From sieloff@ideanet.doe.state.in.usThu May 23 12:52:15 1996 Date: Wed, 22 May 1996 23:23:25 -0500 (EST) From: Roger Sieloff ISDH Reply to: hydro@lists1.best.com To: hydro@lists1.best.com Subject: REDOX, RO & U On Wed, 22 May 1996, Kilroy wrote: > I am quite new in this, As I was looking through a catalog from HANNA (BTW > have any one heard of this company?) some terms came up which I need some > clarification > > a) Oxidation Reduction Potential (ORP) huh?? ;) > > c) Reverse Osmosis (sp?) how does it work? Hapless newbie in a state of Chemical Confusion? This looks like a job for WONDER WARTHAWG! Redox Potentials: "Oxidation/Reduction Potential". Essentially the principle behind why metals corrode, batteries work and pH meters measure pH. Let's start with Granny's heirloom silver serving tray and h'ourderves consisting of Deviled eggs. The silver atoms in the tray are in energy state #1. Pure, bright, beautiful (reduced) Silver metal. Those deviled eggs are loaded with brimstone (sulfur) in energy state #2. State #1 is higher than state #2. Two shiny Silver angels meet the the yellow Sulfur dog of Satan and and fall from grace, their fellow angles tarnished by the black stain of silver sulfide. Specifically, two Silver atoms toss away a high energy pair of electrons, grab hold of a molecule of hydrogen sulfide gas, strip off the hydrogen to regain their electrons and become a black deposit. Silver is oxidized and hydrogen reduced - the original electron pair get shunted through the butler holding the tray and back down to the center of the earth, a hudge reservoir of negative charge. No, the butler feels nothing - there is not that much of a difference in energy state #1 and #2 for the reaction to be at all that spontaneous. (He gets zapped later on the golfcourse when that vast reservoir of negative charge flows back into the positivly charged atmosphere as lightening). Moral of the story: Aside from the fact Satan frequently gets two for the price of one and hurls lightening at heaven from the center of the earth (and I bet you thought it was the OTHER way around!), the difference in energy state #1 and energy state #2 is the REDOX POTENTIAL. If there is a BIG difference then there is a BIG reaction. Car batteries work by continiously oxidizing lead metal to lead sulfate (there's that old Devil again!), starting your car in the morning by producing a whole lot of electricity. On the way to work, the engine drives a little dynamo called the alternator which mechanically feeds electricity back into the battery, turning that lead sulfate back into lead metal, enabling you to start your car the next morning. pH meters are a complicated electrical circuit consisting of mercury, silver and their chloride salts. As far as I can tell, the unit works according to the fact the resistance of one of the electrodes is dependant on the hydrogen ion concentration (pH) of the solution being measured and the other (the reference electrode) is not. The diffrence in electrical potential between the two electrodes is read as pH, accurate only if one uses a solution of known pH to establish the fact that "potential difference such-and-such corresponds to a pH of 4.0" (dip electrodes and twist the calibration knob to 4.0). BTW: the unit works due to a SEMIPERMIABLE MEMBRANE (next lecture) and the electrodes must be kept constantly wet. Also, the unit is temperature dependant, so even a calibrated unit at 10 C might be inaccurate at 40C. Semipermiable Membranes: Take a bowl, fill with water and glass marbles. Pour contents through a sieve into another bowl. Marbles have been separated from water! Semipermiable membranes work exactly the same, only on a molecular level. Granted, gravity is not the force at play here, rather something called brownian movement. Imagine a big amusement park with lots of adults and kids walking around in a more or less random fashion. The kids are supposed to stay with their parents, but naturally DON'T. Still, there is a fairly homogenious mix of adults and kids. Now introduce a fence whose slats are kid sized. Kids slip through and adults can't. Sooner or later there will be a big bunch kids on one side of the fence and a group of irate adults bellowing at them from the other side. Now if giant aliean shrimp from another planet suddenly materialize and capture all the kids separated convieniently on one side of the fence sooner or later another bunch of dumb little kids are going to squeeze through the fence the next day to repeat the process. (P.T. Barnum said there's a sucker born every minute...) Another story, another moral: The point here is that if a membrane has openings in it allowing some molecules to pass through, they will, but not the ones too big to get through. This process is completly natural and requires no outside energy to work. Nature abhores a vacuum - if the vacuum has a bag, you eventually aquire quite a collection of lint. After WWII it was found ordinary celophane used to wrap things could separate small molecules such as urea from blood - hence the development of the first dialysis machine for people with no kidneys. A very important form of Reverse Osmossis employs thin sheets of teflon, that inert plastic used to coat non stick pots and pans. The teflon will only allow water molecules to pass through, hence a "RO" unit can produce absolutly pure water at a fraction of the cost of a distiller (it takes an enormous amount of expensive energy to boil water). That's the way it is, Roger L. Sieloff