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15 Cl and Salinity

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15 Cl and Salinity

     Computer

     15

     Chloride and Salinity

    INTRODUCTION

    Chloride

    Chloride, in the form of the Cl ion, is one of the major inorganic anions, or negative ions, in

    saltwater and freshwater. It originates from the dissociation of salts, such as sodium chloride or calcium chloride, in water. + NaCl(s) (aq) + Cl(aq) Na

    2+ CaCl(s) (aq) + 2 Cl(aq) Ca2

    These salts, and their resulting chloride ions, originate from natural minerals, saltwater intrusion into estuaries, and industrial pollution.

    There are many possible sources of manmade salts

    that may contribute to elevated chloride readings.

    Sodium chloride and calcium chloride, used to salt ; River streambeds with roads, contribute to elevated chloride levels in salt-containing minerals streams. Chlorinated drinking water and sodium-

    chloride water softeners often increase chloride levels ; Runoff from salted roads in wastewater of a community.

    ; Irrigation water returned to

    streams In drinking water, the salty taste produced by

    chloride depends upon the concentration of the ; Mixing of seawater with chloride ion. Water containing 250 mg/L of chloride freshwater

    may have a detectable salty taste if the chloride came ; Chlorinated drinking water from sodium chloride. The recommended maximum

    level of chloride in U.S. drinking water is 250 mg/L. ; Water softener regeneration

     Salinity

    Salinity is the total of all non-carbonate salts dissolved in water, usually expressed in parts per thousand (1 ppt = 1000 mg/L). Unlike chloride (Cl) concentration, you can think of salinity as a +measure of the total salt concentration, comprised mostly of Na and Cl ions. Even though there +2+2are smaller quantities of other ions in seawater (e.g., K, Mg, or SO), sodium and chloride 4

    ions represent about 91% of all seawater ions. Salinity is an important measurement in seawater or in estuaries where freshwater from rivers and streams mixes with salty ocean water. The salinity level in seawater is fairly constant, at about 35 ppt (35,000 mg/L), while brackish estuaries may have salinity levels between 1 and 10 ppt. Since most anions in seawater or brackish water are chloride ions, salinity can be determined from chloride concentration. The following formula is used: salinity (ppt) = 0.0018066 Cl(mg/L)

    A Chloride Ion-Selective Electrode can be used to determine the chloride concentration, which is converted to a salinity value using the above formula.

    Salinity can also be measured in freshwater. Compared to seawater or brackish water, freshwater +has much lower levels of “salt ions” such as Na and Cl; in fact, these ions are often lower in 2+concentration than hard-water ions such calcium (Ca) and bicarbonate (HCO). Because salinity 3

     15 - 1 Water Quality with Vernier

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     Chloride and Salinity

    Method 1: CHLORIDE CONCENTRATION AND SALINITY (ISE)

    Materials Checklist

    ___ computer ___ Low Standard (10 mg/L Cl) ___ Vernier computer interface ___ High Standard (1000 mg/L Cl) ___ Logger Pro ___ Very High Standard (20,000 mg/L Cl)

    ___ Chloride Ion-Selective Electrode ___ tissues

    ___ wash bottle with distilled water ___ beaker for wastewater

     Advanced Preparation

     The Vernier Chloride Ion-Selective Electrode (ISE) must be soaked in the Chloride High Standard solution (included with the ISE) for 1530 minutes. Important: Make sure the electrode is not resting on the bottom, and that the small white reference contacts are immersed. Make sure no air bubbles are trapped below the electrode.

    Testing Procedure

    1. Position the computer safely away from the water. Keep water away

    from the computer at all times.

     ISE 2. Prepare the Chloride Ion-Selective Electrode (ISE) for data soaking collection. for travel a. The ISE should be soaking in the High Standard. Make sure that it

    is not resting on the bottom of the container, and that the small white reference contacts

    are immersed.

    b. Plug the ISE Sensor into Channel 1 of the Vernier interface. 3. Prepare the computer for data collection by opening the file “15 Chloride ISE” from the

    Water Quality with Vernier folder of Logger Pro.

    4. You are now ready to calibrate the Calcium ISE.

    First Calibration Point

    a. Choose Calibrate ; CH1: Chloride ISE from the Experiment menu and then click

    . b. Type 1000 (the concentration in mg/L Cl) in the edit box.

    c. When the displayed voltage reading for Reading 1 stabilizes, click .

    Second Calibration Point

    d. If you are testing a freshwater sample, place the tip of the electrode into the Low Standard (10 mg/L Cl). If you are testing a seawater- or brackish-water sample, place the tip of the electrode into the Very High Standard (20,000 mg/L Cl). Be sure that the electrode is

    not resting on the bottom of the bottle and that the small white reference contacts are

    immersed. Make sure no air bubbles are trapped below the electrode.

     15 - 3 Water Quality with Vernier

     Computer 15

    e. After briefly swirling the solution, hold the ISE still and wait approximately 30 seconds for

    the voltage reading displayed on the computer screen to stabilize.

    f. Enter 10 (the concentration of the Low Standard) or “20,000” (the concentration of the

    Very High Standard), depending on which one you used.

    g. When the displayed voltage reading for Reading 2 stabilizes, click , then click

    .

5. You are now ready to collect chloride concentration data.

    a. Rinse the ISE with distilled water and gently blot it dry.

    b. Place the tip of the probe into the stream at Site 1, or into a cup with sample water from

    the stream. Make sure the ISE is not resting on the bottom and that the small white

    reference contacts are immersed. Make sure that no air bubbles are trapped below the ISE.

    c. Click to begin data collection.

    d. Click to begin a 10 s sampling run. Important: Leave the probe tip submerged for

    the 10 seconds that data is being collected.

    e. When the sampling run is complete, stop data collection and record the mean chloride

    concentration value on the Data & Calculations sheet.

    6. Return to Step 5 to obtain a second reading.

    7. Determine the salinity value of your sample (in ppt). Use this formula to calculate the salinity, based on the chloride concentration, Cl

     salinity (ppt) = 0.0018066 X Cl(mg/L)

     Record this value in the Data & Calculations table (round to the nearest 0.1 mg/L Cl).

     15 - 4 Water Quality with Vernier

     Chloride and Salinity

    DATA & CALCULATIONS

    Method 1: Chloride Concentration and Salinity (ISE) Stream or lake: ____________________________ Time of day: ____________________________ Site name: ________________________________ Student name: __________________________ Site number: ______________________________ Student name: __________________________ Date: ____________________________________ Student name: __________________________

    Reading Chloride Salinity (mg/L Cl) (ppt)

    1

    2

    Average

Column Procedure: A. Record the chloride concentration value (in mg/L Cl) from the computer.

    B. Calculate salinity, using the formula:

     salinity (ppt) = 0.0018066 ; Cl(mg/L).

    Field Observations (e.g., weather, geography, vegetation along stream) ___________________________

     ____________________________________________________________________________________

     ____________________________________________________________________________________

     ____________________________________________________________________________________

Test Completed: ________________ Date: ______

     15 - 5 Water Quality with Vernier

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    Method 2: SALINITY USING A CONDUCTIVITY PROBE

    Materials Checklist

    ___ computer ___ wash bottle with distilled water

    ___ Vernier computer interface ___ tissues

    ___ Logger Pro ___ 5 ppt salinity standard

    ___ Vernier Conductivity Probe ___ 10 ppt salinity standard

    ___ 100 mL graduated cylinder ___ small paper or plastic cup (optional)

    ___ 500 mL bottle with lid

Collection and Storage of Samples

    1. This test can be conducted on site or in the lab. A 100 mL water sample is required. 2. It is important to obtain the water sample from below the surface of the water and as far away

    from shore as is safe. If suitable areas of the stream appear to be unreachable, samplers

    consisting of a rod and container can be constructed for collection. Refer to page Intro-4 of

    the Introduction of this book for more details.

    Testing Procedure

    1. Position the computer safely away from the water. Keep water away from the computer at all

    times.

    2. Prepare the Conductivity Probe for data collection.

    a. Plug the Conductivity Probe into Channel 1 of the Vernier interface.

    b. Set the switch on the probe box to the 0-20000 µS range (0-13 ppt salinity). 3. Prepare the computer for data collection by opening

    “15 Salinity Conductivity” from the Water Quality with

    Vernier experiment files of Logger Pro.

    4. You are now ready to calibrate the Conductivity Probe.

    ; If your instructor directs you to use the calibration

    stored in the experiment file, then proceed to Step 5.

    ; If your instructor directs you to perform a new

    calibration for the Conductivity Probe, follow this

    procedure:

    First Calibration Point

    a. Choose Calibrate ; CH1: Conductivity (ppt) from the Experiment menu and then click

    .

    b. Perform the first calibration point by placing the Conductivity Probe into the 5 ppt salinity

    standard. The hole near the tip of the probe should be covered completely.

    c. Type 5 (the salinity value) in the edit box.

    d. When the displayed voltage reading for Reading 1 stabilizes, click .

     15 - 6 Water Quality with Vernier

     Chloride and Salinity

    Second Calibration Point

    e. Place the Conductivity Probe into the 10 ppt salinity standard solution. The hole near the

    tip of the probe should be covered completely.

    f. Type 10 (the salinity value) in the edit box.

    g. When the displayed voltage reading for Reading 2 stabilizes, click , then click

    .

    h. Rinse the electrode with distilled water and gently blot it dry with a tissue.

5. You are now ready to collect salinity concentration

    data.

    a. Place the tip of the electrode into a cup with

    sample water from the body of water you are

    testing. The hole near the tip of the probe should

    be covered completely.

    b. Monitor the salinity concentration value in the

    meter.

    ; If the value is less than 13 ppt (the upper range

    of the Conductivity Probe), record this value on the Data

    & Calculations sheet. If you did a 1/4 dilution of the

    sample, be sure to multiply this value by 4!

    ; If the salinity value is greater than 13 ppt, go to

    Step 7.

6. Repeat Step 5 to obtain a second reading.

    7. If your salinity value is greater than 13 ppt, then the sample had a salinity value that is beyond

    the upper range of the Conductivity Probe. You will need to dilute the sample to a salinity

    value that is within the range of the probe. To do this,

    a. Use a graduated cylinder to add 100 mL of the water sample to a 500 mL bottle.

    b. Add 300 mL of distilled water to the sample in the bottle, screw the lid on, and shake it

    well.

    c. Important: The sample has been diluted by a factor of 100 / 400 or ?. Any salinity

    measurements made on this sample will have to be multiplied by a factor of 4 to account

    for this dilution.

    d. Repeat Step 5 to obtain a second reading.

     15 - 7 Water Quality with Vernier

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    DATA & CALCULATIONS

    Method 2: Salinity Using a Conductivity Probe

    Stream or lake: _____________________________ Time of day: ___________________________ Site name: ________________________________ Student name: _________________________ Site number: ______________________________ Student name: _________________________ Date: _____________________________________ Student name: _________________________

    Reading Salinity

    (ppt)

    1

    2

    Average

Column Procedure:

    A. Record the salinity concentration as determined by interpolation of the calibration curve.

    Field Observations (e.g., weather, geography, vegetation along stream) __________________________

     ____________________________________________________________________________________

     ____________________________________________________________________________________

     ____________________________________________________________________________________

Test Completed: ________________ Date: ______

     15 - 8 Water Quality with Vernier

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