Alkalinity in Natural Waters - Lab Report Example

ALKALINITY IN NATURAL WATERS Chemistry Lab Report 07 May Introduction The alkalinity of a water sample is defined as the capacity for solutes it contains to react with an acid and neutralize it (Stumm & Morgan, 2012). The alkalinity property of a sample can be determined by titration using a strong acid with the endpoint of the titration giving the pH at which almost all the solutes responsible for the alkalinity have reacted. The end-point pH used in the titration process is a function of the various solute species that contribute to the sample’s alkalinity and their concentrations. The ionic strength and temperature affect the end-point values of a particular sample. An endpoint pH range of 4.0 - 4.6 is specified for methyl orange by the analytical procedures while other indicators are specified at an endpoint pH range of 4.5 - 5.1. The endpoint pH of phenolphthalein indicator is specified at 8.3. A number of solute species contribute to water alkalinity. The primary source of carbon (IV) oxide species that produce ground water or surface water alkalinity is the carbon (IV) oxide gas fraction of the atmosphere (Kruse et al., 2013). Various ways are used to express the alkalinity property quantitatively. The alkalinity property is commonly reported in terms of the equivalent amount of calcium carbonate. Other expressions are milliequivalents per liter, where a milliequivalent per liter is l/50 times mg/L Calcium carbonate. Dissolved carbon dioxide species, carbonates, and bicarbonates are the ones known to produce alkalinity in almost all natural waters. Hydroxides, borates, silicates, organic ligands such as propionates and acetates are the principal noncarbonated contributors to water alkalinity. Biological Oxygen Demand denoted as BOD is defined as the measure of oxygen quantity consumed by microorganisms such as bacteria during the decomposition of organic waste. The presence of organic matter such as sewage, food waste, dead plants, grass, leaves, manure or even grass clippings in a water supply often accelerate the action of bacteria on them. Breakdown of these waste matter by the bacteria require oxygen that is obtained from the available dissolved oxygen. The levels of BOD begins to decline the moment decomposition process begins. The presence of phosphates and nitrates in a water body can contribute high levels of BOD. Chemical oxygen demand denoted as COD is defined as the measure of oxygen amount present in water used for chemical oxidation of available pollutants (Almeida et al., 2012). COD helps to determine the amount of oxygen required for oxidation of organic matter present in a water sample, under specific conditions of temperature, time, and oxidizing agent. The indirect way of measuring the quantity of organic matter in a water sample is to use the chemical oxygen demand test. Most applications of the COD test is in the determination of the amount of organic pollutants present surface water, and thus making COD an essential tool to measure the quality of water. The most common expression of COD is milligrams per liter (mg/L). The expression mg/L indicates the mass of oxygen used per liter of solution. The main objectives of this lab were to determine the quantity of alkalinity that a water sample contains as well as the chemical oxygen demand in a given water sample. The experiment also aimed at determining the biological oxygen demand in a given water sample. 2.1 Methodology 2.1.1 Materials Horse waste Burette (50 mL) pH meter pipette (50 mL) Standardized 1.06N H2SO4 Methyl orange and phenolphthalein indicators Erlenmeyer flasks (250 ml) Magnetic stir plate Spectrophotometer Test tubes Dissolved oxygen meter Columns 2.2 Procedure for Alkalinity test of water samples In 250 Erlenmeyer flask containing 100 mL of tap water, 50 mL of the sample was pipette and the pH of the solution measured. The solution was then stirred using a magnetic stir bar to achieve homogeneity. The burette was filled with the standardized sulfuric acid and the initial reading recorded. Four drops of phenolphthalein indicator were added to the solution and the solution titrated with the standardized acid. The final value on the burette was recorded, and the volume of the acid used calculated. The solution’s pH was then measured. At the end of the first titration, three drops of methyl orange indicator were added to the solution and titrated to the red-orange end-point of methyl orange indicator. The volume of the acid used, and the pH of the solution were recorded. 2.3 Procedure for Chemical oxygen demand Into two test tubes, 2.5 mL of the sample was pipette followed by 1.5 mL of the digestion solution and 3.5 mL of the standardized sulfuric acid. The digestion solution and acid were also added to the third test tube containing deionized water at the same volume. The mixtures were swirled to mix and then placed in a preheated digestion block at 150oC for 2 h. The solutions were then cooled for 45 minutes before being run in the spectrophotometer for COD determination. 2.4 Procedure for Biological oxygen demand The BOD level of one of the two samples in a test tube was recorded using dissolved oxygen meter immediately after being prepared. The second sample was placed in an incubator at 20oC for five days. After five days, another BOD level was recorded, and the difference between the two BOD values was used to determine the BOD level of the sample in mg/L. 3. Results Figure 1: A graph of Alkalinity vs Weeks Figure 2: A graph of COD vs Weeks Figure 3: A graph of BOD vs Weeks Figure 4: A graph of ORP vs Weeks Calculations Total alkalinity, mg/L as CaCO3 = (Total volume of acid used for titration × Normality of standardized acid × 50000 mg/eq)/ Total volume of sample titrated = (7.9 mL × 1.06 N × 50000mg/eq)/ 5.99 mL = 69.9 mg/L as CaCO3 Set 1: BOD = (Initial Dissolved Oxygen – Final Dissolved Oxygen) × (300/mL) = (8.92 - 1.3) × 300/mL = 2286 mg/L Set 2: BOD = (Initial Dissolved Oxygen – Final Dissolved Oxygen) × (300/mL) = (8.92 - 4.7) × 300/mL = 1266 mg/L Average BOD = (2286 + 1266) mg/L/2 = 1776 mg/L 4. Discussion The total alkalinity of the sample was found to be 69.9 mg/L as calcium carbonate, a value which agrees with literature values that set total alkalinity of surface water at < 200 mg/L as CaCO3 (Varol et al., 2012). The presence of anions such as phosphates, carbonates, hydroxides, and bicarbonates in the sample were responsible for the increased alkalinity in the analyzed sample. These species combine with hydrogen ions in the sample to form other compounds hence reduce the concentration of hydrogen ions in the sample leading to increasing pH level. The alkalinity in column three remains almost constant from the first week to the last one while it increases in the other five columns (fig. 1). The biological oxygen demand level in column three remained almost constant for the nine weeks of the experiment (fig. 3). It increased significantly in column five and decreased significantly in column four. Incubation of the samples accelerates the multiplication of the microorganisms in the samples. An increase in the decomposition of the organic matter leads to high demand for oxygen resulting in increased BOD levels. Increased BOD levels mean a decrease in the dissolved oxygen levels because the microorganisms present in the sample use the available oxygen. In the third column, the chemical oxygen demand levels increased steadily from the first day of the experiment to the last day. COD in the other five columns increased for the first two three weeks, declined between the fourth and the seventh week and finally increased to the last week (fig. 2). Oxidation of the organic matter present in the sample to products such as carbon (IV) oxide and water is responsible for the increase in COD levels (Almeida et al., 2012). In comparison to the set up in columns two, three, four, five, and six, the values of chemical oxygen demand, levels recorded in the first column were the least. The oxidation-reduction potential of the sample in column three increased between week three and four, declined between week four and six and then increased with a decrease in the pH of the solution. The same trend is observed in the other five columns. The oxidizing chemical species in the sample pulls electrons from the cell membrane, destabilizing the cell membrane and making it leaky hence the fluctuation in the ORP values. The conductance value increases significantly in all the columns from the first day of the experiment to the last day. Conductance is the inverse of conductivity and, therefore a decrease in conductivity leads to an increase in conductance. Electrons responsible for the conduction process are lost during the reduction-oxidation process leading to the decrease in conductivity. Conclusion The total alkalinity of the sample was determined to be 69.9 mg/L as CaCO3 a value that conforms to the literature one for surface waters. The biological oxygen demand value was found to be 1776 mg/L while the chemical oxygen demand value was determined to be 2178.5 mg/L. The oxidation-reduction potential value for the sample was determined to be -117.7 mV and the leachate volume was 33 mL. The COD levels in column three increased significantly throughout the nine weeks. Chemical oxygen demand helps to determine the amount of oxygen required for oxidation of organic matter present in a water sample, under specific conditions of temperature, time, and oxidizing agent. The BOD levels in column three remained constant for the nine weeks while it increased steadily in column five and decreased in column four. The oxidation-reduction potential of all the columns increased within the first two weeks of the experiment, declined between the fourth and the six and eventually increased. References Almeida, C. A., González, P., Mallea, M., Martinez, L. D., & Gil, R. A. (2012). Determination of chemical oxygen demand by a flow injection method based on microwave digestion and chromium speciation coupled to inductively coupled plasma optical emission spectrometry. Talanta, 97(2012), 273-278. Kruse, N. A., DeRose, L., Korenowsky, R., Bowman, J. R., Lopez, D., Johnson, K., & Rankin, E. (2013). The role of remediation, natural alkalinity sources and physical stream parameters in stream recovery. Journal of Environmental Management, 128(2013), 1000-1011. Stumm, W., & Morgan, J. J. (2012). Aquatic chemistry: Chemical equilibria and rates in natural waters. Hoboken, NJ: John Wiley & Sons. Varol, M., Gökot, B., Bekleyen, A., & Şen, B. (2012). Spatial and temporal variations in surface water quality of the dam reservoirs in the Tigris River basin, Turkey. Catena, 92(2012), 11-21. Read More