Salinity. For this laboratory we will use a simpler definition: The total amount of solid material in gms dissolved in 1 kg of seawater

Transcription

1 Salinity Equipment needed for each student group: 1 internal wave tank with dividers, stirrers and solutions 1 burette stand 1 50 ml burette 1 5 ml autopipette ml Erlenmeyer flask 1 800ml beaker 3 aluminum dishes ml graduated cylinders 1 saltwater hydrometer (scale g/cm3) eyedroppers For general use: refractometers drying oven plastic gloves ml graduated cylinders safety goggles YSI 85 oxygen, salinity temperature meters top loading balances AgNO 3 silver nitrate solution (50 gms AgNO 3 made up to 1000 ml volume using distilled water) K 2 CrO 4 potassium chromate solution (3.5 gms K 2 CrO 4 made up to 1000 ml volume with distilled water) DISCUSSION: Definitions A formal definition of salinity is given by Gross (5th edition): the total amount of dissolved solids in seawater in parts per thousand (ppt) by weight when all the carbonate has been converted to oxide, the bromide and iodide to chloride, and all organic matter is completely oxidized For this laboratory we will use a simpler definition: The total amount of solid material in gms dissolved in 1 kg of seawater In oceanography, the salt content or salinity is expressed in parts per thousand (ppt) rather than parts per hundred. Parts per hundred is the simple percentage sign we all know (%). Parts per thousand, however, has an extra 0 and is written as o /oo. This convention is used because the salinity of seawater varies from 0% to about 3.8% solid materials. Using ppt merely expands this range to larger numbers (0 ppt 38 ppt) without changing the value. In this laboratory our salt water is artificial, and composed of table salt (NaCl) mixed in various proportions with water to produce the necessary salinity. In the open ocean there are many more elements then just the two we use in this lab; however, the methodology you will be taught will work for artificial as well as natural seawater. Page 1 of 17

2 DISCUSSION: Salinity Measurements The water s salt content is one of the primary reasons that oceanography is different from limnology (fresh water). Biologists, for example, study the control different salinities have on the distributions and physiology of certain fish populations. Many oceanic species don t usually live in brackish waters as adults, but may spend part of their juvenile life cycles in estuaries where food may be more available as they are maturing (i.e., Gulf shrimp). On the other hand, some fish have the ability to migrate from high to low salinity environments (i.e., salmon) and other have the ability to go from freshwater to high salinity environments (i.e., freshwater eels) by regulating their internal salt balance. Water circulation is also affected by salinity. In estuaries, fresh water can float on saltier, denser ocean water and even set up a current pattern, which is based on the difference in density between the two water types. In the deep ocean, high salinity water is often more dense, especially if it is colder, and will sink and flow along the ocean floor under the influence of gravity. The salt content can even affect the way sediments are deposited on the ocean floor. Fine sediments settling in salt water may adhere to one another and settle at a more rapid rate than the individual particles. This process of adhering is flocculation. Your T.A. will show you an example of flocculation by shaking up identical sediments in two jars, one filled with freshwater and one filled with saltwater. They will do this at the beginning of the lab, and by the end of lab you will observe any differences. This laboratory is concerned not so much with why we have to measure salinity, but with the problems associated with determining the property. Some of the methods are easy, but imprecise, while others involve more complex chemical measurements. One of the main things we should remember, we always have to balance the number of samples we have to analyze and the importance of the accuracy of our determination to our research program with the time and resources we can spend on each sample. In other words, we need to maximize the cost - benefit ratio when selecting the technique we will use in the field. For example, it may be important for some oceanographers to know the value of salinity to the third decimal place (i.e., ppt) while others may only be concerned with the first decimal place (i.e., 24.0 ppt). Since we will be using several hydrometers, refractometers and YSI 85 meters, it is important to calibrate them against each other. This has been done prior to your lab. Each has either a serial number or some other identifying number. This is done to aid us in the precision of our measurements. To aid us in accuracy, we would need to compare our measurements to standards maintained by the National Bureau of Standards. In this lab, we are mainly interested in having comparability among groups, so we have concentrated on precision. Some instruments can be adjusted, or zeroed, others, such as hydrometers, cannot. This laboratory presents five analytical approaches to salinity determination: 1. evaporation of seawater 2. conductivity of seawater (done indirectly using nickel electrodes) 3. determination of density by hydrometer and graphically converting it to salinity 4. refraction of seawater 5. titration of Cl - ion Page 2 of 17

3 SET UP: 1. In the provided containers, obtain approximately liters of the three sample solutions: Known, Unknown A and Unknown B. 2. Proceed with the given exercises at your bench. EXERCISE 1: Salinity By Evaporation According to the informal definition, salinity is the total amount of dissolved solids (or salts) in one kilogram of seawater. One way to measure salinity, then, is to evaporate 1 kg of seawater. We can shorten this process by taking a small portion of a kilogram and removing the water content. 1. In this laboratory exercise you will be measuring the amount of salt in three samples: known sample (35 ppt) unknown sample A unknown sample B 2. Before weighing out three aluminum dishes to the nearest 0.01 gm on a top loading balance (Fig. 1), mark each dish on the bottom with a pencil (to make an indentation) with a code to identify your group and which of the three samples is in the dish. Record the weights in Table 1 on your forms. FIGURE 1 3. Transfer 5 mls of each of the three salt solutions to the aluminum dishes using the autopipette. Make sure to use a different pipette tip for each individual salt solution. Although the autopipette is set to dispense exactly 5 ml of solution, it is not necessary to have exact volumes because you will be weighing each sample. Make sure you note the petri dish number marked on the bottom. Page 3 of 17

4 4. Weigh each aluminum dish with the water and record in Table 1 (Fig. 1). 5. Put all the samples in the forced-air-drying oven at 100 o C until the water has evaporated. This will take at least one hour. 6. Weigh each aluminum dish (dish+salt) before you leave the lab. Again, record the data in Table 1 and calculate the weight percent salt and then convert to parts per thousand for comparison with the other methods. Study the following example. Then determine the salinity of each of your samples, using the information you have entered in Table 1. sample dish# empty wt dish+water wt of water dish+salt wt of salt known To get the %salt = weight of salt x 100 =.13 x 100 = 2.5% weight of water 5.13 To get the ppt ( ) salt = weight of salt x 1000 =.13 x 1000 = 25 ppt weight of water 5.13 TABLE 1 sample dish# empty wt dish+water wt of water dish+salt wt of salt known unknown A unknown B salinity (ppt) salinity (ppt) salinity (ppt) known unknown A unknown B Answer the following questions: A. Assume there was some sediment in the sample just evaporated. Would this have affected the salinity as you measured it? Explain. Page 4 of 17

5 B. Could you have evaporated the water out of the seawater at room temperature? If you could, would this have affected the salinity measurement? Explain. EXERCISE 2: Electrical Conductivity Of Seawater Figure 2 illustrates the 2 types of instruments we will use to measure the conductivity of our samples. These instruments both measure the electrical conductivity of a solution through a set of four pure nickel electrodes. Electrical conductivity of a solution depends on the rate at which the ions or molecules move around. This rate is a function of temperature. The rate increases with temperature. FIGURE 2 1. Place the conductivity probe into the container holding the sample of Known salinity to verify the salinity. Allow the reading to become reasonably stable before writing down a value (it shouldn t change more than +.05 ppt). The instrument also measures temperature and makes the appropriate adjustment. Record your value in Table Rinse the probe with distilled water and blot dry. Now place the conductivity probe into the container holding the sample of Unknown A to measure the salinity. Record your value in Table Again, rinse the probe with distilled water and blot dry. Finally, place the conductivity probe into the container holding the sample of Unknown B to measure the salinity. Record your value in Table Rinse the probe with some water and blot dry. Place the probe into its compartment for safe keeping. Page 5 of 17

6 TABLE 2 Salinity (ppt) Known sample Unknown A Unknown B Answer the following question: 1. Is warmer or colder water a better conductor of electricity? Why? EXERCISE 3: Salinity By Hydrometer The density of seawater is controlled primarily by the water temperature and its salinity. Therefore, if we measure the temperature and density, we should be able to determine the unknown factor: salinity. One way to measure density is to make use of the formula: density = mass/volume We could take an exact volume of water and weigh it, then divide the mass by the volume. Unfortunately, there are a lot of errors involved with this type of density determination. We need an exact volume measure to the third place (.001 ml) and weight to the third place (.001 gm), both of which are not east to measure with simple laboratory equipment. Instead of measuring mass and volume directly, we make use of one of Archimedes principles: a floating body will displace a volume of water equal to its own mass. If we use a fixed mass, such as a hollow glass ball, it will sink down into the water until it displaces its own mass. If we have denser water, it will sink less (float more). This same principle is used in testing antifreeze levels in your car, using a hydrometer containing small plastic balls of different densities, which sink or float depending on the concentration of the antifreeze. There are also hydrometers manufactured to measure seawater density. In this case, the hydrometer stem is graduated so that as it sinks and displaces its own mass, and the level to which it sinks is equal to the seawater density (Fig. 3). As density of the seawater increases, the volume of the displaced seawater decreases (the hydrometer sinks less in the higher density fluid). In Fig. 3 the higher salinity solution is represented by the darker color. You will note that the hydrometer floats higher in the saltier solution than it does in the less salty solutions (lighter colors). This is because the higher the salinity, the higher the density at a constant temperature. A practical example of this is the fact that it is easier for a person to float in the ocean than it is in freshwater. In very salty bodies of water, such as the Dead Sea, it is even easier to float than it is in the ocean because of its higher salinity. Page 6 of 17

7 FIGURE 3 First you have to learn how to read the hydrometer. Fig. 4 below shows an enlarged view of the hydrometer stem. Depending on which hydrometer you will be assigned there are four to five major divisions: each of these major divisions is divided into 10 parts, and each is further divided in half (see Fig. 4 below). Page 7 of 17

8 FIGURE 4 1. Pour ml of each of the three sample solutions into graduated cylinders. Insert the hydrometer into each sample and obtain a density reading. Take the temperature as well. Record the data in Table The relationship between salinity, temperature and density is shown in Fig. 5. By measuring the density on the hydrometer stem as well as recording the temperature of the seawater, we can use this graph to interpret the salinity. As an example, if you examine Fig. 5, we have plotted the results of measuring the density of with a temperature of 20 o C. The resulting salinity is approximately 19 ppt. 3. Look up the appropriate salinity using Fig. 5. Repeat for each of the samples. TABLE 3 sample TEMPERATURE ( o C) DENSITY (g/cm3) SALINITY (ppt) known unknown A unknown B Page 8 of 17

9 A. A fully loaded ship moves from a fresh water port and heads over to the Red Sea, an area known for its high salinities. Will the ship ride higher or lower in the Red Sea compared with the port it left? Explain. B. Examine Fig. 5. Given the range of temperatures in the open ocean, say -1 to 35 o C, and the range of salinity, say ppt, which will generally be more important in determining the density of seawater? ` FIGURE 5 Page 9 of 17

10 EXERCISE 4: Salinity By Refractometer Salinity and temperature also affect the speed of light in seawater. We ve all noticed how a pole pushed into the water appears to bend. This bending is a function of the difference between the speed of light in air versus its speed in water. If we could detect very small angles, the pole would appear to bend at a slightly different angle in saltwater than freshwater. If we have the proper instrumentation, we can actually measure the bending or refraction of light in seawater. A refractometer (Fig. 6) can precisely measure the amount of refraction that is caused by the salt content. The instrument that you will be using is temperature compensated. This means that the temperature effects on refraction can be ignored for these measurements. 1. Carefully rinse the face of the prism with distilled water, and then dry using a soft tissue. Then using the eyedroppers, drop one or two drops of the known sample from the compartment of your tank on the prism face. Slowly lower the cover down on top of the drops. Be careful during these measurements, as refractometers are expensive. FIGURE 6 2. Facing the light or a window, look through the refractometer and note where the intersection lies between the upper shaded portion and the lower clear portion of the scale (Fig. 7). This boundary represents the salinity. Record the reading to the nearest ppt and enter it in Table 4. Page 10 of 17

11 FIGURE 7 3. Repeat steps 1 and 2 for each of the unknowns. TABLE 4 sample SALINITY (ppt) known unknown A unknown B Answer the following questions: A. Under what conditions would the refractometer method be the only procedure you could use to determine salinity? B. Consider the four methods for determining salinity you have examined so far in the laboratory: 1. evaporation, 2. conductivity, 3. hydrometer, and 4. refractometer. Which takes the most time? Which are the most difficult aboard ship? Page 11 of 17

12 EXERCISE 5: Salinity By Chemical Titration The last method this salinity laboratory demonstrates is one that is very precise (will replicate well) and accurate (close to the true salt content). It s a bit complicated and normally would be more appropriate for a chemical laboratory; however, we introduce it here because for decades it was the standard technique in oceanography for determining salinity. As you proceed through this last exercise, imagine doing it at sea under very rough conditions with the oceanographic ship pitching and rolling. In this titration laboratory we will use a silver nitrate solution to precipitate out the chlorides in a known volume of artificial seawater. Although natural seawater is much more complex than a mixture of NaCl (table salt) and distilled water, the analytical technique is the same. The reaction is as follows: NaCl + AgNO 3 NaNO 3 + AgCl sodium silver sodium silver chloride nitrate nitrate chloride (precipitate) By adding excess AgNO3 to a NaCl solution (artificial seawater), you cause the chloride to precipitate as silver chloride and settle to the bottom. It s then possible to dry and weigh the AgCl precipitate, and from the weight determine the amount of chloride in the sample. First, we have to calculate the percent of chloride in silver chloride. This is done in the following manner: atomic weight of Cl - x 100 = percentage Cl - in AgCl atomic weight of AgCl 35.4 x 100 = 24.7% Cl - in AgCl Since we know there is 24.7% chloride in AgCl, we can determine salinity. For example, if we took 1000 gms of seawater and precipitated out 100 gms AgCl, how many grams of Cl - are in the sample? 100gms x 24.7% = 24.7 gms of Cl - Chemical oceanographers tell us that chlorides make up about 55% of the total salts (salinity) in seawater. Therefore, salinity = 100/55 x Cl - or salinity = 1.8 x Cl - Since you measured 1000 gms of seawater, your total salts or salinity would be: salinity = 1.8 x 24.7 gms = 44.5gms/1000 gms = 44.5 ppt Page 12 of 17

13 The problem with drying and weighing the AgCl precipitate is that it takes too much time, and the precipitate weight is a function of the oven temperature you use for drying. (Note: similar problems occur when we determine salinity by evaporation) So rather than drying and weighing silver chloride, chemical oceanographers can use the same reaction to measure salinity by knowing exactly how much silver nitrate was used during the analysis. They do this by adding another chemical reagent, potassium chromate, which acts as an indicator to tell us exactly when all the Cl in the seawater has combined to form silver chloride. The potassium chromate indicator remains colorless as long as there is Cl present, but the instant the last of the Cl is bound up as silver chloride, the solution turns orange. The orange color occurs when the extra chromate ion combines with the silver ion to form silver chromate. At this point you might throw up your hands and say Enough - I don t really understand all this chemistry! In fact, you don t need to in order to do the salinity titration outlined below. Just remember that the chlorinity is only part of the total salts (55%). The following two reagents will be made available for the titration analysis: AgNO3 solution (50 gms AgNO3 made up to 1000 ml volume) and the K2CrO4 solution (3.5gms K2CrO4 made up to 1000 ml volume). The experimental setup is shown below in Fig. 8. FIGURE 8 Page 13 of 17

14 You will need to calibrate the silver nitrate solution against the known salinity sample. 1. The person who will be doing the actual titration must wear safety goggles and disposable gloves. 3. Transfer exactly 5 ml of the known salinity sample from the compartment on your tank into a 125 ml Erlenmeyer flask using the pre-set autopipette. 4. Add 5 ml of the potassium chromate solution to this same flask using the labeled autopipette.. 5. Since silver nitrate will temporarily stain your skin brown, the T.A. will fill the 50 ml burette with the silver nitrate solution, pouring it carefully down a funnel into the top of the burette. It only has to be done once for this entire set of four titrations. It doesn t have to be filled to the zero mark; just note the starting and end point readings for each titration. (Fig. 8) 6. Put a magnetic stirrer bar in the flask and place it on the stirrer plate. Adjust the spin rate to properly mix the solution. Record the starting level of the silver nitrate in your burette. Now add the silver nitrate solution drop by drop from the burette until you note an abrupt color change from milky yellow to orange. Your TA will show you a video. 7. Record endpoint reading of silver nitrate in your burette in Table Remove the stirrer using the magic wand and dispose of the waste in the labeled container. Rinse the stirbar and the flask with a little water and dispose of the water again in the labeled container. Place the stirbar back in the flask and proceed. 9. Repeat the analysis for the known sample (i.e. do it twice!), and calculate the average number of milliliters of silver nitrate used for the titrations, you know that the salinity here is 35 ppt and will use this information to set up the ratios that follow. 10. Titrate the unknown samples in a manner similar to that described above, and enter all data in Table 5. Do each of the unknowns only once. 11. Calculate the salinity of your unknown samples in the following manner: milliliters of AgNO 3 to titrate known = milliliters of AgNO 3 to titrate unknown known (ppt) unknown (ppt) If it took 10 mls to titrate the sample of known salinity (35 ppt), what is the salinity of the unknown if it took 7 mls to titrate the sample? 10 ml AgNO3 = 7 ml AgNO3 35 ppt X 10X = 7 x 35 ppt x = 24.5 ppt Page 14 of 17

15 Another way to write the equation is: unknown salinity = #ml AgNO 3 (unknown) x known salinity #ml AgNO 3 (known) TABLE 5 Sample initial burette mark final burette mark mls AgNO3 salinity used known 35 known 35 Unknown A Unknown B average Answer the following questions: A. Suppose that your glass burette was not exactly round, and that instead of delivering 1 ml per graduated division, it really delivered 1.1 ml. Could you still use it for a salinity titration against a sample of known salinity? B. You transferred exactly 5 mls of seawater to an Erlenmeyer flask with a pipette. Then you went to talk to someone in another lab group for 15 minutes, and some of the seawater evaporated before you could begin your titration analysis. Would you have to start all over, or could you use the evaporated sample? Explain. EXERCISE 6: Salinity and Density Are Related So far in this lab, we have been measuring the salinity of three solutions, Unknown A, Unknown B and Known (35ppt). In the lab on Density, we will examine the effects of both salinity and temperature on density. While both parameters have an effect, we will see that the range of temperature throughout the worlds oceans is greater than the range of salinity. This means that in most regions of the ocean, temperature has a greater effect on density than does salinity. Page 15 of 17

16 To explore the connection between salinity and density we will use a long thin tank with a divider in the middle and the saltwater solutions from the previous exercises. 1. Put the divider securely into the grove of the tank, dividing it in half. 2. In 2 Erlenmeyer flasks, collect 250 ml of the unknown samples from the previous exercises. In order to remember which flask holds which solution, add a few drops of different food coloring to each. This will also help you see the results of your exercise. You can write down your colors on your form under Exercise Based upon the salinities you measured in the previous exercise, determine which solution is more dense, Unknown A or Unknown B. Now predict what will happen when the divider is removed. Write down your predictions by answering questions A and B in the spaces provided. 4. While holding the center divider, pour the two solutions into the two separate halves. This works best if you keep the solution level on each side of the divider at about the same height. 5. Pull the divider out of the groove, while holding onto the tank. Try to be as steady and smooth as possible. Observe how the two fluids mix. Record your observations on your form by answering question C. under Exercise Now you will empty the tank, and repeat steps 1 through 5 using whichever solution was MOST dense and the Known (35 ppt) solution. 7. Your TA will now show you a demonstration with a larger tank and ask questions based on your results. Unknown A vs Unknown B: Unknown A color Unknown B color A. Which is more DENSE? B. What will happen when the divider is removed? Will the fluids remain separated or become completely mixed together? Page 16 of 17

17 C. Described what happened when you removed the divider. What does this tell you about the densities of these solutions? Unknown vs Known 35ppt: Unknown color Known (35ppt) color A. Which is more DENSE? B. What will happen when the divider is removed? Will the fluids remain separated or become completely mixed together? Which solution will flow to the bottom this time? C. Described what happened when you removed the divider. What does this tell you about the densities of these solutions? Record the given salinities for each exercise in Table 7 on your forms. Known Unknown A Unknown B TABLE 7 Evaporation Conductivity Hydrometer Refraction Titration (Exercise 1) (Exercise 2) (Exercise 3) (Exercise 4) (Exercise 5) Page 17 of 17