THE HUDSON RIVER: BASIC CHARACTERISTICS OF NATURAL WATER

PRE-LABORATORY ASSIGNMENT:

Before the lab experiment begins, you may collect a water sample from the Hudson River. Contact your lab instructor before collecting the sample for location and instructions.

OBJECTIVES: 

To apply the titration (acid/base), potentiometric, and electromechanical methods in environmental analysis.
To determine the basic characteristics of natural water.

INTERDISCIPLINARY ASPECTS:

hydrology, microbiology, toxicology, and physics

BENCHMARKS:

The origin of alkalinity and acidity in water.
Environmental effects of river pollution.
Chemical principles of the detection method used.
The "natural" levels of alkalinity, basicity, Dissolved Oxygen (DO), Total Dissolved Solids (TDS), and conductivity for river waters.

ACIDITY AND ALKALINITY

Acidity, as applied to natural water and waste water, is the capacity of the water to neutralize OH-. Acidic water is not frequently encountered, except in cases of severe pollution. Acidity generally results from the presence of weak acids, significant concentrations of CO2, H2PO4-, H2S, proteins, and fatty acids. Acidic metal ions, particularly Fe3+, may also contribute to acidity.

Alkalinity is the capacity of water to accept H+ ions. Alkalinity is important in water treatment and in the chemistry and biology of natural waters. Highly alkaline water often has high pH,  and generally contains elevated levels of dissolved solids. Alkalinity serves as a pH buffer and reservoir for inorganic carbons. It helps to determine the ability of a water system to support algae growth and the growth of other aquatic life. Thus, it can be used as a measure of water fertility. 

Alkalinity can be expressed as:

[alk] = [HCO3-] + 2[CO32- ] + [OH-]

The range of alkalinity values for natural waters is typically between 30 and 500 mg of CaCO3/L, with higher values occurring in regions that have alkaline soils. Rainwater usually has very little total alkalinity, less than 10 mg/L, since it has contact with few minerals. Surface waters generally have total alkalinities of less than 200 mg/L. Ground water's total alkalinities are frequently much higher, sometimes over 1000 mg/L, due to increased pressure of CO2 from microbial degradation of organic matter underground. Seawater has a total alkalinity of typically 200 - 500 mg/L, due to the high concentrations of dissolved bicarbonate ions.

PRINCIPLES OF THE METHOD

Total acidity is determined by titration with base to the phenolophtalein endpoint (pH= 8.3)

Free mineral acid is determined by titration with base to the methyl orange endpoint (pH= 4.3)

Phenolphthalein alkalinity is determined by titration with acid to the pH at which HCO3- is the predominate carbonate species (pH= 8.3)

Total alkalinity is determined by titration with acid to the methyl orange endpoint (pH= 4.3) where both bicarbonate species have been converted to CO2.

REAGENT LIST:

0.01 M HCl
0.01 M NaOH
Phenolphthalein
Methyl orange
Buffer solutions for pH 4, 7, and 10

GLASSWARE AND ACCESSORIES:

-    two 250 mL beakers, buret, stand, 100 mL graduated cylinder

-    pH meter, glass combination electrode, stirring plate, stirring bars

PROCEDURE:

CALIBRATION OF pH METER

Touch Setup on the main screen. Touch pH to access the pH Setup screen.
Calibrate the instrument at pH 10, 7 and 4, using standard buffer solutions. (follow the instructions for the specific type of pH-meter).
Wash the electrode with a jet of distilled water, then insert it into the buffer solutions. Use the minimum amount of buffer, just enough to cover the glass bulb.

DETERMINING TOTAL ACIDITY AND ALKALINITY

To determine total acidity, prepare your burette by filling it to the top of the cylinder with standard 0.0100 M NaOH (Be sure to write down the EXACT Concentration form the bottle you used to fill your burette.
Check that there are no air bubbles in the burette tip. If there are, allow a few milliliters of base to flow out.
In your lab notebook, make a table with two columns to monitor the pH as you titrate the HCl sample with base.
Measure 100.0 mL HCl with a graduated cylinder, and place it into a clean 250.0 mL beaker.
Add 4 drops of phenolphtalein indicator to the beaker.
Immerse the electrode into the beaker, stir the solution, allow the reading to stabilize for about 2 minutes, and then read the display to obtain the initial pH of your sample. Note this value in your table.
Swirling the sample gently, titrate the solution with 0.01 M NaOH. In your notebook, record the pH, the burette reading, and the amount of base added at regular intervals.
Watch for color changes. NOTE: phenolphtalein turns pink at endpoint (pH 8.3).
Clean and prepare your burette by filling it to the top of the cylinder with standard 0.0100 M HCl (Be sure to write down the EXACT Concentration form the bottle you used to fill your burette.
Follow the same titration procedure to determine the total alkalinity of the base using 4 drops of methyl orange as the indicator (endpoint occurs around pH 4.3). 
Plot the titration curves (mL of Acid Added vs. pH).

MEASUREMENT OF ALKALINITY IN SAMPLE

Prepare your burette by filling it to the top of the cylinder with standard 0.0100 M HCl (Be sure to write down the EXACT Concentration form the bottle you used to fill your burette.
Check that there are no air bubbles in the burette tip. If there are, allow a few milliliters of acid to flow out.
In your lab notebook, make a table with two columns to monitor the pH as you titrate the sample with acid.
Measure 100.0 mL of your water sample with a graduated cylinder, and place  into a clean 250.0 mL beaker. Immerse the electrode into the beaker, stir the solution, allow the reading to stabilize for about 2 minutes, and then read the display to obtain the initial pH of your sample. Note this value in your table.
Swirling the sample gently, titrate the solution with 0.01 M hydrochloric acid. The chemical reaction you are monitoring is:

CaCO3 + 2 HCl -----> H2CO3 + CaCl2

In your notebook, record the pH, the burette reading, and the amount of acid added at regular intervals.
Plot the titration curve (mL of Acid Added vs. pH).
Calculate the total and phenophtalein alkalinity for your sample as mg/L of CaCO3.

SAMPLE ANALYSIS DATA SHEET:

Sample

aliquot volume

[mL]

pH

volume of acid used to pH=8.3

[mL]

volume of acid used to pH=4.3

[mL]

phenolopht.

alkalinity

[mg/L]

Total alkalinity

[mg/L]

aliquot 1

           

aliquot 2

           

aliquot 3

           

CONDUCTIVITY AND TDS (Total Dissolved Solids)

Conductivity is the ability of a solution to pass current. It follows that the amount of current flowing is proportional to the number of ions present in the conducting solution. Therefore, a measure of the conductivity will give a direct reading of the concentration of ions in the solution. Conductivity gives a means of monitoring the build up of contaminants and also the success of water treatment programs.

Various ions in solution contribute to a total conductivity reading. The presence of organic alcohols and sugars will give inadequate conductivity results. Some materials reduce the accuracy of the technique by coating the sensor. Temperature effects are also sources of experimental inaccuracies.

Total Dissolved Solids (TDS) is a measurement of the total concentration of ionic species in a water sample. The presence of possible soluble mineral salts may affect some types of vegetation and fish, possibly affecting their survival in a particular habitat (Klein, 1959). In general, fresh water has less than 1,500 mg/L of TDS, brackish water has between 1500 and 5000 mg/L TDS, saline water has above 5000 mg/L, and seawater has a TDS content of 30,000-40,000 mg/L.

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PRINCIPLES OF THE METHOD

A solution of pure water has a very high resistance to the flow of electricity because, the ions that are present are in trace quantities. The electrical resistance of the water is lowered by dissolution of an ionic compound in the solution. The ions provide a pathway for the flow of electrons through the solution. The conductance of a solution is simply the reciprocal of the resistance and is a measure of the solution’s ability to conduct electricity. The unit of measure for conductance is the Siemen, S.

The instruments used to measure total dissolved solids is a conductivity meter, and an electrical circuit called a Wheatstone bridge. Conductivity is the quantity that is usually recorded as a measure of total dissolved solids. It is conductance per unit distance between the two parallel plates, usually expressed as Siemens per centimeter (S/cm). Very pure deionized water has a typical conductivity of less than 1 mS/cm, whereas rainwater can have conductivity in the range of 20-40 mS/cm. Unpolluted surface waters have conductivities in the range of 30-400 mS/cm. The effluent from the treatment plants may have a conductivity between 300 - 1000 mS/cm, due to the ionic substances in treated waste water.

The relationship of conductivity in mS/cm to total dissolved solids in mg/L depends on the identity of ions in the solution. A rough guide to the conversion between mS/cm and mg/L for natural waters is that conductivity in microsiemens per centimeter is about 110-115% of the dissolved solids in mg/L.

REAGENT LIST:

Conductivity standards (A or B; 1413 mS or 12.88 mS).
TDS standards (A or B; 706 mg/L or 6.44 g/L).

GLASSWARE AND ACCESSORIES:

-    three 25 mL beakers

-    Conductivity meter, TDS sensor

PROCEDURE:

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CALIBRATION OF CONDUCTIVITY AND TDS SENSOR and MEASUREMENT

Make sure the clear plastic shield is in place when calibrating or measuring. When measuring, make sure the solution is above the cell chamber rings and below the vent hole.
Make sure the cell chamber is bubble-free when measuring. To reduce air bubbles, immerse the probe in calibration solution at an angle and then rise to a vertical position.
Press mode, read, cal or M to turn on the meter and start calibration/measurement.
Hold the sensor in the air and press cal.  Cal I is displayed (the meter should read 0.00 uS for conductivity. Press mode again, the meter should read 0.00 mg/L for TDS). After endpoint the display automatically updates to the calibrated value shown, and the temperature at which reading was taken.
Rinse with distilled water or rinsing solution between calibration/measurements.
Place the sensor in either the Conductivity standard A or B solution; press cal - cal 2 is displayed( the meter should read 1413 mS or 12.88 mS or conductivity. Press mode again, the meter should read 706 mg/L or 6.44g/L for TDS). After a stable value has been obtained, the display automatically updates to the calibrated value show, or the temperature at which reading was taken.
Put TDS meter in your sample and press read again to start new measurement.

DISSOLVED OXYGEN (DO)

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Oxygen, making up one -fifth of the earth’s atmosphere, is one of the most important elements. The maximum concentration of dissolved oxygen in water is governed by Henry’s Law. Its actual concentration depends on such factors as pH and temperature. The saturated amount of oxygen at 25 oC is 8.32 g/L. When DO is below 5 mg/L, the survival of fish and other aquatic life become problematic. Oxygen, when present in water, is consumed not only by living organism (for respiration), but also by organic molecules present in the water system. The latter needs oxygen for their oxidation and chemical transformation. Thus, the level of oxygen in water systems is important because it controls many aspects that maintain ecological balance. For instance, in the top layer of water, where the concentration of oxygen is expected to be higher, sulfur is present as sulfate ions and nitrogen is present as nitrates. Whereas, at the bottom of the water system, we expect the presence of sulfides, methane and ammonia. These species are responsible for odor emitted from water systems when the tide is low.

PRINCIPLES OF THE METHOD

The dissolved oxygen meter consists of an electrode that is shielded from the sample solution. The shielding is made possible by a membrane that is selectively permeable to oxygen. When oxygen diffuses through the membrane and reaches the electrode, a transfer of electrons takes place between the electrode and the oxygen molecule.

When oxygen reacts with hydrogen ions supplied by a buffer solution contained within the membrane, it referred to as a cathode reaction (given by the following equation):

O2(aq) + 4 H+ + 4 e ----> H2O

Anode reaction:

2Pbo (s) ---- > 4e + 2 Pb2+ (aq)

The transfer of electrons causes a current to flow through a current-measuring device consisting of an ammeter and a reference electrode. The amount of current is proportional to the amount of dissolved oxygen in the sample. The electrode is calibrated by using solutions of known concentration of oxygen. Concentrations of dissolved oxygen are reported in mg/L.

REAGENT LIST:

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Dissolved oxygen standard (zero oxygen solution)

GLASSWARE AND ACCESSORIES:

-    three 25 mL beakers

-    Dissolved oxygen meter

PROCEDURE:

CALIBRATION OF DO METER and MEASUREMENT

Before using the DO, remove the wetting cap from the tip of the sensor.
Unscrew the membrane cap from the sensor. Polish the silver/gold tip carefully, using the electrode cleaning compound of silver polish, paying particular attention to the gold cathode.
Rinse tip with DO electrolyte, and fill the membrane cap (avoiding air bubbles).
Hold the sensor vertically and gently screw the membrane cap onto the sensor, allowing surplus electrolyte to run out.
Fit sensor to the meter and allow 1 hour for polarization.
Press mode, read, cal or M to turn on the meter and start calibration/measurement.
Place the sensor in the zero oxygen solution and press cal and cal 1 is displayed. After the value has stabilized, the display automatically updates to the calibrated value shown and the temperature at which the reading was taken.
Rinse the probe with distilled water or rinsing solution between calibration/measurement.
Hold the sensor in the air 10 mm above fresh water; cal - cal 2 is displayed (the meter should show 100% of O2). Once the reading has stabilized, the display automatically updates to the calibrated value shown and the temperature at which the reading was taken.
Press read again to start new measurements in 100 mL of your sample.
After use, replace the wetting cap.

SAMPLE ANALYSIS DATA SHEET:

Sample

aliquot volume

[mL]

pH

Conductivity

[mS]

TDS

[mg/L]

DO

[%]

aliquot 1

         

aliquot 2

         

aliquot 3

         

REPORT:

Prepare a laboratory report containing:

    1. An introduction with an objective on the experimental work.
    2. Experimental procedures and analytical methods used.
    3. Results of the experiments, with the titration curve and error analysis.
    4. Discussion of the data, BENCHMARKS, and the environmental effects excess pollutants have on the basic parameters of natural water.

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