Materials Included In Kit

Additional Materials Required

Prelab Preparation

Sodium chloride solution, 0.5%: Use a balance to weigh out 0.5 g of sodium chloride. Transfer to a labeled volumetric flask and dilute to 100 mL with deionized water.

Sodium chloride solution, 2.0%: Use a balance to weigh out 2.0 g of sodium chloride. Transfer to a labeled volumetric flask and dilute to 100 mL with deionized water.

Sodium chloride solution, 4.0%: Use a balance to weigh out 4.0 g of sodium chloride. Transfer to a labeled volumetric flask and dilute to 100 mL with deionized water.

Unknown water sample 1: Use a balance to weigh out 1.0 g of sodium chloride. Transfer to a labeled volumetric flask and dilute to 100 mL with deionized water.

Unknown water sample 2: Use a balance to weigh out 3.5 g of sodium chloride. Transfer to a labeled volumetric flask and dilute to 100 mL with deionized water.

Safety Precautions

Silver nitrate is corrosive and highly toxic; avoid contact with eyes and skin. Silver nitrate will stain skin and clothes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The leftover solutions contain a silver chloride precipitate and sodium nitrate in solution. Filter the precipitate through filter paper, place the filter paper into a plastic bag, and place in the garbage according to Flinn Suggested Disposal Method #26a. Flush the filtrate down the drain with excess water according to Flinn Suggested Disposal Method #26b. Excess fluorescein and sodium chloride may be stored for future use or rinsed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b. Excess silver nitrate may be stored for future use or treated according to Flinn Suggested Disposal Method #11, Procedure B, silver disposal.

Lab Hints

• Enough materials are provided in this kit for 30 students working in groups of three or for 10 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• Use local, natural sources for the unknown samples. Adjust the three known samples to bracket the likely concentrations of the unknown samples.
• To convert percent to parts per thousand, multiply by 10. To convert percent to parts per million, multiply by 10,000. The unit, parts per thousand (also seen as ppt or ‰) is the unit most often referenced in oceanography and biosalinity agriculture references. The unit ppt has recently been replaced by the unit practical salinity units, or psu. A special instrument called a CTD (conductivity-temperature-depth) is used to measure psu.
• Students with access to a computer spreadsheet program such as Excel^®, may use the program to graph the data table using the XY (scatter) chart and then insert the linear (least squares) the trend-line.

Teacher Tips

• For a realistic scenario refer to Well Wishes: A Case Study on Septic Systems and Well Water featured in the Journal of Chemical Education, Volume 81, Number 2, February 2004, pages 218–220 or visit the authors’ website at http://www.stolaf.edu/people/walczak/cases.html (accessed May 2007).

Answers to Prelab Questions

1. During which season would the lake closest to your school have the highest saline concentration? Explain.

   Generally speaking, in the United States the highest saline concentration will occur in the late summer when there is less rain and higher temperatures (more evaporation).

2. Dehydration and crop failure can both be attributed to osmosis. Explain how a high salt concentration causes dehydration and crop failure. Reference your textbook, if necessary.

   Osmosis is the movement of water molecules from an area of high concentration to an area of low concentration. Water inside the plant or animal cells will move from inside the cell to the salty area. The loss of intercellular water causes the cell membranes to shrivel and eventually to die because the cell will not have enough water to carry on its normal processes.

Sample Data

Answers to Questions

1. Graph the titration results for the standard solutions: Plot the average number of drops of 1 M silver nitrate added on the y-axis versus the concentration of sodium chloride (%) on the x-axis. Using a straightedge, draw a best-fit straight line through the origin and the data points.
   {10841_Answers_Figure_1}
2. Using the graph, determine the concentration of sodium chloride in the two unknown samples.

   According to the graph, unknown sample 1 contains 3.4% salt and unknown sample 2 contains 1.2% salt.

3. Classify the unknown samples as fresh, brackish, marine or brine based on the concentration of sodium chloride. Explain.

   Unknown sample 1 contains 3.4% salt, which is within the range for marine water. Unknown sample 2 contains 1.2% salt, which is within the range for brackish water.

4. The water in one location along an estuary was sampled every hour for 24 hours. The 24 samples were analyzed using the titration method used in this experiment. The data table below lists the concentration of sodium chloride for each water sample. Based on these results, determine the approximate times of high tide and low tide that day.
   {10841_Answers_Table_3}

   High tide occurred at 4 am and 4 pm. Low tide occurred at 11 am and 10 pm. During high tide the estuary water is primarily ocean water and therefore it has the highest concentration of sodium chloride. During low tide the estuary water is primarily fresh water from the inland watershed and therefore during low tide the sodium chloride levels are at the lowest point of the day. [Tide information is from the lower Hudson River, Poughkeepsie, New York for July 4, 2007, as predicted at www.saltwatertides.com (accessed June 2007).]

References

Harris, Daniel. Exploring Chemical Analysis; 1997; W. H. Freeman & Co., New York, pp 110–113.

Introduction

Approximately 70% of the Earth’s crust is covered in water, with oceans accounting for 97% of all the water. Saltwater or saline water, like that found in the ocean, contains significant amounts of dissolved salts, including sodium chloride, potassium chloride and calcium chloride. Given that there is so much water on Earth, why is there so much concern about “water shortages” all across the world?

Concepts

• Salt toxicity
• Salinity
• Calibration curve

Background

The Earth has a limited amount of freshwater. Freshwater contains less than 1% dissolved salt, usually sodium chloride, and is found in rivers, lakes, groundwater and in subterranean aquifers. All naturally occurring water has some material dissolved in it. Solids, liquids or gases from the soil, precipitation or the atmosphere may dissolve in the water by erosion, mixing and diffusion, respectively. The difference between freshwater and saline water is that saline water contains more dissolved salts (see Table 1). Ocean water contains about 3.5% dissolved salts, including sodium chloride, magnesium chloride, calcium chloride and potassium chloride. The concentration of salt in ocean water and saline lakes varies by location and by season. For example, a hot, shallow area of the ocean may contain 3.8% dissolved salts while a cool area with a lot of glacier melt may contain 3.2% salt. Aquatic plants and animals have adaptations that allow them to survive in freshwater or saltwater environments. Most terrestrial plants and animals must ingest freshwater in order to survive.

At high concentrations, sodium chloride is toxic to most terrestrial plants and animals, including humans. Ingesting large amounts of salt creates an electrolyte imbalance in humans. The kidneys attempt to “flush out” the excess sodium chloride, resulting in severe dehydration that can lead to organ failure and death. In the United States, drinking water must contain less than 0.4% chloride (see Table 1). The drinking water standard is mandated by the United States Environmental Protection Agency. This level can be difficult to achieve in areas of the country where large deposits of sodium chloride occur. Many areas of the United States contain significant amounts of salt. In fact, the United States is the largest producer of salt in the world. States, such as New York, Michigan, Kansas, Texas, California, Nevada and Utah, currently have active salt mining.

The amount of sodium chloride present in water and soil also affects the ability of areas with high salt concentrations to produce enough food to survive. Whether this is because the area is arid and the only source of irrigation water is the ocean, or because the ground itself contains large amounts of sodium chloride, traditional crops and grasses fail to thrive in excessively salty areas. A few species of plants flourish in salty environments. Plant biologists and geneticists study these salt-tolerant plant species in order to develop new crop species for the areas with high salt content.

In order to determine the amount of salt in water the sample is titrated with silver nitrate. The silver ion in silver nitrate reacts with the chlorine ion in salt to form silver chloride, a solid. The silver and chlorine react in a one-to-one ratio. Therefore, if the amount of silver added to the water sample is known, the amount of chlorine ion can be calculated. The exact point, called the endpoint, at which the chlorine has been completely reacted, is difficult to determine without the use of an indicator. The negatively charged indicator fluorescein changes from yellow to pink when it absorbs onto the positively charged crystal surface of the silver chloride.

Experiment Overview

The purpose of this experiment is to determine the salinity (concentration of salt) of two unknown samples. Different sources of naturally occurring or simulated water may be tested including local rivers, lakes, streams, oceans or drinking water. Three samples containing known amounts of salt will be analyzed using micro-titration techniques. The titration results for the known samples will be plotted on a graph to obtain a saline calibration curve, which will then be used to determine the concentration of the two unknown samples.

Materials

Prelab Questions

1. During which season would the lake closest to your school have the highest saline concentration? Explain.
2. Dehydration and crop failure can both be attributed to osmosis. Explain how a high salt concentration causes dehydration and crop failure. Reference your textbook if necessary.

Safety Precautions

Silver nitrate solution is toxic and irritating to body tissue; avoid contact with eyes and skin. Silver nitrate also will stain skin and clothes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines.

Procedure

1. Rinse the 24-well reaction plate with deionized water and shake the water from the wells. Use cotton swabs to dry each well. Place the reaction plate on a sheet of white paper.

Standard Solutions

2. Rinse the syringe with the 0.5% sodium chloride solution.
3. Measure and add 1.0 mL of the 0.5% sodium chloride solution to each of three adjacent wells, A2, B2 and C2.
4. Using the thin-stem pipet, add one drop of 0.1% fluorescein to wells A2, B2 and C2. Stir with a clean, plastic toothpick.
5. Using a micro-tip pipet, slowly add the 1 M silver nitrate one drop at a time to the 0.5% sodium chloride solution in well A2. Stir with a plastic toothpick after each drop is added. Count the exact number of drops that must be added for the solution to turn the peachy-pink in color seen in well A1. Record the number of drops of silver nitrate in the data table on the Salinity Worksheet.
6. Repeat step 5 two more times for the 0.5% sodium chloride solution in wells B2 and C2. Use a clean toothpick to stir each solution. Record the number of drops of silver nitrate needed for each sample (trial) in the data table on the Salinity Worksheet.
7. Repeat steps 2–6 four more times, using first the 2.0% sodium chloride solution (wells A3, B3 and C3) and then the 4.0% sodium chloride solution (wells A4, B4 and C4), followed by unknown sample 1 (wells A5, B5 and C5) and unknown sample 2 (wells A6, B6 and C6). Record all data on the Salinity Worksheet.
8. Calculate the average number of drops of silver nitrate added for each known solution and each unknown sample. Record the average number of drops for each sample on the Salinity Worksheet.
9. Return any extra silver nitrate, fluorescein, and sodium chloride solutions to the teacher.
10. Place several thicknesses of paper towels in a large, flat pan. The pan may be shared.
11. Pour the contents of the reaction plate onto the paper towels and allow the liquid to soak into the towels and air dry.
12. Rinse the wells with water.
13. Fold the paper towels and place into a resealable plastic bag and dispose in the garbage.

Student Worksheet PDF

10841_Student1.pdf