WHAT'S IN SEA WATER?

By Rick Ford

Ocean water is commonly called saltwater, and for good reason. Ocean water typically contains about 3.5% salts, or 35 parts per thousand. It can be as low as 20 parts/thousand (in the mouths of rivers, e.g.) and in places like the Red Sea has been measured as high as 42 parts/thousand. Here where we are it has been running nearly exactly 35 parts/thousand every day. The amount of salinity affects the density of water (saltier it is the more dense it is) which in turn affects how fast sound travels through it (more dense it is, the faster sound travels). Since our sonar mapping devices use sound waves to measure depth, this density is important to us out here, and we keep an eye on it.

The salts in ocean water are mainly made up of 9 ions: sodium (Na), Magnesium (Mg), Calcium (Ca), Potassium (K), Chlorine (Cl), and Sulphate (SO4). Of these, Sodium and Chlorine are by far the most abundant, so as you might guess, the most prevalent salt is NaCl, or just plain table salt that is usually put into food. But there are other salts as well. Na, Mg, Ca, and K all form positive ions; Cl and SO4 both form negative ions. Positive and negative ions attract (much like a magnet) and bond together in a process called ionic bonding. Any time this happens the result is a salt. So, with these nine ions, any of the positive ions can (and do) bond with either of the negative ions, forming a wide array of salts (KCl, CaCl2, etc.), and all of these are in ocean water.

The addition of salts in ocean water makes it just slightly more dense than fresh water. That means that some things that would sink in a lake would float in the ocean.

Why can't you just drink it?

It's common knowledge that you can't drink ocean water. A sip or two won't hurt you, but you cannot live on the stuff day after day. Here's why.

Water goes in and out of cells by a process called osmosis. Simply put, if there is too much water in a cell there is a buildup of pressure and the water gets squeezed out of the cell through the cell membrane (the cell's skin, if you will). If there is not enough water in a cell, the resulting low pressure will cause water to be sucked into it, again through the cell membrane. The key to this process working is that the cell membrane is set up to allow freshwater in and out of it.

Since salt water has salts in it and is slightly more dense than freshwater, this normal in/out flow is disrupted. Salt water won't flow in, and the salts themselves actually attract and draw water out of the cells, literally causing the cells to dry up (dehydrate) in a short time.

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