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Next: Lab-3 Field Procedure Up: Water Quality Measurements (Lab Previous: Lab-3 Background


Lab-3 Activities

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

* Temperature
measured using pH meter or dissolved oxygen meter, or thermometer
* pH
``acidity'' of the water, measured using pH meter, or pH paper (yields low precision measurement)
* Turbidity
the clarity of the water, measured using portable turbidimeter (Hach 2100P)
* Basic Quality
we will use swimming pool test strips to measure hardness, Cl, pH, and alkalinity
* D.O.
Dissolved Oxygen content. Measured using a hand-held dissolved oxygen meter

* Electrical Conductivity
see ``Other Parameters''. Measured using the D.O. meter. Record both the first reading (units are ``$\mu S$'') 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).

* Salinity
calculated by the D.O. meter based on temperature and conductivity measurements. Or use Pocket TDS meter.

Figure 4.6: Portable turbidity meter (see manufacturer). Cover opens for insertion of sample container.
Image 2100p

Calibration of Instruments

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 (Fig. 4.7).

Figure 4.7: Turbidimeter controls and display. Press ``READ'' when ready to take a measurement. After Hach Model 2100P Turbidimeter Instruction Manual, 1997, p. 24.
Image turb_controls2

Use the following procedure when measuring the turbidity of your sample:

  1. 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.

  2. Wipe the cell with a soft, lint-free cloth to remove water spots and fingerprints.

  3. Press I/O - the instrument will turn on. Place the instrument on a flat surface. Do not hold the instrument while making readings.

  4. 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.

  5. Set automatic range by pressing the RANGE key. The display will show ``AUTO RNG''.

  6. Select signal averaging (reports average of 10 measurements) by pressing ``Signal Average'' key, display should show ``SIG AVG''

  7. 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 turns off.

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 ( ${\rm ^\circ C }$) is displayed in the lower center. pH meters generally require frequent calibration in the field, and calibration is part of this lab.

Figure 4.8: ph Meter display and controls.

Image ph_controls

pH Calibration

The procedure for pH meter calibration is:
  1. 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.
  2. Turn on device (press I/O) key
  3. 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
  4. Press ``CAL'' button to enter calibrate mode, display will flash
  5. Rinse electrode and place into the 4.0 buffer.
  6. 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.
  7. 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.
  8. The display will remain fixed momentarily, then P2 will be displayed in the lower field. The meter is now ready for the second buffer (7.0).
  9. 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.
  10. The meter will automatically enter measure mode and MEASURE appears above the main display.
  11. 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.

pH Measurement

The procedure for making a pH-temperature reading is:
  1. rinse the electrode tip in deionized water
  2. depress the dispenser button on the top of the electrode until a click is heard (releases reference electrolyte at tip of electrode)
  3. wait until the readings become steady (``READY'' indicator shows, and meter beeps once)
  4. record results (including temperature)
  5. 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.

Figure 4.9: Pour-through cell arrangement for spectrophotometer measurements. After Hach DR/2010 Spectrophotometer Instrument Manual, p. 10, 1997.

Image pour_thru_cell

The procedure for making a nitrate analysis via spectrophotometry is:

  1. Obtain at least 50 ml of filtered sample
    1. 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)
    2. place clear 250 ml sample jar (these have light blue lids) under funnel to catch filtrate
    3. pour sample into filter until water level is close to the top
    4. wait for sample to gravity drain into clear jar
    5. repeat, after second filter-full you should have enough sample to analyze for nitrate

  2. Prepare spectrophotometer
    1. make sure drain hose from pour-through apparatus leads to a large-volume waste container
    2. rinse out pour-through-cell pour 25-50 ml of de-ionized water into the pour-through funnel and allow it to drain
    3. Enter the stored program number for high range nitrate nitrogen (NO3-N). Press 355 ENTER. The display will show Dial nm to 500.
    4. 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$_3$'' for expected high range ``HR'' of concentration)
  3. Prepare sample for measurement
    1. 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)
    2. Put on rubber gloves, move DOWNWIND from the class
    3. Tear open one NitraVer 5 Nitrate Reagent packet, push the sides together to form a square opening along the top
    4. Add entire contents of packet to the mixing bottle, cap firmly
  4. Follow measurement program:
    1. Press SHIFT (release) then TIMER, display will show 1 minute countdown
    2. Shake the cell vigorously until the timer beeps
    3. Press SHIFT then TIMER again. Spectrophotometer will begin five minute reaction period countdown
    4. 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.
    5. When the timer beeps, the display will show mg/L NO3-N HR
    6. Press ZERO (this measures and sets the blank concentration). The display will show: Zeroing ? then, 0.0 mg/L NO3-N HR.
    7. Pour the reacted sample into the pour-through funnel
    8. wait until the water level in the funnel stops falling
    9. Press READ The display will show: ``Reading'' then the result in mg/L nitrate nitrogen (NO3-N) will be displayed.
    10. 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.

Figure 4.10: Dissolved oxygen-electrical conductivity-temperature meter (see manufacturer). Black probe is inserted into stream (or sample container) for measurement.
Image ysi85

* On/Off
Press the ON/OFF button to turn on the meter. The instrument will 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.
* Temperature ( ${\rm ^\circ C }$)
always displayed as the last line in the readout
* 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 $\rm\frac{ft}{sec}$. Wait until changes in reading are less than 1% over 15 seconds to record your reading.
* Dissolved Oxygen $\rm\frac{mg}{L}$
a measurement of oxygen in $\rm\frac{milligrams}{liter}$ (same as parts-per-million)
* Conductivity
a measurement of the conductive material in the liquid sample without regard to temperature (units are micro-siemens or $\mu S$). Be sure the two holes near the cable on the sensor are submerged; these make the conductivity measurement
* Specific Conductance
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 ${\rm ^\circ C }$(units are micro-siemens or $\mu S$)
* Salinity
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:

  1. open the bottle and shake out one strip. DO NOT put wet fingers into the bottle, you'll ruin the rest of the strips.
  2. dip the strip into the water and remove immediately
  3. hold the strip horizontally for 15 seconds, DO NOT shake off excess water from the strip
  4. 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 up previous contents
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.