Investigating Water Pollutants and Water Analysis
Objectives:
- Understand the importance of a safe water supply.
- Gain a basic understanding of water pollution and water analysis.
- Measure pH and other characteristics of water samples.
- Compare the properties of different water samples.
- Understand treatment methods of contaminated water.
- Explore various prevention methods of water pollution.
Background:
Water is an essential compound for all living things and is found in all parts of the environment. The earth’s water supply exists as surface water such as oceans, lakes and rivers, underground water, solid water in glaciers and the polar ice caps, and in the atmosphere as water vapor. Throughout history, access to a safe water supply has been an important part of human life. Water pollution has always been a problem for mankind, but as the population continues to grow it becomes a bigger problem every day. The development of sewer systems and water treatment facilities has improved the lives of many people, but there are still many problems that exist.
Water pollution can be caused by many sources, both natural and man-made. Many deadly diseases caused by bacteria, parasites and viruses have been spread through water supplies in the past, but these are very rare in developed countries today. The major threat to the water supply in industrialized countries is from toxic chemicals that can be introduced to the water in different ways. Some of these sources include wastes from industry and municipal sewage systems, pesticide and herbicide run-off from farms, illegal dumping, and leaking storage tanks.
Fortunately, there are many things that can be done about water pollution, the most important being prevention. However, even when the greatest care is exercised, some pollution always exists. There are many different water analysis techniques that are used to test water for various pollutants. These tests can warn people about a dangerous water supply. Water treatment facilities are able to remove or neutralize many of the contaminants. In order to do this efficiently, however, the water being treated must be tested to know what kinds of problems there are that need to be corrected.
Water pollutants are typically measured in parts pre million (ppm), parts per bioolion (ppb), and even parts per trillion (ppt). Although these quantities of pollutants cannot be typically seen, water quality laboratories can detect these extremely small concentrations in water. This measurement system has been in use since the early 1900s and uses the notation of “grams of element in grams of water”. There are 1000 grams of water in one liter, so if we have one thousand liters of water, there are a million grams. Therefore, if one gram of an element, such as nitrogen, is dissolved in a million liters of water, we would end up with a solution with 1 ppm of nitrogen.
Water treatment plants typically clean water by taking it through a series of aeration, coagulation, sedimentation and filtration processes as well as chemical disinfection. “Aeration” is the addition of air to water. It allows gases trapped in the water to escape while adding oxygen to the water. During the “coagulation” process, the dirt and other suspended solid particles are chemically “stuck together” into “floc” (i.e. large particles) so that they can be removed. “Sedimentation” is the process that occurs when gravity pulls the particles of floc (clumps of alum and sediment) to the bottom of the water collection site. “Filtration” through sand and pebbles removes most of the impurities remaining in the water after coagulation and sedimentation have taken place. During the final treatment phase, chemical disinfectants are added to kill any disease-causing bacteria and microorganisms.
In this investigation, you will test unknown water samples for the six major factors that affect water quality.
Dissolved oxygen concentration is a key indicator of water quality. Natural waters are saturated with oxygen. However, high levels of bacteria from sewage pollution or large amounts of decomposing plants decrease oxygen levels. The ideal dissolved oxygen concentration in water is over 10 ppm.
Copper in drinking water is often the result of copper leaching from rock weathering. Other major sources include corrosion of brass and copper piping and the addition of copper salts used to treat algae growth. The maximum amount of copper allowed by the EPA in drinking water is 1.3 mg/L. High doses of copper consumption can lead to liver damage or anemia. Copper can be removed from the water by filtering it through activated charcoal.
Nitrates are the most common contaminants found in water. Nitrates occur in water as a result of seepage through nitrate-bearing rocks or soils. Nitrates may also come from septic systems, fertilizers or pollution of organic wastes. A condition known as “cyanosis” or “blue baby syndrome” occurs in infants who drink water contaminated with nitrates. Nitrates (NO3) are converted to nitrites (NO2) in the body. Because hemoglobin has a stronger affinity for nitrites than for oxygen, if enough nitrates are consumed by infants, they will turn “blue” due to oxygen starvation. The EPA maximum limit for nitrates is set at 10 ppm. Nitrates can be removed from drinking water through a process known as “reverse osmosis” or by a nitrate selective anion resin water conditioner.
Phosphates are nutrient chemicals containing phosphorus and are essential for all life on earth. A major source of phosphates is fertilizers and industrial waste discharges. Excessive amounts of phosphates can cause algal blooms in ponds and lakes. Acceptable phosphate concentration is less than 4 ppm.
Hardness is caused by excessive calcium and magnesium in water. Hard water is found in over 80% of the United States. Hardness causes scale deposits in water pipes, increases soap consumption, and affects the taste of the water. Water with a calcium and magnesium level of over 85 ppm is considered hard. Hardness is removed by treating water with a conditioner or water softener which removes the calcium and magnesium from the water. The conditioners or water softeners replace the calcium and magnesium with minerals that do not form scale deposits.
Chlorine is the most widely used disinfection agent of municipal water supplies. A safe chlorine concentration in drinking water is between 0.1 and 0.4 ppm. Chlorine exists in water in its free state or can combine with organic compounds, forming potentially carcinogenic compounds. To remove chlorine from water, it can be passed through an activated charcoal filter.
The term “pH” is used to indicate “acidity” or “alkalinity” of a given solution. All natural waters typically fall within 6.0 and 8.0 pH range. PH levels below 7 are considered “acidic” while those above 7 are “alkaline”. A pH of 7 is considered “neutral”. Acidic water is typically caused by industrial waste contamination. The pH of acidic water can be raised by adding a base, such as sodium hydroxide. The lower the pH of water, the greater the corrosive tendency of the water, which also contributes to iron staining problems. The ideal pH range for water use in the home is between 7 and 9.
Safety & Disposal:
Be sure to follow proper lab safety techniques as directed by your teacher. Always wear safety gloves, goggles, and a lab apron, to protect your eyes and clothing when working with any chemicals or whenever handling collected soil and water samples. Be sure to keep your ands away from your face and mouth.
Disinfect your area and dispose of any waste materials at the end of the investigation as directed by your teacher. Reacted water samples may be disposed of by flushing them down the drain, followed by copious amounts of water. Always wash your hands after handling the tablets and before leaving the laboratory.
Materials Needed Per Group:
- 2 Chlorine test tablets - 2 Copper test tablets
- 4 Dissolved Oxygen test tablets - 1 Graduated cylinder, 100mL
- 2 Hardness test tablets - 2 Nitrogen test tablets
- 2 pH test strips - 2 Phosphate test tablets
- 3 Pipets, plastic - 1 Thermometer
- 2 Vials with cap, 10mL - 100mL Water sample, collected
- 100mL Water sample, unknown (assigned)
Activity 1: Testing the Dissolved Oxygen
What to do…
You will use the colorimetric method to measure the dissolved oxygen content in water. During such a method, a reagent is added to a water sample which is oxidized. A color change is then produced in proportion to the amount of oxygen present in the water sample. This color is compared to a key provided.
The percentage of dissolved oxygen saturation is an important measurement of water quality. Cold water can hold more dissolved oxygen than warm water. Dissolved oxygen is typically measured in ppm and can be converted to percentage saturation using the following chart:
Collecting Water Samples
The sample collection technique used is important in dissolved oxygen measurement. Collected water samples should be tested for dissolved oxygen levels as soon as they are collected. In this exercise, you will compare the dissolved oxygen content of two water samples. Water can be collected from a fish aquarium, pond, lake or any other local water source.
Using a thermometer, measure the temperature of the water you are about to test. To collect your samples, fully submerge a small vial into the water sample. Carefully remove the vial from the water sample. Ensure that the sample vial is completely full and that it does not have any bubbles. Proceed immediately with the testing procedure, as soon as the sample is collected.
Procedure:
1. Drop two dissolved oxygen test tablets into the vial with your sample water. The water should overflow when the tablets are added.
2. Screw a cap on the vial, ensuring that there are no air bubbles in the water sample.
3. Gently mix the sample by inverting the vial over and over until the tablets have completely dissolved. This will take 4-5 minutes.
4. Wait 5 additional minutes for the color to develop fully in your water test sample.
5. Hold the vial upright and compare the color of the water test sample to the “Dissolved Oxygen Color Chart” to obtain your result.
6. Locate the temperature of the water sample on the “% Saturation Chart”. Locate the Dissolved Oxygen ppm result of the water sample at the top of the chart. The percentage saturation of the water sample can be found when the temperature row and the Dissolved Oxygen column intersect. Record your results in Data Table 1.
Activity 2: Testing for Copper
What to do…
The amount of copper in water is measured by adding an indicator reagent to the water sample. The test reagent combines with copper to produce a blue color. This color is then compared to a key provided.
Procedure:
1. Fill a test vial with 10mL of your collected water sample.
2. Drop one copper test tablet into the vial with your water sample.
3. Screw the cap on the vial. Gently mix the sample by inverting the vial over and over until the tablet has disintegrated. Bits of material may remain suspended in solution.
4. Hold the vial upright and compare the color of the water test sample to the “Copper Color Chart” provided. Record your results in Data Table 1.
Activity 3: Testing for Nitrates
What to do…
The nitrate content in solution is measured using the indicator reagents contained in a nitrate test tablet. The zinc in the tablet reduces nitrate into nitrite which in turn reacts with the indicator, chromotropic acid to produce a pink color in proportion to the amount of nitrites present.
Procedure:
1. Fill a test vial with 10mL of your collected water sample.
2. Drop one nitrate test tablet into the vial with your water sample.
3. Screw the cap on the vial. Gently mix the sample by inverting the vial over and over until the tablet has disintegrated. Bits of material may remain suspended in solution.
4. Wait 5 minutes for the red color to develop.
5. Hold the vial upright and compare the color of the water test sample to the “Nitrate Color Chart” provided. Record your results in Data Table 1.
Activity 4: Testing for Phosphates
What to do…
You will test a water sample collected from a local pond, river, lake or even from a water puddle after a rainfall.
Procedure:
1. Fill a test vial with 10mL of either your collected water sample.
2. Drop one phosphate test tablet into the vial with your water sample.
3. Screw the cap on the vial. Gently mix the sample by inverting the vial over and over until the tablet has disintegrated. Bits of material may remain suspended in solution.
4. Wait 5 minutes for the blue color to develop.
5. Hold the vial upright and compare the color of the water test sample to the “Phosphate Color Chart” provided. Record your results in Data Table 1.
Activity 5: Testing for Hardness
What to do…
Water hardness is measured by adding an indicator tablet to the sample of water. If the water is “soft” a blue color develops. But, when calcium or magnesium are present in a “hard” water sample, the indicator combines with them to form a red-colored solution. The hardness water test you will perform is qualitative and will indicate whether the water sample is considered “soft” or “hard”.
Procedure:
1. Fill your test vial with 10mL of your water sample.
2. Add one hardness test tablet.
3. Screw the cap securely on the vial and shake gently until the tablet has fully disintegrated.