MadSci Network: Science History |
Dear Tonya, The chemistry of salt water is well understood and the techniques for measuring it have expanded over the years. In the early 1960's salt content was most simply measured by titrating with silver nitrate (AgNO3) solution with a yellow potassium dichromate indicator. As the silver and chloride reacted to form insoluble silver chloride (AgCl), the indicator would change from a bright canary yellow to a faint rose pink color at the end point. By knowing the concentration of the silver nitrate, the volume of silver nitrate titrated into the sample, and the volume of the original salt water sample, the concentration of the salt could be determined. The following formula relates the concentrations and volumes: C1 x V1 = C2 x V2, where: C1 = Concentration of the silver nitrate delivered V1 = Volume of silver nitrate delivered C2 = Concentration of the salt in the original salt water sample, and V2 = Volume of original salt water sample. The concentration of salt can also be detemined by mesuring the electrical conductivity of the water sample. Pure water is a poor conductor of electricity, but if you dissolve salt in it, the water will conduct electrical current more easily. By referring to a standard chart of conductivity vs. salt concentration, the salt content can be easily determined. You mentioned a reftractometer. These were also in use in the early 1960's. As the salt content increases in a water sample, the index of refraction increases. That is, the more salt dissolved in water, the slower light travels through it. This is measured by determining how much a beam of light is bent as it passes from air into the salt water or from glass into the water. A hydrometer measures the density of the salt water solution. The more salt dissolved in the water, the more dense it is, or the more dense it is. A hydrometer floats in the water with a claibrated scale sticking out above the surface. More dense liquids will cause the hydrometer to float higher in the fluid. A less dense liquid will cause it to sink lower in the fluid. The specific gravity can be directly measured from the scale. A liquid's density can also be measured with a volumetric flask and an analytical balance. This laboratory technique has been used for hundreds of years. A known volume of water, say 100 cc., is accurately weighed on an analytical balance. If the net weight is 103 gm., the density is 103 gm/100 cc. or 1.03 gm/cc. for a specific gravity of 1.03. Pure water has a specific gravity of about 1.00 depending on the temperature. This difference is easily measured in the laboratory. Saltiness of sea water vaires by depth and by latitude on the earth. Different seas have slightly different salt contents from the main world ocean. The average surface salinity of all the world's oceans is about 3.5%. Saltier water is slightly denser than fresher water and will sink as some of the water is evaporated from the surface. The Medeterranian Sea, for instance loses 100,000 tons of water per second to evaporation, increasing the salininty of that body of water to about 3.9%. The Red Sea is saltier still, at 4.0%. The Dead Sea is about 10% salt by weight. Beacuse of this evaporation process, the saltier water left after evaporation, sinks. As alinity increases, density increases, and water with a high alinity will tend to sink to a level that achievess an equilibrium. Thus, surface water in an ocean has a uniform salinity to a depth of a few hundred meters. On the average, at a depth of about 500 to 600 meters there is a sudden increase in salinity at aboundary called the halocline. This increased slainity continues for several hundred feet and then slowly increases very slowly to the bottom. Contrary to popular belief, the salinity in the oceans did not slow increase with rain water flowing over the exposed land masses, dissolving salt as it went. Much of the chlorides in the ocean are believed to have come from volcanoes and the outpouring of the gases that accompany eruptions. The temperature also changes as the depth increases. The surface water is the warmest, and therefore the least dense. As one goes deeper into the sea, the temperature decreases. There is a boundary layer at about 800 meters where the temperature has reached a somewhat stable low temperature that continues to the bottom. The coldest water in the ocean is on the very bottom. The surface temperatures range from about 32 deg. F. in the high latitudes to about 82 deg. F. near the equator. I hope this helps. Life Nature Library, The Sea, Time Life Books, New York, 1963. Introductory Oceanography, Joseph Weisberg, McGraw Hill Book Co., New York, 1974. The Book of Popular Science, The Sea, Grolier Book Co., 1967.
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