Experiment 14
How healthy is my water?
Pre-Lab Assignment
- Prepare chemicals and containers needed for water sampling field trip.
- Read the lab thoroughly.
Purpose
The purpose of this experiment is to determine the concentration of common species in water to determine the water’s quality and to determine of the water is safe for humans and other organisms
Homework - Sampling the Water Supply
Checklist for the Field Trip
By the end of the field trip you should have done the following:
1. Written down general observations about the site in your notebook.
2. Taken water samples in carefully labeled bottles.
3. Taken any field measurement desired (temperature, etc)
Making Field Observations
The first goal when you arrive at the site is to familiarize yourself with it. This is best done walking around the site. Write down your observations in our laboratory notebook. This may be your only chance to visit the site so write down as thorough a description as you can. e.g., "approximately 6 feet out from the large boulder downstream from the west side of the bridge." At a minimum, note the following:
1. Take particular note of any potential pollution sources such as animals, birds, human activities, pipes that might indicate a pollutant source, debris, and the behavior of any aquatic life.
2. Note the condition of the water. Is it clear? Cloudy? Smelly? Foamy?
3. If you do not already have a map of the site, sketch one in your notebook. Indicate the exact location you will be taking your sample on your map. You will also need print a map from the internet of the location either before or after taking the sample.
4. If you have access to a camera, you may want to take photos of the area.
Labeling the Sample Bottles
In order to avoid confusion during sample analysis in the laboratory, careful labeling is extremely important. Some do's and don'ts follow:
•DO write your name on the bottles and number them. Use the numbers to refer to your samples in your notebook.
•DON'T use water-soluble ink to write on the sample bottles. Use an indelible marking pen instead(Sharpie) or a pencil. There is nothing more frustrating than tohave your label dissolve!
•VERY IMPORTANT! Label the sample bottle with indelible ink before immersing it in the water. Pens do not write well on wet surfaces, and water-soluble inks will disappear instantly in water.
Taking the Sample: A Few Tips
•Remember that your fingers are "dirty" in the sense that they will add ionic substances to the water sample. Do not touch the inside of the sample container or the container lid.
•When taking a water sample from a moving source, position the bottle so the mouth of the bottle is pointing upstream and your fingers (holding the bottle) are downstream of the bottle opening. If the water is not moving such as from a pond, try to keep your fingers far from the bottle lip.
•When taking a water sample, first rinse the sample bottle with a bit of the sample.
This is a good habit to get into for both sampling and carrying out the analytical procedures.
Be sure to fill your bottle to the very top, leaving no room for air. You may have to use a cup or other container to help in this.
Ideally you will collect your sample in such a way that you can cap the bottle while it is still submerged. That means that you must be able to reach into the water with both arms and the water must be deeper than the sample bottle. If this is not possible, do your best not to add atmospheric oxygen by agitating the sample.
Figure 1 Ideal collection of water sample.
Preserving and Storing the Sample
It might not always be possible to analyze a sample as soon as you arrive back in the laboratory and for this experiment, you will likely keep your sample for several weeks. Itisimportant to store the sample correctly so that no changes occur in the sample. The presence of oxygen (if the sample bottle is not completely full) and bacteria in a sample are the biggest culprits for changing the concentrations of different analytes. Filtration of the sample through 0.45 m pore-size filters will remove most bacteria and slow the degradation of the sample. This will be done on this first day of the lab. Until then, store your sample in your refrigerator.
Sample maybe stored safely using the following guidelines
•Storage before filtration should be at 4o C for no more than 2 days.
•Storage after filtration should be at 4°C for no more than 30 days.
Lab Day 1-
Laboratory Measurements of Water Samples Composition
pH of water sample
As you are aware, pH is a measure of the acidity or basicity of a substance. The table below shows the pH of some common solutions. Notice that the pH of rainwater is significantly lower than that of pure water. Why is this so?
As rain falls through the air, carbon dioxide in the air dissolves in the rainwater and reacts with the water to form carbonic acid.
CO2(g) + H2O(l) H2CO3 (aq)
Carbonic acid then reacts with water to produce hydronium ions, whichlowers the pH of rainwater to approximately 5.7.
H2CO3 (aq) + H2O(l) H3O+ + HCO3-
It is important to note that this slightly acidic pH value is for “pure” rainwater. Pollution from the air or from the ground may lower the pH even further to form “acid rain”. Also recall the fact that for each change of one pH unit, the actual change in concentration of hydronium ions that has occurred is a very large tenfoldchange! For example, a solution of pH 5 has 10timesthe concentration ofhydronium ions as a solution of pH 6
Effects of pH on the Qualityof Natural Waters
The pH of natural waters has major consequences for the organisms that live there. Acidification is a particular problembecause more human inputs into natural systems are acidic than basic. A decrease in pH much below the neutral value of7.0 can result in a variety of effects on lakes, rivers, and streams and their inhabitants. For example,
1. The solubility of many minerals increases at lower pH values.In areas where there are naturally high concentrations of metal ores, the result can be a release of toxic metal ions (e.g., AI+3, Pb+2, etc.) into the environment.
2.A low pH dissolves the calcium from the shells of crustaceans and mollusks, weakening them and making the animals more susceptible to physical damage as well as to predators and dis ease.
3. Acid disturbs the balance in ion uptake by fish. Fish require a balance of sodium, potassium, calcium, and chloride ions in their blood. In acidic waters, sodium ions are lost through the gills and cannot be replaced quickly enough to maintain the desired level in the blood. When the balance of ions is disturbed beyond a certain point, the fish die. Although some species of fish are more tolerant to waters with low pH than others, none can survive in waters with a pH much below 4.5.
pH of Drinking Water
Humans and animals can tolerate fairly large extremes in the pH of their drinking water. Consider the fact that most soft drinks have a pH between 2 and 4. Raw (untreated) public water supplies typically have a pH between 4 and 9, with the majority having a pH between 5.5 and 8.6. After treatment, most public water supplies have a pH between 6.9 and 7.4. The acceptable pH range for drinking water as dictated by the Public Health Service Act is 6.5-8.5.
The major health problem related to the pH of drinking wateris that very acidic solutions increase the solubility of metals such as lead, copper, zinc, and iron. Ifpipes in plumbing systems are made of these metals, water with a low pH will corrode the pipes, and the metal ions will contaminate the drinking water. Of particular concern is lead, since lead is a cumulative toxin. The ancient Romans used lead to make pipes to carry water, hence the word "plumbing" is derived from the Latin word for lead, plumbum(chemical symbol, Pb). The pipes in many water distribution systems built before 1986 contain lead, and if the pH is less than 7, lead leaches out ofthe pipes and into the drinking water. Thus it is in the best interest of the public health if the pH of drinking water is not acidic.
Procedure for Determining the pH of Water
Computer Setup and pH Sensor Calibration
Materials
To calibrate the pH Sensor you will need the following: wash bottle, distilled water, three beakers (50-150 mL in size), buffer solutions of high pH (e.g. pH 7) and low pH (e.g. pH 4), pH Sensor.
1. Turn on Science Workshop interface and the computer if they are not already on.
2. Connect the plug of the pH Sensor to Analog Channel A on the interface.
3. On the desktop, open the program Data Studio, then select Open Activity, and then open the file phmeters.sws in the Chem 1B folder on the desktop.
4. Put distilled water into the wash bottle and into one of the beakers. Put about 50 mL of the pH 4 buffer solution in one of the other two beakers and about 50 mL of the pH 7 buffer solution into the third beaker.
5. Remove the pH electrode from its storage bottle of buffer solution.
6. Use the wash bottle to rinse the end of the electrode. If the pH electrode has not be soaking while stored, soak the electrode in a beaker of distilled water for 10 minutes.
7. In the Experiment Setup window, click Calibrate Sensors at the top.
8. Place the pH sensor in the pH 4 buffer.
9. Find were it says “Calibration point 1” and in the “Standard Value Box” check that the value is set to 4.000. If is does not say 4.000, change it to 4.000.
10. When the voltage stabilizes, click on “ Read from Sensor”
11. Thoroughly rinse the pH electrode with distilled water and dry it with a tissue.
12. Place the pH sensor in the pH 7buffer.
13. Find were it says “Calibration point 2” and in the “Standard Value Box change the value to 7.000.
14. When the voltage stabilizes, click on “ Read from Sensor”.
15. Click OK to end the calibration.
16. Thoroughly rinse the pH electrode with distilled water from the wash bottle and dry gently.
17. Place in the pH meter in your sample and record its pH.
Determination of pH of water sample
1. Take the pH of your unfilteredsample by filling a clean100-mL beaker with enough sample to immerse the electrode into the beaker. Stir the solution, allow the reading to stabilize for about 1minutes, and read the display to obtain the pH of your sample. Note this value in your notebook.
2. When done measuring the pH of your water, you may pour it back into your sample bottle to save for further analysis.
Note: If the pH of your sample is below 4.5, there is no need to determine total alkalinity, since the sample will have noacid neutralizing capacity. If the pH is above4.5, you will need to determine the total alkalinity of your sample in the next lab.
Filter your water sample
Before continuing your analysis, your sample will need to be filtered to remove any bacteria that may effect the concentrations of ions. This will be done with a 0.45 m filter as demonstrated by your instructor. It will be necessary to “pre-filter” your sample through cheesecloth or a course filter to remove large particles first in most situations.
Total Dissolved Solids
Purpose
To determine the concentration of total dissolved solids in a solution by filtering the sample, evaporating the water at 103-105°C, and weighing the residue remaining after evaporation of the water.
What are total solids and why are they important?
The total amount of dissolved chemical species in water is called total dissolved solids, abbreviated TDS, and is a good general measure of the concentration of ionic substances in water. In general, fresh water has less than 1,500 mg/L ofTDS, brackishwater between 1500 and 5000 mg/L TDS, and saline water above 5000 mg/L TDS.
Seawater has a TDS content of 30,000-40,000 mg/L. The amount of dissolved solids in the water is called the water’s salinity and has units of mg of solids per gram of solution. The oceans and some inland lakes or seas such as the Great Salt Lake in Utah, Mono Lake in California, or the Dead Sea between Israel and Jordan have a large quantity of dissolved solids (mostly sodium chloride or calcium carbonate) and thus have high salinity.
Fresh water in rivers, streams, and rainwater typically has very low salinity. Estuarine waters, where rivers and streamsmix with ocean water, have intermediate salinity that decreases
from the mouth of the river inland. Although most estuarine species can tolerate some variation in salinity, they are best adapted to a particular zone of salinity. When the salinity of a body of water increases significantly over a short period of time, many species die. Thus, it is important to ensure minimum levels of in stream flow to maintain the ecological health of the plants and animals that live in or adjacent to a river or stream. This is particularly critical in delta areas where a river empties into the ocean, since these areas are important breeding grounds for many aquatic species.
The table to the right lists the major ionic constituents of an average river. In river water, dissolved solids consisting of calcium, chlorides, nitrate, phosphorus, iron, sulfur, and other ions particles that will pass through a filter. Samples will also contain suspended solids include silt and clay particles, plankton, algae, fine organic debris, and other particulate matter. These are particles that will not pass through a filter.
The concentration of total dissolved solids affects the water balance in the cells of aquatic organisms. An organism placed in water with a very low level of solids, such as distilled water, will swell up because water will tend to move into its cells, which have a higher concentration of solids through the process of osmosis. Similarly, an organism placed in water with a high concentration of solids will shrink somewhat because the water in its cells will tend to move out. This will in turn affect the organism's ability to maintain the proper cell density, making it difficult to keep its position in the water column. It might float up or sink down to a depth to which it is not adapted, and it might not survive. In addition, a high concentration of total solids will make drinking water unpalatable and might have an adverse effect on people who are not used to drinking such water. Levels of total solids that are too high or too low can also reduce the efficiency of wastewater treatment plants, as well as the operation of industrial processes that use raw water.Total solids also affect water clarity. Higher solids decrease the passage of light through water, thereby slowing photosynthesis by aquatic plants. Water will heat up more rapidly and hold more heat; this, in turn, might adversely affect aquatic life that has adapted to a lower temperature regime.
Total solids measurements can be useful as an indicator of the effects of runoff from construction, agricultural practices, logging activities, sewage treatment plant discharges, and other sources. As with turbidity, concentrations often increase sharply during rainfall, especially in developed watersheds. They can also rise sharply during dry weather if earth-disturbing activities are occurring in or near the stream without erosion control practices in place. Regular monitoring of total solids can help detect trends that might indicate increasing erosion in developing watersheds. Total solids are related closely to stream flow and velocity and should be correlated with these factors. Any change in total solids over time should be measured at the same site at the same flow.
Sources of total solids include industrial discharges, sewage, fertilizers, road runoff, and soil erosion. Total solids are measured in milligrams per liter (mg/L) or ppm.
Gravimetric Determination of Total Dissolved Solids (TDS)
We will determine the total dissolved solids by taking a known volume of the sample of filtered water, and carefully evaporate the water. When all the water has evaporated, a dry residue will remain consisting of the constituents that were previously dissolved in the water. The dry residue can be weighed in order to determine the weight of dissolved solids in mg per liter of water (ppm).
Procedure
1. Label a clean 150-mL beaker with your sample number and name and place it in a drying oven at 103-105°C for at least 1hour. Remove the beaker from the oven, place it in a desiccator until cool, and then weigh the beaker to the nearest 0.1 mg, recording the weight in your laboratory notebook .
2. Place 50.0 mL of the filtered water sample into the clean, dry beaker.
3. Place the beaker on wire gauze over a Bunsen burner and heat the sample to just below boiling to reduce the volume of the sample to approximately 10 mL. Do not allow the sample to boil over or splatter.
4.Allow the beaker to cool and place it in the oven at 103-105°C until the nextlaboratory period.
5. Day two- Remove the beaker from the oven and place it in a desiccator to cool to room temperature, then reweigh to the nearest 0.1mg, recording the weight in your laboratory notebook.
Calculations
Calculate the total dissolved solids in milligrams of solids per liter of solution.