The Official Koiphen "The Effects of pH on ORP Readings" Thread

[stephen]

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.

[BruceP]

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!

[Roddy Conrad]

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.

[stephen]

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.

Please do Roddy. I did a lot of tests using muratic acid but I am sure the accuracy would not be as precise as yours. But it was accurate enough to see the dramatic effect that pH has on ORP readings

[Zac Penn]

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?

[stephen]

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?

Much more oxidizing which would be good.

[farne230]

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

[Rick Gippner]

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 Gippner]

One should be able to run the formula backwards and predict the ORP.

[stephen]

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

[Rick Gippner]

um, 39.727181 if I do the math correctly

pH 8.5
ORP 275.00
DO 6.2

[Rick Gippner]

I just noticed no DO in your formula. If I drop the DO from mine:

33.227181

[stephen]

um, 39.727181 if I do the math correctly

pH 8.5
ORP 275.00
DO 6.2

I get 33.23 which is almost exactly the same as mine. DO has already been factored into this particular equasion.

[Rick Gippner]

I get 33.23 which is almost exactly the same as mine. DO has already been factured into this particular equasion.

I thought it was (ORP + 205)/29.58 + (pH x 2) + DO . no?

[stephen]

I thought it was (ORP + 205)/29.58 + (pH x 2) + DO . no?

Different variation of equation:

rH2=ORP/29 + 2 x pH + 6.67

275/29=9.48 plus 17 plus 6.67 = 33.15

[Rick Gippner]

I thought it was (ORP + 205)/29.58 + (pH x 2) + DO . no?

Different variation of equation:

rH2=ORP/29 + 2 x pH + 6.67

275/29=9.48 plus 17 plus 6.67 = 33.15

I see said the blind man

[Rick Gippner]

Then I would agree that rH2 is a better route as in theory one could have water devoid of O2 and still have a high factor, no?

Edit: Drat have to go to work now

[stephen]

I see said the blind man

It doesn't really matter which equasion that you use as long as you use the same one when comparing.

[stephen]

Rick, for example, I believe that your water would be more oxidizing than water with a pH of 7 and an ORP of 360mV

[Rick Gippner]

Rick, for example, I believe that your water would be more oxidizing than water with a pH of 7 and an ORP of 360mV

This is why I agree that you formula without DO is incomplete. Your water is, what, 75-80 degrees? Mine is 50 and therefore more DO.