The total amount in grams of solid material dissolved in 1 kg of seawater.

Transcription

1 MS20 Laboratory Objectives To understand the definition of salinity To learn the sources and sinks for salt in the ocean To measure the salinity of a water sample by mass evaporation To measure the salinity of a water sample using its electrical conductivity To measure the salinity of a water sample using its density To measure the salinity of a water sample using its refractive index To measure the salinity of a water sample by chemical titration Introduction Salinity is a measure of the total dissolved salts in seawater. This sounds simple enough, but measuring salinity becomes a problem when we realize that some of the salts do not simply dissociate into ions but chemically react with water to form complex ions, and some of the ions include dissolved gases such as CO 2 (which converts to carbonic acid, H 2 CO 3, bicarbonate, HCO 3 -, and carbonate, CO 3 2- ). Duxbury (1971) defines salinity as: "The total amount in grams of solid material dissolved in 1 kilogram of seawater when all the carbonate has been converted to oxide, all of the iodine and bromine have been replaced by chlorine, and all the organic matter has been completely oxidized." For this laboratory we will use a simpler definition: The total amount in grams of solid material dissolved in 1 kg of seawater. Normally, salinity is expressed in parts per thousand (ppt), often written as o / oo. It is also sometimes abbreviated as per mil, much the same as parts per hundred is abbreviated as percent (%). For example, if you had 1000 g of seawater that contained 35 grams of dissolved salt, you would have a 3.5% salt solution or a salinity of 35 parts per 1000 (35 º/ ºº ). In this laboratory our salt water is not seawater, it is a solution of table salt (NaCI) dissolved in de-ionized water. We only have two elements of the 11 commonly found in seawater (and the dozens that are present in trace amounts); however, the methodologies we use here will work for natural seawater as well as for our artificial seawater. Revised on 2/21/2005 Page 1 of 21

2 Why is the sea salty? Why do we care? The salt in the ocean comes primarily from the weathering of rocks on land. When rocks are exposed at the surface of the Earth, minerals are weathered chemically to produce various solid compounds as well as free ions such as calcium (Ca 2+ ), potassium (K + ), and sodium (Na + ). Volcanic eruptions produce CO 2, hydrochloric acid (HCl), and sulfur dioxide (SO 2 ), all of which are soluble in water. These solutes are brought to the ocean by precipitation and by river transport. Evaporation of water concentrates these materials, bringing the salinity of the oceans well above the salinity found in rivers and lakes. Some materials (such as calcium and potassium) are gradually removed from the ocean by organisms that use them as nutrients. Other materials are lost through sediment interactions: the sediments at the ocean s bottom behave much like a water filter, adsorbing various ions associated with salinity and locking them away. These processes of adding salt to the oceans, concentrating it through evaporation, and removing it through various chemical and biochemical processes, have been active on Earth for billions of years; the average salinity of the ocean is therefore a function of the rates of these various processes, and has changed little through Earth s history. The water's salt content is one of the primary reasons that study of the ocean is different from the study of fresh water lakes (known as limnology). Biologists, for example, are concerned with the range of salinities at which an organism can live. Many species don't usually live in brackish water as adults, but may spend part of their juvenile life cycles in estuaries where food may be more available as they are maturing. On the other hand, some fish have the ability to migrate from high to low salinity environments 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 the most dense and will sink and flow along the ocean floor under the influence of gravity. Physical oceanographers, by knowing the salinity and temperature fields of the ocean, can make predictions about the velocity and direction of these density-driven currents. The salt content can even affect the way sediments are deposited on the sea bottom. Very fine clay minerals tend to clump together (a process known as flocculation) in saline environments. These particle aggregates have a greater ratio of mass to surface area, leading to much more rapid settling rates. Measuring Salinity In this laboratory we will investigate five analytical methods for determining salinity. Some methods are straightforward and some are complicated, some are simple but imprecise and some are difficult but precise. Revised on 2/21/2005 Page 2 of 21

3 As you perform these tests, pay particular attention to the results the different methods produce different levels of precision and accuracy, since the components of salinity vary in their effects on the different measurement techniques. Care in your technique should produce repeatable results, but do not expect that each technique will produce exactly the same numbers. Methods of salinity determination: evaporation of seawater conductivity of seawater measurement of water density by hydrometer refraction of light through seawater titration of the chloride ion (CI - ) For each method you will be measuring the salinity of three samples: one known sample, whose salinity is 35º/ ºº ; and two samples, A and B, whose salinities are to be determined by you. A. Determination of salinity by evaporation As we have defined salinity as the total mass of dissolved salts (measured in grams) in one kilogram of seawater, the most straightforward way to measure salinity is to measure exactly one kilogram of seawater, evaporate the water, and weight the salt that precipitates out. Evaporating a full kilogram of water would take more time than we have today however, so we will shorten the process by evaporating a small fraction of a kilogram. 1. Label three 250 ml beakers for the three samples: 35 o / oo, and unknowns A and B. Fill each beaker to about 200 ml from the dispensers on the side of the room, making sure that you have the correct sample in each labeled beaker. 2. Label three Petri dishes (with the same labels as the sample beakers) and weigh each to the nearest 0.01 gram. Record the masses of the dishes in Table A on the answer sheet. 3. Using a pipette, transfer about 10 ml of each of the three salt solutions to the corresponding labeled Petri dishes. Weigh each Petri dish with the water to the nearest 0.01 gram and record the masses on the answer sheet. Determine the mass of the water samples by subtracting the weight of the dish only, and record the masses. 4. Carefully bring the Petri dishes to the back of the room, where your instructor will place them in the drying oven. Leave them in the oven until dry this will take the majority of the lab period. You will be doing exercises B through E while waiting for the samples dry. 5. Once the samples are dry allow them to cool for a few minutes, then weigh each Petri dish and record the results on the answer sheet (dish + salt). Subtract the masses of the dishes to determine the mass of each of the salt samples and record the results on the answer sheet. Revised on 2/21/2005 Page 3 of 21

4 6. Determine the salinity of each sample using equation 1 (below). Record your answers on the answer sheet. B. Determination of salinity by electrical conductivity Contrary to common belief, pure water is not a very good conductor of electricity. It becomes a good conductor when soluble salts are added, due to the presence of positive and negative ions that can donate and accept electrons. The transfer of these electrons is what allows water to conduct, and the higher the concentration of ions in the solution the better the conductivity (the tap water in your house has enough ions present to be a good conductor). (1) Figure One: Electrical conductivity as a function of salinity. Pure water (such as distilled or deionized water) does not conduct electricity well enough to complete a circuit and light a small light bulb. Water with dissolved salts contains both positive and negative ions that complete the circuit. The higher the salinity, the greater the conductivity, and the brighter the bulb. This can be seen in Figure One, in which a water solution is used to bridge an electrical connection. The greater the conductivity of the solution, the more light produced by the light bulb. One way to measure the conductivity of the solution would be to measure the brightness of the bulb against some standard. Fortunately we can directly measure the conductivity of the water with a conductivity meter (Figure two). Electrical conductivity in water is a function not just of salinity but also of temperature, so we will need to consider temperature in our measurements as well. Conductivity meters can be made to measure a variety of salinity conditions. Unfortunately devices that can handle the high salt concentrations found in the world s oceans (and in today s lab) are quite costly. The device we will be using today is only accurate at much lower salinities so we must dilute our samples by a known amount in order to make our measurements. Revised on 2/21/2005 Page 4 of 21

5 Figure Two: Using the conductivity meter. Pour the sample, diluted to 25% strength, into a 250 ml beaker. Measure and record the temperature, then place the probe into the sample to take the conductivity reading. Remember to multiply your reading by four, since you diluted the sample by a factor of four. 1. Measure exactly 25 ml of the 35 o / oo water into a 25 ml graduated cylinder. Carefully pour this water into your 100 ml volumetric flask. Fill the volumetric flask exactly to the measurement line with deionized water from your squirt bottle. NOTE: It is important to dilute the sample using only deionized water, as tap water contains ions such as sodium and calcium that will alter the salinity of your sample and skew the results of the experiment. Remember that both the graduated cylinder and the volumetric flask are calibrated so that the bottom of the fluid meniscus should be at the measurement line for the most accurate measurement. 2. If you have been careful the sample in the volumetric flask now has exactly ¼ the salinity of the original sample. Transfer the sample to a 250 ml beaker. Measure the water s temperature (allow seconds for the thermometer to equilibrate) and record the temperature in Table B on the answer sheet. Revised on 2/21/2005 Page 5 of 21

6 3. Measure the electrical conductance of the water (the units of conductance are mmhos/cm the reciprocal of electrical resistance). Place the meter into the beaker (be sure that the probe sensors are completely submerged) and allow at least 30 seconds for the readout to equilibrate. Record the conductance on your answer sheet, remembering to multiply your measurement by a factor of four (since you diluted the sample by a factor of four). NOTE: The conductivity meter is a valuable instrument handle it carefully. Have a small beaker of deionized water at your station and use it to rinse the probe sensors between measurements. 4. Repeat steps 1-3 for the unknown samples A and B, recording the temperatures and conductivities in Table B on the answer sheet. 5. Figure Three is a graph that relates salinity in parts per thousand (horizontal axis) to conductivity in mmhos/cm (vertical axis). The sloping lines represent the relationship between conductivity and salinity at several known temperatures. Use this graph to determine the salinity values for each of your samples and record these values in Table B on the answer sheet. NOTE: Since we have only included lines for temperatures at 5 degree increments, you will be expected to interpolate your results. For example: if your water temperature was 17.5 C, use your measured conductivity and the graph to determine the salinity if the water were 20 C and if the water were 15 C. As 17.5 C is halfway between 15 C and 20 C, the salinity value is halfway between the two values you determined for these temperatures. Revised on 2/21/2005 Page 6 of 21

7 Figure Three: Conductivity as a function of salinity and of temperature. Lines show the relationship between conductivity and salinity at 5 degree increments between 5 C and 25 C. C. Determination of salinity by density using a hydrometer Remember from the isostasy laboratory that density is simply the ratio of a sample s mass to its volume. The density of seawater is a function of both water temperature and salinity, thus if we measure both the temperature and the density of a water sample we should be able to determine its salinity. To determine density we could take an exact volume of water and weigh it, then divide the mass by the volume. Unfortunately it is difficult to determine both mass and volume to the precision we desire with the laboratory equipment we have available to us. Revised on 2/21/2005 Page 7 of 21

8 Instead of measuring mass and volume directly, we return to Archimedes' principle: "a floating body will displace a volume of water equal to its own mass." If we use a fixed mass that is less dense than the water sample, such as a hollow glass ball, it will sink down into the water until it displaces its mass. The greater the density of the water the higher the sphere will float. Hydrometers are devices designed to measure fluid density. The hydrometer s mass is precisely fixed and it is concentrated at the bottom of the tube (like a buoy) so that the hydrometer will always float upright. The narrow stem is precisely graduated so that as the device sinks and displaces its own mass, the level to which it sinks is equal to the seawater density (Figure Four). As density of the seawater increases, the volume of the displaced seawater decreases (the hydrometer sinks less in the higher density fluid). Figure Four: Hydrometer function. As the density of the fluid increases, the hydrometer displaces less water. Since the upper tube is very narrow this difference in volume corresponds to a significant change in depth. The density of the fluid can be read from the graduations on the stem. Figure Five below shows a close-up view of the hydrometer stem: there are labeled divisions representing 5/1000 gram/ml (1.010, 1.015, and so on); these are divided into five parts (1/1000 g/ml) by secondary unlabeled divisions; these are divided into halves (0.5 g/ml) by tertiary unlabeled divisions. If you are careful using this device you can determine density to a precision of 5 parts in 10,000 (1/2 milligram/ml). NOTE: The entire hydrometer is sealed in thin glass, and most of the mass is located in the metallic base. This device is carefully calibrated, very fragile, and very expensive. Handle your hydrometer carefully, as the tiniest crack in the glass makes it worthless for measuring density. Revised on 2/21/2005 Page 8 of 21

9 Figure Five: Reading density from the hydrometer. The major lines are labeled to 5 parts in 1000, the secondary unlabeled lines to 1 part in 1000, and the tertiary unlabeled lines to 5 parts in 10,000. This device can be read to the fourth decimal place. 1. Pour approximately ml of water from the known (35 o / oo ) salinity sample into a 250 ml graduated cylinder. 2. Measure and record the water temperature in Table C on the answer sheet. 3. Carefully lower (do not drop) the hydrometer in the graduated cylinder until it floats. If there is not enough water to allow the hydrometer to float, more water can be added until the hydrometer rises from the bottom. 4. Read the density of the water from the graduated lines on the hydrometer and record this value in Table C. 5. Rinse and dry the hydrometer and the graduated cylinder with deionized water. 6. Repeat steps 1-5 for the unknown samples A and B, recording the temperatures and the density values in Table C. 7. Figure Six (below) is a graph relating water density (vertical axis) to salinity (horizontal axis). It can be read like Figure Three, as it has lines representing the density-salinity relationship at several fixed temperatures. Determine the salinity of your sample by finding the salinity that crosses the lines representing your measured density and temperature. As before you may need to interpolate between the temperature lines. Record your salinity values in the appropriate space on Table C. Revised on 2/21/2005 Page 9 of 21

10 Figure Six: Density as a function of salinity and of temperature. Lines show the relationship between density and salinity at 5 degree increments between 5 C and 30 C. D. Determination of salinity by refractometer The speed of light in a vacuum is 2.998x10 5 kilometers/second. Even in intergalactic space however, photons of light periodically interact with stray atoms, molecules and dust fragments that slow it down by some negligible amount. Even the thickness of Earth s atmosphere is not enough to decrease this velocity by more than a fraction of a percent. In water however, the speed of light decreases to 2.25x10 5 km/second and in glass it decreases even more, to about 2.00x10 5 km/second. This decrease in light velocity is due to the increase in density of the medium, as photons are constantly being absorbed and re-emitted by atoms that they encounter. Revised on 2/21/2005 Page 10 of 21

11 When light moves from one medium to another in which the velocity is different, the light will bend, or refract, by an amount that is proportional to the difference in velocity between the two media. Since the density of water increases with. We will take advantage of this property by measuring the amount of that refraction in water samples. This is possible using a device called a refractometer. A refractometer can precisely measure the amount of refraction that is caused by the density. The instrument that you will be using is temperature compensated. This means that the temperature effects on refraction can be ignored for these measurements, and you will be able to read the salinity directly from the refractometer. The refractometer consists of a blue-tinted glass prism and a beveled, clear plastic cover lens, each with a known refractive index. When a thin layer of water is placed between these two lenses it will refract light through an angle that depends upon the salinity of the water sample. When you look through the eyepiece you will see a calibrated scale with an upper blue-shaded region and a lower white region. The boundary between these two regions will cross the scale at a value representing the sample s salinity (Figure Seven). Figure Seven: The refractometer. Place a drop of the water sample between the two lenses and lower the upper lens into place. Look through the eyepiece and aim the refractometer toward a light source. The boundary between the upper blue-shaded region of the scale and the lower white region crosses the calibration scale at a value that represents the salinity of the sample. Before you begin, carefully rinse the face of the prism and the cover lens of the refractometer with deionized water, and dry with a cloth towel. Place a drop of deionized water on the lens and close the cover. Look through the eyepiece. The boundary of the shaded region should cross the scale at the zero line. If it does not, inform your instructor so that she/he can adjust it or assign you a different refractometer. If you have a problem reading the scale you can sharpen it using the focus ring. Revised on 2/21/2005 Page 11 of 21

12 1. Using an eyedropper, place one or two drops of your known (35 o / oo ) sample onto the prism face as shown in Figure Seven. Close the prism cover being careful not to trap any air bubbles in the water on the prism face. 2. Holding the prism toward the light, look though the refractometer and note where the intersection lies between the upper shaded portion and lower clear portions of the scale. This boundary represents the salinity. Record to the nearest part per thousand (º/ ºº ) and enter in Table D on the answer sheet. 3. Rinse the prism and cover plate in deionized water and dry with a cloth towel. 4. Repeat steps 1-3 for each of the unknowns and record the salinities in Table D. E. Determination of salinity by chemical titration NOTE: This section of the laboratory will be done for you as a demonstration by your instructor. You will be responsible for the results. Also, in a few weeks you will be responsible for performing a titration yourself, so pay close attention to how it is done. The final method of salinity determination that we will demonstrate is very precise (that is, it will replicate well) and accurate (it produces a result that is very close to the true salinity). It is a bit complicated and normally would be more appropriate for a chemistry laboratory; however, we introduce it here because for decades it was the standard technique in oceanography for determining salinity. In this section we will use a silver nitrate solution to precipitate out all of the chloride in a known volume of artificial seawater. Although natural seawater contains a much wider array of ions than a mixture of NaCl and fresh water, the analytical technique is the same. When we add a solution containing silver nitrate (that is, Ag + ions and NO3 - ions) to a solution containing sodium chloride (that is, Na + ions and Cl - ions), the chloride ions in the solution will react with the silver ions to produce a solid precipitate, silver chloride (and leaving the sodium and nitrate ions in solution): (2) Revised on 2/21/2005 Page 12 of 21

13 Hypothetically we could then dry and weigh the precipitate AgCl, and from the weight determine the amount of chloride in the sample. But first we have to calculate the mass percentage of chloride in silver chloride. This is done in the following manner: (3) From this we can see that AgCl contains 24.7% chloride ions. If we precipitate 100 grams of AgCl from 1000 grams of seawater that would represent 24.7 grams of Cl -. Chloride represents a majority of the negative ions present in seawater, but not all. Fortunately the proportion of chloride is consistent. We know from chemical oceanography that chlorides make up about 55% of the total salts (salinity) in seawater. Therefore if we know the chlorinity (the mass of chloride present) we can determine the overall salinity: Salinity = 100/55 x mass Cl - = 1.8 x mass Cl - (5) In the above example we measured 1000 grams of seawater and found 24.7 grams of chloride so the salinity can be determined: Salinity = 1.8 x mass Cl - = 1.8 x 24.7g Cl - = 44.5 º/ ºº (6) The problem with drying and weighing the AgCl precipitate is that it takes a lot of time, and the mass of the precipitate is a function of the oven temperature you use for drying, since the AgCl can hold onto a significant amount of water even when it has seemingly dried out (similar problems occur when we determine salinity by evaporation). So rather than drying and weighing the silver chloride, we can use the same reaction to measure the salinity by knowing exactly how much AgNO 3 was used to precipitate all of the chloride. We can do this by adding another chemical reagent, potassium chromate (K 2 CrO 4 ), which acts as an indicator to tell us exactly when all of the chloride in the seawater has reacted to form AgCl. 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 (CrO 4 2- ) combines with the silver ion to form a complex ion called silver chromate. (4) The following two reagents will be made available for the titration analysis: AgN0 3 solution (50 grams of AgNO 3 dissolved into a 1000 ml solution) and the K 2 Cr0 4 solution (3.5 grams of K 2 Cr0 4 dissolved into a 1000 ml solution). The experimental setup is shown below in Figure Eight: Revised on 2/21/2005 Page 13 of 21

14 Figure Eight: Set up for chemical titration. Note that the burette reads from top to bottom. Your instructor will perform the following procedure as a demonstration. You are responsible for observing, recording all of the numbers, going through all of the calculations and answering questions about the procedure. The first part of the process requires calibrating the AgN0 3 solution against the 35 o / oo salinity sample: 1. Pour approximately 20 ml of the 35 o / oo sample into a 50 ml beaker. 2. Using a graduated 5 ml pipette, transfer exactly 5 ml of this 35 o / oo sample into a 125 ml Erlenmeyer flask. 3. Add 5 ml of the K 2 CrO 4 solution to the same flask. Place a magnetic stirrer in the flask and set it under the burette on top of a stirring hot plate (Figure Eight). 4. Fill the 50 ml burette with the AgNO 3 solution, pouring it carefully down a funnel into the top of the burette. This can be done once for this entire set of four titrations. Record the starting point in Table E. 5. Turn on the stirrer (not the heat) to a low setting. Add the AgN0 3 solution slowly from the burette until you detect an abrupt color change from milky yellow to orange. 6. Record the new burette reading (the end point) in Table E. Revised on 2/21/2005 Page 14 of 21

15 7. Repeat the analysis for the known sample, again recording the starting and ending points on the burette in Table E (the starting point for the second analysis should be the same as the ending point for the first analysis). 8. Repeat steps 1-6 to titrate the unknown samples, recording the data in Table E. 9. Calculate the salinity of unknown samples A and B using the average amount of AgNO 3 required to titrate the 35 o / oo using the equation below: Rearranging the above equation: (7) (8) Revised on 2/21/2005 Page 15 of 21

16 Name Lab Section Date. MS20 Laboratory: Answer sheet: record all data in the appropriate metric units (centimeters, grams, etc.). Remember to use significant figure rules and to indicate appropriate units (if the scale reads 13.4 g, your answer is not 13.4, but 13.4 g (or 13.4 grams). A. Determination of salinity by evaporation TABLE A: SAMPLE MASS OF EMPTY DISH MASS OF DISH + WATER MASS OF WATER MASS OF DISH + SALT MASS OF SALT SALINITY OF SAMPLE 35 o / oo A B Assume there was "dirt" (fine silt or clay particles) in the sample you just evaporated. How would this have affected the salinity as you measured it? Could you have evaporated the salt out of the seawater at room temperature? If you could, would this have affected the salinity measurement? Explain. Revised on 2/21/2005 Page 16 of 21

17 B. Determination of salinity by electrical conductivity TABLE B: SAMPLE TEMPERATURE ( C) ELECTRICAL CONDUCTANCE (MMHOS/CM) SALINITY ( O / OO ) 35 o / oo A B Is warmer or colder water a better conductor of electricity? Why might this be true? Water at a temperature of 5ºC and 40 º/ ºº will have the same conductivity as what salinity water at 25ºC? Revised on 2/21/2005 Page 17 of 21

18 C. Determination of salinity by density using a hydrometer TABLE C: SAMPLE TEMPERATURE ( C) DENSITY (GRAMS/ML) SALINITY ( O / OO ) 35 o / oo A B A fully loaded ship moving from a fresh water port heads over to the Persian Gulf, an area known for high salinities. Will the ship ride higher or lower in the Gulf compared with the port it left? Explain. Why might the Persian Gulf be significantly more saline than other ocean basins (hint: it is the same reason that the Mediterranean Sea is more saline than the rest of the Atlantic, and the same reason that the Dead Sea is the saltiest body of water on Earth)? Does a ten degree change in temperature have a greater or lesser effect on density than a 10 o / oo change in salinity? Revised on 2/21/2005 Page 18 of 21

19 D. Determination of salinity by refractometer TABLE D: SAMPLE SALINITY ( O / OO ) 35 o / oo A B Under what conditions might the refractometer method be the only procedure you could use to determine salinity? Of the four methods for determining salinity you have examined so far in this laboratory: Evaporation Conductivity Hydrometer Refractometer. Which takes the most time? Which method might the most difficult to do aboard ship? Revised on 2/21/2005 Page 19 of 21

20 E. Determination of salinity by chemical titration TABLE E: SAMPLE INITIAL READING FINAL READING ML OF AgNO 3 SALINITY OF SAMPLE 35 o / oo 35 o / oo average A B Suppose that your glass burette were 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? Explain! You transferred exactly 5 ml of seawater with a pipette to an Erlenmeyer flask. Then you went for a cup of coffee, and some of the seawater evaporated before you could began your titration analysis. Would you have to start all over, or could you use the evaporated sample? Explain! Revised on 2/21/2005 Page 20 of 21

21 Name Lab Section Date. MS20 Laboratory: 1. Record the given salinities for each exercise. 35 o / oo A B (A) EVAPORATION (B) CONDUCTIVITY (C) HYDROMETER (D) REFRACTION (E) TITRATION 2. Consider the various methods of determining salinity. Which is the most accurate, which the least, and why? (Define accuracy as being close to the "true" value). 3. The silver nitrate titration actually directly measures the chlorinity of the samples, not the salinity. A more exact relationship between salinity and chlorinity is given by: Salinity = x Chlorinity Calculate the salinity of the following samples given the chlorinity: a. Cl = 10 ppt b. Cl = ppt c. Cl = 35 ppt d. Cl = 100 ppt 4. Sodium and chlorine are only two of the major ions in seawater. What are the others that make up 99% of the total salts? (hint: see your class text) Revised on 2/21/2005 Page 21 of 21