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Limnology

By Martha Rivera,2014-01-20 01:53
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Limnology

LAB 4: AQUATIC CHEMISTRY

    CONDUCTIVITY, PH, AND ACID NEUTRALIZING

    CAPACITY

GOALS:

    1) Learn to calibrate the pH meter and measure pH, temperature, and REDOX

    2) Learn to calibrate the conductivity meter and measure conductivity Note the conductivity

    meter is also on the oxygen meter

    3) Understand the units of pH and what pH means in natural waters and what the readings mean

    4) Understand what alkalinity means and how to measure

BACKGROUND:

    Stream organisms are influenced by the physicochemical features of the water body. Physical

    features that are important to the biota include temperature and light. Temperature affects many

    things but of most concern is the effect on dissolved gases in the water (e.g., dissolved oxygen), and

    the fauna of streams because most of these organisms have no internal control of body temperature.

    Light influences the temperature of the water and primary productivity of the ecosystem through its

    influence on photosynthesis. The greatest source of heat in freshwater is from solar radiation. This is

    especially true in streams that are not shaded by a tree canopy. In streams that are densely shaded,

    transfer of heat from the air and through flows of groundwater may be more important in

    determining water temperature than solar radiation. Temperatures in streams may vary daily (called

    the diel temperature flux), especially in streams without canopy cover.

    There are a myriad of chemical features of water that are important to the biota. The

    dissolved oxygen concentration is important because of its role in plant and animal metabolism. The

    inorganic carbon complex (includes such things as dissolved CO and relatives), which regulates the 2

    acidity, alkalinity, and pH of most streams, is also important to the biota. Inorganic nutrients, such as

    nitrogen and phosphorus, are also important because of their roles as major cellular components of

    the biota. These two inorganic nutrients may limit primary production via photosynthesis.

Temperature, dissolved oxygen concentration, conductivity, and pH

    In this exercise, we will study how temperature, dissolved oxygen concentration,

    conductivity and pH differ between our study reaches. Temperature, dissolved oxygen concentration,

    conductivity and pH are thought to influence the biota and interact with each other and other

    characteristics in the following general ways:

    Temperature: influences the growth and reproduction of organisms; influences the amount of

    dissolved oxygen in water and often the amount of oxygen needed by organisms

Dissolved oxygen concentration: The concentration of dissolved oxygen (often abbreviated DO)

    is controlled by the interplay between a stream and the atmosphere and by the balance between

    photosynthesis and plant and animal respiration. Conversely, the distribution of organisms and

    the numerous aspects of stream chemistry are strongly influenced by the available DO.

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The amount of oxygen dissolved in water is normally measured as milligrams of oxygen per liter

    of water (mg O/L). The maximum amount of O that will dissolve, i.e. complete saturation, is a 22decreasing function of water temperature and also depends on atmospheric pressure. For example, at oomean sea level and a temperature of 0C, oxygen has a solubility of about 14 mg/l. At 35C, the

    solubility is about 7 mg/l. Consequently we can compute streams maximum possible oxygen

    content from its elevation and temperature. However, stream ecologists are far more interested in

    knowing what a stream’s actual oxygen content is, and how this varies with time.

CONDUCTIVITY

     Conductivity is a measurement that we can use to summarize the chemistry of the water.

    Conductivity is one of the major monitoring tools in our National Forests and Parks, especially in

    wilderness areas. A conductivity of less than 10 means the pH is probably less than 6, a danger

    level. It is much easier to measure conductivity than pH, especially in dilute water.

     Conductivity meters measure the ability of a water sample to conduct electricity. While pure

    water is resistant to passage of an electrical current, dissolved ions in the water reduce this resistance

    and electron flow increases. The amount of current conducted is proportional to the number of ions 2+2++,+-2-2-in solution (notably Ca, Mg Na K, CO, SO, and Cl) and is expressed in mhos (reflecting 34the fact that conductivity is the recipricocal of resistance which is expressed in ohms) or the

    equivalent unit of µS/cm (S standing for Siemens).

    Water Source Conductivity µS/cm

    Mountain lakes in granite 1-30

    Rain water 10-15

    Lakes pH>7 >100

    Local Lakes 250-390

pH

     The pH has a major influence on the overall chemistry of lake waters and on the distribution

    of organisms within those waters. pH results from a complex interaction of dissolved ions and in

    some cases organic compounds.

     +pH = - log [H]

    Recall that water can dissociate into hydrogen and hydroxyl ions:

     +-HO ? H + OH 2

    The product of the concentrations of these two ions is a constant, called the dissociation -14constant or K. K = 10. ww

     +--14[H][ OH] = 10

    At neutrality, concentrations of hydrogen and hydroxyl ions are equivalent:

     +-[H] = [OH] +-+-14[H][ OH] = 2 [H] = 10 +-7[H] = 10

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    pH is a unit for reporting hydrogen concentration on a base 10 logarithmic scale. That is,

     ++[H] = log[1/ H] pH = -log1010

     y(It may help to remember that the equation logx=y can also be written as 10=x. 10-pH+Therefore, 10=[H].)

     +A high pH means that [H+] is low; low pH means that [H] is high.

    At neutrality, then

     -7pH = - log[10] = -(-7) = 7 10

    For practice, determine pH for the following ion concentrations.

     +-4(1) [H] = 0.0001 = 10 +(2) [H] = 0.1 -(3) [OH] = 0.0001

     The pH of natural waters may range from 3.0 to 12.0.

ACID NEUTRALIZING CAPACITY (ANC)

     ANC, the buffering capacity of a given water sample, is defined as the amount of acid that

    must be added to change its pH to standardized levels. Most current research reports this buffering

    capacity as ANC, but the historical literature and some modern literature reports it as alkalinity.

    The standardized pH levels are based on the dissociation of carbonic acid (HCO) to bicarbonate 23-2-(HCO) and carbonate (CO) ions in water. This dissociation occurs in two steps. 33

     ++-2-HO + CO ? HCO???CO + H ??CO + 2 H 222333

Which ions are present at a given time depends on pH:

     -Relation between pH and the relative proportions of inorganic carbon species of CO, HCO, and 232-- COin solution. Note that at pH 6-8 bicarbonate (HCO is the most abundant form of carbon 33

    [from Wetzel (1983).]

    Laboratory 4 3 of 13

     -2-HCO and CO are the buffers in the system. One way to look at the chemistry is to consider the 33+above equation. As H is added to the system, reactions are driven to the left until all of the +carbonate and bicarbonate buffers are used up. At this point, additional H drives the pH down.

     +2- In more detail: At high pH, there are lots of CO ions and few H ions. As long as both 3-2-ions are present, they react immediately to form ?CO. This removes CO ions from the system 33+2-and keeps [H] relatively constant. As the supply of CO ions is exhausted, pH begins to drop. In 3+-the range of pH 7-10, HCO ions take over and react with H to form HCO. When there is no 323+--2-more HCO, H ions added to the system are no longer buffered by HCO and CO, and pH 333+lowers proportionally with additional H ions.

     For most lakes we will be concerned with total ANC, which is reported as milliequivalents -3-6-1(meq, ie. 10) or microequivalents (µeq, ie. 10) per liter (l ). In the early literature and in some recent cases where alkalinity has been reported rather than ANC the units of mg/l of calcium

    carbonate (CaCO) are used. This unit is based on the mistaken assumption that most of the 3

    buffering capacity of water is derived from dissolved CaCO. Conversion between mg/l CaCO and 33

    µeq/l is straightforward and can be found in Wetzel and Likens (1991).

     ANC measurements provide a measure of the total ions in water available to take up

    Hydrogen ions. Ions include carbonate, bicarbonate, organic anions, silicates, arsenates, and

    aluminates. Even though ANC determination techniques were developed as basic lake chemistry

    measures, they provide a good estimation of the susceptibility of a lake or stream to the effects of

    acid deposition. ANC levels less than 200 µeq/l are vulnerable to acid rain; levels of 600-4000 µeq/l

    are considered "safe".

     We will determine ANC at the inflection point for bicarbonate, around pH 4.6. Occasionally

    you may see reports of phenolphthalein alkalinity, which is ANC determined at the inflection point

    for carbonate, around pH 8.3. The name derives from earlier systems of ANC determination that

    used color indicators to mark a particular point in the titration. Phenolphthalein has a color shift

    around pH 8.3. Other indicators were used that changed color around pH 4.6. Indicator-based

    techniques must be evaluated with caution and should be avoided in any current, precise work.

LITERATURE CITED:

    References

    Gran, G. 1952. Determination of equivalence point in potentiometric titrations. Analyst 77:661-671. Soranno, P.A. and S.E. Knight. 1992. Methods of the Cascading Trophic Interactions Project. 2nd ed. University of Wisconsin,

    Madison. pp.23-24.

    Wetzel, R.G. and G.E. Likens. 1991. Limnological Analyses. 2nd ed. Springer-Verlag. pp.107-128.

    Zimmerman, A.P. and H.H. Harvey. 1979. Final report on sensitivity to acidification of waters of Ontario and neighboring sites for

    Ontario Hydro. University of Toronto. pp.6-14.

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    MATERIALS AND SUPPLIES

    IN THE FIELD

     Tape Measures 100m, 50m, 25m

     Meter Sticks 4

     Flow Meter and Staging rod

     Dissolved Oxygen and Conductivity Meter

     pH ORP meter

     Camera

     GPS Unit

     Water Bottles Acid Washed 1 Liter Bottles 1 for chlorophyll and 1 for nutrients

     2 liter plastic bottles (for ANC)

     2 liter plastic bottles for chlorophyll and nutrients

     First Aid Kit

     Stream Maps

    IN THE LAB

     squirt bottle of distilled water

     beaker for rinse water

     datasheets

     pH meter

     25ml erlenmeyer flask

     pH buffer 4.0

     pH buffer 7.0

     10ml beakers

     ANC titration stand with graduated burette

     Funnel

     magnetic stirrer and stir bar

     supply of 0.05 N HCl

     50ml volumetric flask

     100ml glass beaker or similarly sized container

     datasheets

    Filtration Materials Materials:

     Pump

     1 liter filtration flask

     filtration tower

     graduated cylinder

     Three plastic 250 ml water sample bottles per sample

     Squirt bottle with distilled H2O

     Black Film Canisters (35mm) for chlorophyll filters

     forceps

     glass fiber filters (Whatman GFF 47 mm) Note you need to have several preweighed filters

    for biomass

    Laboratory 4 5 of 13

PROCEDURE:

In the Field:

    MEASURING CONDUCTIVITY

     A simple probe and electronic meter can be used to measure conductivity. The meter should obe calibrated with a standard solution of KCl at 25 C. A 0.001 N solution of KCl will have a

    specific conductance of 1410 µS/cm. Additional concentrations for KCl standard solutions and their specific conductance are given in Wetzel and Likens (1991).

Conductivity Procedures

    1. Collect a water sample for conductivity determination by filling a bottle or jar completely oand allow sample to come to room temperature (25 C).

    2. Immerse the bottom 1/4 to 1/3 of probe into the water sample. Not fully immersing the probe

    in the water sample may result in abnormally low readings.

    3. As with the DO meter you must determine the appropriate scale for your sample.

    4. Read conductivity once the meter stabilizes.

    5. Rinse probe between samples.

MEASURING pH

    1) Calibrate the pH meter using two points in the pH range:

    a) See Appendix A or Handout in class on how to calibrate

    b) Be sure to choose two buffers that bracket the value of pH that you anticipate for your

    sample (e.g. 4.0 and 7.0 for a lake with a pH of 6.0).

    c) Rinse the electrode with distilled water now and every time you are about to place the

    probe into a sample or buffer especially when chaning from buffers to water samples

    (Do you know why? Beside to prevent cross-contamination.)

    2) Immerse the electrode in the sample to be analyzed. Read pH after the meter stabilizes.

    3) Rinse the electrode with distilled water now and every time a sample or buffer is changed.

    This prevents cross-contamination. 4) Cover the end of the pH probe with the black plastic cap moistened with buffer after use

    (probes do not function when they dry.)

IN THE LAB:

    Today’s lab work will consist of two major tasks:

    1) filter water samples for later silica and chlorophyll analysis and

    2) Measure ANC

Filtration Procedure (try to minimize exposure of samples to light and do not overfill filtration

    flask):

    1. Wash filter flask, tower, plastic bottles with distilled water.

    2. Attach the filter flask to the pump with the tubing, put filter onto tower, and put tower onto

    flask.

    3. Shake sample bottle, and using a graduated cylinder, measure out a defined volume of

    sample. 300 ml may be adequate for highly productive systems, while 1000 ml may be

    needed in less productive systems. Record the filter volume.

    4. Pour a small amount of the sample from the graduate cylinder in the filter tower and let it

    filter disconnect the vacuum line and swirl the water in the flask and dump. WHY DO

    THIS?

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    5. Filter the rest of the sample.

    6. Pour filtrate (what was filtered) into labeled sample bottle (Group, Date, Stream, Lattitide

    and Longitude, Sample Type filtered water) to be analyzed at a later time.

    a. One sample should be placed in the fridge for Silica Analysis

    b. Two samples should be placed in the freezer for later analysis 1 for nitrogen and

    one for phosphorous

    c. One sample of unfiltered water should also be placed in the ridge.

    d. These samples should be stored in the freezer for later analysis.

    7. Then put the filter back on the filtration flask and rinse filter tower with distilled water.

    8. Unseat tower and use forceps to fold the filter in quarters, dirty side in. Place the folded

    filter into a labeled 35 mm Black Film Canister (Labels should include Lake, Date, Depth

    and Group Initials).

    9. Repeat for one more sample note you should have 2 filters for each sample

    e. The second sample should be filtered onto a preweighed filter and recored filter

    weight and the volume filtered through the filter!!!!!

    10. . Make sure there is enough unfiltered water to freeze about 150 ml

    11. Freeze filters until you are ready to perform Chl a analysis

    12. Place the biomass filter in a tin pan and place in the drying oven at 100C

MEASURING ANC

    Materials: (for each group of students)

    ? 1 pH probe

    ? ANC titration stand with graduated burette

    ? Funnel

    ? magnetic stirrer and stir bar

    ? supply of 0.05 N HCl

    ? 1 50ml volumetric flask

    ? 1 100ml glass beaker or similarly sized container

    ? 1 squirt bottle of distilled water

    ? 1 beaker for rinse water

    ? datasheets

     The basic technique for measuring ANC involves monitoring the pH of a sample as acid of

    known concentration is added. ANC is determined as the amount of acid needed to reach standard levels associated with the dissociation of HCO It is important to note that endpoint 23.

    values are only approximate. For precise measurements one should actually determine an inflection

    point in the graph of acid added versus pH of the sample; this inflection point should occur near 4.6,

    representing the exhaustion of the buffering capacity provided by the presence of the bicarbonate ion.

ANC Procedures

    1. Place 0.05 N hydrochloric acid (HCl) into a burette.

    2. Using a volumetric flask, measure out 50 ml of water sample (V) into a small beaker or jar. s

    Add a small stir bar.

    3. Arrange the sample, the burette, and the electrode of a pH meter so that the sample can be

    gently stirred with a stirrer as acid is added. Stir sample continuously.

    4. Record initial pH.

    5. Titrate sample SLOWLY with acid with volumes of 0.1 or 0.2 ml at a time. After EACH

    addition, record pH once it has stabilized, and keep track of the cumulative volume of acid

    Laboratory 4 7 of 13

    ). At pH=4.5, continue titrating but use even smaller volumes added since you began (Va

    (0.05 ml). Stop when the pH reaches 3.5. Try to have at least 10 data points between pH 4.5

    and 3.5.

    6. Rinse electrode well and place into distilled water. Rinse glassware with distilled water and

    dry on rack.

ANC Calculations y 1) Calculate the first Gran function F1 for each data point (many calculators have an xkey which

    you will find useful)

     -pH F1 = (V + V) * 10 (units are in equivalents) sa

    Where:

     V is the volume of the sample (50 ml or 0.05 l) s

     V is the cumulative volume of acid added a

     Report volumes (V) in liters, not ml !

    2) Plot pH versus the volume of acid added on the top half of your graph paper.

    3) Plot F1 versus the volume of acid added on the bottom half of your graph paper. See example

    below.

    4) Extrapolate the linear portion of the plot by drawing a "best fitting" line through your data points.

    Determine where this line crosses the x-axis -- this point is the x-intercept or equivalence point.

     -15) Calculate alkalinity (in µeq l):

     6 (normality of acid) * (x intercept in liters) * 1 *10 µeq/ N ANC, µeq/l=

     (sample volume in liters)

ANC titration curves from Wetzel and Likens (1991). Top panel: pH vs. volume of acid added. -6Lower panel: Gran function (F1) (units are equivalents,1 x 10) vs. volume of acid added. Also

    shown is the best fit regression line for the last six data points. The line has a slope equal to the

    normality of the acid used for the titration (0.0001 eq/ml = 0.1 N) and has an intercept of 0.125 ml.

    99887766pHpH554433221100

    1515

    1010F1F1

    55

    0000.10.20.300.10.20.3

    Acid Volume (ml)Acid Volume (ml)

    Laboratory 4 8 of 13

DATA SHEETS:

Stream _______________ Date _______________ Time _________

Group

     sample: ANC Titration pH & Conductivity

    _________

    Cumulative pH F1 (Gran sample pH conductivity

    Volume of Acid Function)

    Added (mL) (eq)