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#1
03-11-2008
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The Official Koiphen "The Effects of pH on ORP Readings" Thread
In the last few years I have attempted to minimize the value of ORP readings. For example someone may proudly proclaim that their ORP hovers around the 350mV level while yours is only 250mV. Some will even insinuate that your water is inferior to theirs as a result of this number. This is not necessarily true however. The true value of the reading is dependent upon the pH of the sample tested. Some folks falsely claim and preach that oxygen levels are the greatest factor influencing the ORP of the water. This is not true. Altho oxygen level does play a minimal part in determining the ORP, it is the pH of the water that mostly determines it's level.
In 1923 Clark computed a true relative hydrogen reducing power scale (rH score, aka rH2 score) from examination of both pH and ORP measures based on a a variation of the Nernst Equation. The relative hydrogen, or rH (aka rH2) scale is a scale the measures the reducing power of simple compounds in aqueous solution. The rH scale runs from 0 to 42; 28 is mid-point (balance), below 28 is reducing and above 28 oxidizing. So, rH provides a measure of absolute reducing potential of a substance eliminating the effects of pH. There are several variations of the Clark/Nernst Formula for computing rh out there. I have recently seen Jr. post this one: rH = ORP divided by 29 + (2 x ph). This is Extremely Inaccurate as Dissolved Oxygen was left out of the equation. Reef Central Aquarium website uses a simular equasion: rH=(ORP/29) + (2 x pH) + 6.67 (a constant added for the dissolved oxygen) This appears to be a decent way of calculating rH. The formula that I choose to Calculate my rH is: rH = ((ORP + 205)/29.58) + (2 x pH). It is a bit more complicated but probably more accurate as well. We can discuss more or calculate rH levels if there is interest. Last edited by stephen; 06-29-2009 at 06:21 PM. 
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#2
03-11-2008
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I havent a clue as to what you are talking about, but I always find your threads about water interesting.....
 It's All About The Water!
__________________
Bruce "The significant problems we face cannot be solved at the same level of thinking we were at when we created them." Albert Einstein Every good landscaping project starts with a hole! http://www.worldwidekoiclub.org/ 
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#3
03-11-2008
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I ran an experiment on this subject at work one day. With distilled water as the base, and no oxidizer present, I adjusted pH from 1 to 14 using either caustic or hydrochloric acid. At pH 1 distilled water without any oxidizer runs an ORP of 1200, at pH 14 the ORP of distilled water is near or at zero.
I can look it up at work and publish the entire data set if there is any serious interest in the effect.
__________________
President Truman said, "If you can't stand the heat, get out of the kitchen." And he said if you want a friend on the AKCA board, better take your dog to the board meeting with you.
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#4
03-11-2008
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Quote:
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#5
03-11-2008
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So if ph adjusts your orp value up or down...A 300 orp reading at a ph of 8, is much better than an orp reading of 300 at a ph of 7. Correct?
__________________
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#6
03-11-2008
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Quote:
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#7
03-11-2008
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I would like to know if there is a published PH/ORP curve. One problem with ORP readings is they drift higher. I can say my orp reading were lowest in the morning and highest at night the same as PH readings.
Bob
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#8
03-12-2008
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I for one would like to know more about this. The four dimensional graph of pH, Temp, DO and ORP boggles my mind, but I think there is some kind of of Holy Grail hidden in that number. Some kind of alchemy that turns water into gold is held there, a universal measuring stick if you will.
__________________
Rick I've always wanted to be out, standing in my field  Indecisiveness is the key to flexibilty  Koi keeping teaches humility!  Home  Work  
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#9
03-12-2008
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One should be able to run the formula backwards and predict the ORP.
__________________
Rick I've always wanted to be out, standing in my field Indecisiveness is the key to flexibilty Koi keeping teaches humility! Home Work
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#10
03-12-2008
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The point here is that ORP readings alone do not provide the true oxidizing value of the water. Without pH factored into the equasion it is a pretty worthless number when used to compare the water of 2 different ponds. Unfortuneately you will often hear some constantly spouting off about the ORP reading in their pond. Big deal, they have a low pH
A more meaningful comparison would be attained by using rH2. I will do the current rH2 on my pond. The current pH is 7.7 and the ORP is 324 mV. (324 + 205 = 529) /29.58 = 17.88 + (7.7 x 2 = 15.4) rH2 = (17.88 + 15.4) 33.28 rH2 of 28 is considered neutral, so the above reading indicates that the water is in an oxidizing state. What's yours 
Last edited by stephen; 03-12-2008 at 10:27 AM.
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