beeps, the display will show mg/L NO3-N
- Press ZERO (this measures and sets the blank
concentration). The display will show: Zeroing ? then, 0.0 mg/L
- Pour the reacted sample into the pour-through funnel
- wait until the water level in the funnel stops falling
- Press READ The display will show: ``Reading'' then the result in
mg/L nitrate nitrogen (NO3-N) will be displayed.
- Record the value in your notebook.
Using the YSI-85
The D.O. meter
will be used to measure dissolved oxygen (DO), electrical
conductivity, and salinity. The device measures six different water
parameters, and is intended for use in moving water. The ``MODE''
button on the meter switches between the following in the order given
below. Press the ``MODE'' button and wait for a few seconds for the
mode to move to the next entry.
Note that D.O. is measured by the
rate of consumption of oxygen at the tip of the probe, so it requires
continual movement of water past the tip (an up-and-down motion seems
to work best, keep the probe tip submerged). Stable readings will not be
possible while the temperature of the sample is changing.
Dissolved oxygen-electrical conductivity-temperature meter (see
manufacturer). Black probe is inserted
into stream (or sample container) for measurement.
- Press the ON/OFF button to turn on the meter. The
activate all segments of the display for a few seconds, which will be
followed by a self-test procedure and will last for several more seconds.
The Model 85 will display the cell constant of the conductivity probe
when the self-test is complete (with ``CEL'' shown at the bottom
of the LCD display). Then it will switch to D.O. % mode.
- always displayed as the last line in the
Dissolved Oxygen %
- a measurement of oxygen in percent of
saturation. If you are measuring D.O. in a bottled sample,
stir with the probe so that water moves past it at a
rate of around 1
. Wait until changes in reading
are less than 1% over 15 seconds to record your reading.
- a measurement of oxygen in
(same as parts-per-million)
- a measurement of the conductive material in the liquid
sample without regard to temperature (units are micro-siemens or
). Be sure the two holes near the cable on the sensor are
submerged; these make the conductivity measurement
- also known as compensated conductivity which
automatically adjusts the reading to a calculated value which would have
been read if the sample had been at 25
(units are micro-siemens or
- a calculation done by the instrument electronics,
based upon the conductivity and temperature readings (units are
parts per thousand, or ppt)
Using the Basic Chemistry Test Strpis
These test strips work just like the pH test strips you probably used
at one time in your life. Strip technology has advanced to the point
where they are highly useful for reconaissance field chemistry and
process control. We're using them in this project to allow rapid
determination of geochemistry. To use the strips:
- open the bottle and shake out one strip. DO NOT put
wet fingers into the bottle, you'll ruin the rest of the strips.
- dip the strip into the water and remove immediately
- hold the strip horizontally for 15 seconds,
DO NOT shake off excess water from the strip
- compare the colors on the strip to the chart on the bottle.
Feel free to interpolate if an intermediate color appears. The
pad at the end of the strip corresponds to total hardness.
Next: Lab-3 Field Procedure
Up: Water Quality Measurements (Lab
Previous: Lab-3 Background
GEOS 3110 Professor's Notes, Summer 2007
Dr. T. Brikowski, U. Texas-Dallas. All rights reserved.