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Last updated: 12/30/2023 Version number: 1.54.

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Relative Hydrogen Score, or rH: Clark’s Rendering of the Nernst Equation for Determining Relative Hydrogen (rH, aka rH2) from pH and ORP Measures

Abstract
Discusses a version of the Nernst equation posited by Clark in 1923 for computing a true relative hydrogen reducing power score (rH score, aka rH2 score) from examination of both pH and ORP; often cited by deClerck (aka DeClerk; circa 1950) for beer brewing measures in a brewing textbook.  Includes samples of some widely used rH formulas and the most accepted ones. 

Introduction
The relative hydrogen scale was first proposed by Clark in 1923 to the scientific community (mostly inorganic chemists) as a means of measuring the true reducing power of hydrogen present in an aqueous solution, based upon a variation of the Nernst equation. The relative hydrogen, or rH (aka rH2), scale is a scale which measures true reducing power of simple compounds (very low molecular weight antioxidants) in aqueous solution. Let us note here that complex biochemical reducing agents, for example, antioxidants such as vitamin C, vitamin E, alpha lipoic acid or Coenzyme Q10 (ubiquinone) will not necessarily, when in aqueous solution, influence pH and particularly, ORP, in such a way as to render a true measure of their reducing (antioxidant) power; this is due to the fact that their antioxidant activity is optimal only for certain energy levels or levels of complexity, and outside those levels (e.g., simply dissolved in water), the reducing power may not be “released”.  To return to the topic of rH: In other words, unlike ORP, which yields a rough indicator of the amount of hydrogen reducing power available, but which is highly dependent upon the untoward and undesired effects of pH (e.g., the H+ ion, or hydrogen proton, which, when present in quantity, makes substances acidic), the rH yields a true index of hydrogen reducing power available, fully compensated for pH, to allow removal of all influence of the H+ ion (which, simply speaking, makes a substance acidic if present in quantity.)

More About the Scale
The rH scale employs the same logarithmic Bar scale used to express gas pressure in terms of order of magnitude, and the rH scale runs from 0 to 42; 28 is mid-point (balance), below 28 is reducing, above 28 oxidizing.   An rH of 42 indicates 1 atmosphere (1 ATM) of partial pressure of O and a –42 exponent partial pressure of H, while an rH of zero (0) indicates 1 ATM partial pressure of H (adjusted for skewing effects of the H+ [aka “acid”] ion) and a –42 exponent partial pressure of O.  Therefore, an rH score of 0 would indicate maximum possible reducing (antioxidant) power, while a score of 42 would indicate the maximum possible oxidizing environment.

So, rH yields a hydrogen proton-unbiased measure of absolute reducing potential of a substance; eliminating effects due to pH.  rH is log1/hydrogen partial pressure; a one unit change in rH indicates a 10X change. E.g., a shift on the Barr scale of rH from a score of 27 to one of 26 indicates a 10X increase in reducing power.  rH 28 = H partial pressure (pp) of 10^-28 atmospheres, an rH 11 = H pp is 10^-11 atmospheres, and a 0 rH is the rH value of pure hydrogen at STP.  Again, as noted above, an rH score of 0 would indicate maximum possible reducing (aka “antioxidant”) power, while a score of 42 would indicate the maximum possible oxidizing environment.

Nonetheless, ORP and better, rH, have found some degree of acceptance for many years in the fields of high-end aquarium keeping, wine and beer brewing, food storage and food safety as an indicator or relative antioxidant ability, and, since the mid-to-late 90's; in some sectors of the nutritional antioxidant field as an indicator of the same as well.  It is somewhat important to note that ORP and rH scores have also been used to some extent for many years in the groundwater and waste water remediation fields as well as measures of relative oxidation or reducing ability of water.

Some Notes Regarding the Nominal rH Score Range of 0 to 42
In the section immediately above this one, I mentioned that the nominal range of the relative score is commonly agreed in the field to be 0 to 42, with a midpoint of 28. I did not posit that range, and rather that is the nominal range for relative hydrogen score that has pretty much uniformly been reported in the literature in this field since the 1920s, and I have simply reported that commonly accepted range on this page and on other pages on this website.

However, the reality is that in some cases, particularly when you are dealing with extremes of either pH or ORP (used to compute the rH score), you may find that the relative hydrogen score delivered by the formula that you are using may be a score that is somewhat below 0 (i.e., a score of -0.7, or a score of -2.1), or alternatively, a score that is somewhat higher than 42 (i.e., a score of 45, or a score of 50.2). There are a great number of reasons why this can happen when making measurements in the real world, and just a few of them are listed below, and, in any one situation, one or more of these factors may be relevant:

• when measuring pH in the real world, there are often minor errors of up to 0.4 pH units, and, in some extreme cases, up to 2 pH units, in the measure, dependent upon the probes used, their age, probe contamination and poisoning factors, their cleanliness, and the calibration of the probe and meter, and also, of course, there is the matter of temperature compensation, which is often ignored. These distorted pH readings can and will result in errors in the relative hydrogen score calculated from these readings.
  
• when making measurements of ORP (aka redox) in the real world, there are very often rather major errors, and sometimes extreme errors, in the resultant readings to the extent that errors in real-world ORP readings easily make any minor errors commonly encountered with taking pH measures appear totally pale and minor in comparison. These distorted ORP readings can and will result in errors in the relative hydrogen score calculated from these readings. Although I will make no attempt to provide an encyclopedic list of the reasons for such errors in real-world ORP measures, here are just a few of them:
  
• it is a well-known fact that at least 50% of new ORP probes, fresh out of the package, are incapable of detecting ORP values below 0 (i.e. ORP values of -50, or -200, or -400) without extensive and exhaustive "conditioning", which few users ever bother to perform (and, to be fair, a great many users of new ORP probes are totally unaware of the need for such conditioning of the ORP probes, just as a great number of users of older in-service ORP probes regularly fail to properly clean their probes)
  
• due to the same factor mentioned above, at least 50% of new ORP probes, fresh out of the package, not only cannot detect ORP values below 0, but will also read high for most ORP values in the range of 0 to about 300 or 400, again due to failure to properly prepare and condition the new probe. These new probes may also tend to "pull" to readings in the range of 0 to 300 toward (false) readings in the range of +400 and above, again due to failure to properly prepare and condition new ORP probes.
  
• let us now look at the case of ORP probes which have been properly prepared and conditioned when they were put into service, so that they are able to accurately detect ORP values below 0, and also able to accurately display ORP values in the range of 0 to 400 (since these are the two ranges most commonly blunted or distorted by new probes). The reality is that, in a great many settings, once ORP probes have been in use for a few weeks, unless they were properly stored and cleaned between each use, they will very often, due to a variety of types of contamination and buildup of salts and other compounds on probe components, begin to "pull" readings (usually toward the range of 400 to 800, but I have seen rare cases where contamination and poisoning of ORP probes led to the readings being pulled downward toward the -200 range)
  
• the ORP reading delivered to the measurement and display instrument (and most commonly displayed in millivolts, aka mv) by most ORP electrodes is simply a raw voltage (aka EMF potential), and the reality is that there are several different types of ORP electrodes on the market, and each requires a different offset (i.e., adjustment) to the raw voltage (EMF potential) delivered to the ORP meter before it can be accurately displayed as the true ORP in millivolts (mv). Failure to calibrate the meter/display device for the type of probe in use can result in distortions of the ORP reading by up to 200 mv (and sometimes more); this failure to set up the meter to display readings with the appropriate/accurate offset, in order to display accurate ORP in millivolts, is extremely commonplace in taking real-world ORP measures across the world.
  
• many users of ORP probes fail to realize that it can take many minutes, and sometimes far longer, for an ORP probe, even it is has been properly prepared and conditioned, to accurately display the true ORP of many aqueous solutions, and, because of this factor, more conscientious users of ORP probes will often wait for at least 5 to 10 minutes to allow the ORP probe to "settle", once it has been immersed in the aqueous liquid to be tested.
  
• and, much as was also true for pH measures as noted above, many readings derived from ORP probes have never been compensated for the actual temperature of the liquid being measured; this will make the reading appear either higher or lower than the true value.
  
• there exists, in the literature dating back to the 1920s, a large number or variant formulas or equations for calculating rH score. Each one will yield a slightly different rH score for the same combination of ORP and pH values. The reality is that none of these formulas is perfect when it comes to the real world, and rather, each of these formulas is simply an attempt at coming up with a formula that seems to deliver a "best-fit" approximation of relative hydrogen score in the real world that hopefully closely matches the purely theoretical construct of relative hydrogen score. In reality, the fit to theoretical values is never perfect when looking at computation of relative hydrogen score from real-world ORP and pH measures, and thus, even if you feed the best of the candidate rH formulas only sets of ORP and pH measures that are quite accurate, you will sometimes, particularly when dealing with extremes of pH and ORP values, end up with rH scores that are below zero (that is, in the negative range) or above 42, that is, outside the normal range cited for rH scores. In my own work over the past 10+ years as a consultant in the realms of making accurate ORP measures for values below 200 mv (and particularly in the negative range), and in the realms of ensuring accurate computations of relative hydrogen scores in a variety of settings for a number of clients, I find that the lowest real-world rH score usually encountered at the lowest extreme is usually never below about -4.5, and I find that the highest extreme rH score usually encountered is never above 50, and the reality is that these outlier scores (at both the low high ends of the scale) are very rare, and the vast majority of rH scores derived in the real world, from real-world measures, comfortably fall into the 0 to 42 range commonly cited in the literature on relative hydrogen score.
  

rH or rH2?
In the early days of discussion in the literature of the relative hydrogen score, it seems that there was at least a modicum of effort made to use the abbreviated term "rH2 score" rather than "rH score" for this entity. However, as time passed, the shorthand term "rH score" began to be used just as frequently as the term "rH2 score". It occurs to me that the reasons for the proliferation of the term "rH score" are manifold. First, the term "rH" is easier to both speak and write, and is usually considered an acceptable shorthand, since everyone knows that H2 (the diatomic molecule consisting of two hydrogen atoms) is simply one occurrence, that is, the diatomic molecular form, of hydrogen (H).

But there is another reason for the increased usage of the term "rH score" as well, and that is simply the fact that, in reality, the relative hydrogen score is truly a measure of the relative presence of all simple forms of hydrogen, including both the monatomic elemental form (sometimes called atomic hydrogen, or active hydrogen, the hydride ion, or negative hydrogen ion) and the diatomic molecular form (aka H2). This latter-day shift to more wide-spread usage of the term "rH score" is a response to the fact that, at times throughout its history, the scale in question has been referred to as the rH2 scale, which would tend to imply that the scale is solely indicative of the presence of dissolved diatomic hydrogen, aka H2, in solution. As I have hinted above, this is decidedly not true, and the scale is equally sensitive, if not more so, to the presence in solution of atomic hydrogen or the hydride ion, the latter usually not in free form, but rather "available" in the form of hydride ions, solvated water clusters, hydride ions in water cages, or very low molecular weight antioxidants. Yes, it is true that even some biochemical antioxidants with an MW over 150, such as many forms of Vitamin E, also function as an antioxidant by donating the hydride (H-minus ion) at the right time and place, but many of these molecules are so specific as to where and how they can and will donate the hydride ion that they will usually not influence rH score (or, concomitantly, ORP measures) significantly.

An Important Note About rH (or ORP) and Antioxidant Power
Although it may be noted again later in this paper, I feel that it is extremely important to note that rH is only a rough and correlative indicator of true antioxidative (aka reducing) power of a solution. While it is true that many of the simple hydride antioxidants, such as atomic hydrogen or the hydride in in water cages, or other simple hydride species, and many lighter molecular weight (MW) antioxidants with MW below about 150 will often appreciably reflect their antioxidant activity via a low ORP and low rH score (indicating high antioxidant ability), it is equally true that many higher MW antioxidants, with MW ranging from 150 to over 1,000 (some have MWs of up to 200,000 Dalton units...) will not necessarily have their antioxidant or reducing activity adequately reflected in ORP or rH score. This is because these biochemical antioxidants may be more specifically "tuned" to donate their free electron (or H- ion, as the case may be) in specific biochemical systems or at very specific energy levels, and will therefore not show much effect on ORP and rH score of "plain" unadulterated water.

The Simplified Variants of the Clark/Nernst Formula for Computing rH
There have been a number of variant simplified formulas for computing rH from ORP and pH of aqueous solutions floating around the worlds of science and the related applied science fields for over 40 years.  For the purposes of this paper, a “simplified formula” is any which has been rendered as a simple algebraic equation without any logarithmic functions.  Particularly, there have been a number of variant simplified rH formulas appearing in a number of fields for many years since Clark published his two-part article in 1923  positing this measure based upon the Nernst equation. Some of the fields where rH (aka rH2) has been employed extensively are :

• professional wine and beer brewing
• homebrewing of beer and wine
• nutritional antioxidant research
• within the food processing industry, as a means of assessing redox state (e.g., antioxidant status and spoilage potential) of fruit juices, etc.
• some sectors of alternative health diagnostic systems, especially Bioelectronics by Vincent (BEV), also known as Biological Terrain Analysis (BTA or BT)
• inorganic chemistry of metals (oxidation and reduction)
• biochemistry of metal oxidation or reduction
• soil nutrient measurement and formulation, especially within the realms of organic farming/gardening
• environmental waste water and remediation studies
• ground water studies, including assays of surface waters, underground aquifers, water from deep wells, etc.
• the high-end aquarium world 

rH = ((ORP + 200) / 30) + (2 * pH)

For some samples of rH scores calculated from various combinations of ORP and pH, please see the Samples table on the page on this site entitled Sample rH Scores Calculated from ORP and pH

This is the formula which I normally use for computing rH and which seems to be employed by most researchers whom I know, with extremely minor variations at times.  Since I have no patience for performing hand calculations, I have created a simple Excel spreadsheet with columns for name of batch or test, pH and ORP, and then a fourth column instantly calculates the rH from the ORP and pH. 

Please note that manufacturers (e.g., WTW and others) of digital test equipment for rH, ORP and pH, and of calibration test solutions for ORP and rH, often use a simpler and less accurate formula. This is discussed to some extent in the section below entitled A Wide Range of Variant Formulas.

For some samples of rH scores calculated from various combinations of ORP and pH, please see the Samples table on the page on this site entitled Sample rH Scores Calculated from ORP and pH
 

A Wide Range of Variant Formulas
The Clark/Nernst formula has been converted into a number of other variant simplified rH formulas over the years, some of which yield meaningful and rather accurate numbers and others of which yield near-nonsensical scores.  Some of the mainstream and variant versions, with a few notes, appear below:

Generic:
Per numerous generic sources in a number of fields, the simplified Clark/Nernst equation for deriving rH from pH and ORP is:
  rH = (ORP / 30) + (2 * pH)
This yields a score which is somewhat higher than the true rH reading.

Patrick Flanagan:
Per Patrick Flanagan, the Clark equation for deriving rH from pH and ORP is:
  rH = ((ORP + 200) / 30) + (2 * pH)
Patrick reports that the +200 is to adjust for a pure hydrogen reference standard, as well as temperature.   He reports that the number should be 204 for 75 degrees F. 
Please note that this is the formula which I usually use; see also notes below about the Siegfried Lubke formula, which is essentially identical….

The formula from Siegfried Luebke (aka Lubke or Leubke), the amazing Austrian organic farmer:
Per Bob Pike at Pike Agri-Lab Supplies (organic and sustainable farming supplier), the formula he was given years ago by Siegfried Lubeke, the near magical and mystical farmer in Austria, is:
  rH = ((ORP + 210) / 30) + (2 * pH)
Lubke showed him this exact formula from a German-language book in his library, which is where Luebke found it originally. Bob was under the impression that the + 210 was the rendering of RT for 21 degrees C.  It is not; rather, it is, as Patrick reports, an adjustment for a pure hydrogen electrode reference.
This formula is almost identical to Patrick’s formula from Clark, except for the fact that Patrick employs +200 while Pike/Luebke employ +210 in the numerator of the fraction; this is simply related to temperature adjustment.

Incidentally, while on the topic of Luebke, in November 2003 I received a very helpful note from Steve Diver, the well-known organic farming and biodynamic farming consultant, and I have reproduced part of his letter below:

"Fyi, Siegfried Luebke is the Austrian farmer who developed Controlled Microbial Compost.  He is recognized as a self-taught genius in microbiology.  Referring to him as the mystical farmer from Austria is a bit misleading. The Luebkes never talk about mystical things in their seminars; they are very practical and grounded in agronomics, microbiology, soil science, and organic farming methods.   They use ORP as compost quality parameter.

Fyi, there is a parallel between the microbial power of Clay-Humus, SAM (syntropic antioxidative microbes), BD and so on, in that all microbe farming systems are infused with and emanate a bioenergy field.   Microvita theory based on P.R. Sarkar's work in India may be a unifying thread that links subtle energy practices to biology and physics. 

Fyi, Dr.Alberta Velimirov and other researchers in Europe have used the P-Value test to evaluate electrochemical properties of food.  It is single value derived from pH, redox, and electrical conductivity."  [ed, note: This is extremely similar, if not identical to, the Biological Terrain Assessment (BT, or BTA) score created by followers of the work of Claude Vincent, the French hydrologist turned health trends statistician.]

EIT formula from Microhydrin’s Early Days:
The EIT folks – who did not reveal their exact rH formula -- provided a list of pH/ORP/rH values for various common liquids to RBC, and these values were published on numerous RBC websites.  Most of the calculated rH values agree closely with Patrick’s formula above, but a few, especially at pH’s below 6.5 and ORPs in the negative range, were very far off from all the results from any other equations. I suspect that they attempted to use either the formula from Patrick or from Lubke, but that they sometimes erred on their math; perhaps their program or Excel sheet had a bug…

Gage, O’Dowd and Williams Paper on Biological Removal of Iron and Manganese:
Per Gage and O'Dowd in web PDF file: "Biological Iron and Manganese Removal, Pilot and….", presented at the Ontario Water Works Association conference, 5/3/2001:
  rH = (Eh / 0.029) + (2 * pH)
I suspect there is a major typographic error in the formula in the article, and that the first portion of the right hand part of the equation should read (Eh * 0.029) or (Eh / 29)… as it currently stands, the formula makes little sense, and yields nonsense numbers…!  This formula simply does not seem to work even if ORP is adjusted to yield a "true" Eh score.

Formula Found in Several Places in Aquarium World Websites:
Per formula: Reef Central Aquarium website; the 6.67 constant added is for "dissolved oxygen":
  rH = (ORP / 29) + (2 * pH) + 6.67
Strangely, the +6.67 function does much the same thing as adding +200 or +210 to the ORP in the numerator of the fraction… yielding an rH which is very close to the formulas (e.g., Flanagan, Lubke, etc.) which do this.

From the beer brewing world, largely from an article by deClerk circa the 1950s:
This one is really odd, and the method has been culled and reconstructed from various homebrew beer websites, old beer brewing texts, and a few old posts to homebrew beer list groups or forums, all of which seem to trace their lineage back to an article by DeClerk from the 1950s, apparently published in a beer brewing book. It appears to be a rather complex equation, which involves one measure of temperature and several separate measures of pH performed on the liquid, the latter with several different types of electrodes, each using different reference metals in the electrode. Obviously, the purpose of this painstaking, tedious, puzzling and expensive exercise is to really derive a pH measure and a simulacra of an ORP measure. Perhaps when deClerk originated this measure in the 1950's, good ORP electrodes were hard to come by... Things are much easier nowadays, as we have access to good reliable pH electrodes and ORP electrodes.

Unmodified Nernst/Clark Formula, from Several Papers:
For reference purposes, Dr. Patrick Flanagan and Cory Stephanson report that the original Clark formula, from Clarks' 1923 paper is:
    Eh = 1.23 – ((RT / F) pH) – (RT/ 4F) * ln (1 / Po)

For some samples of rH scores calculated from various combinations of ORP and pH, please see the Samples table on the page on this site entitled Sample rH Scores Calculated from ORP and pH

Notes:

where ORP or Eh or Eo or E = ORP in millivolts, 1.23 refers to potential of oxygen under 1 atm pressure, rH is defined as the negative logarithm of the oxygen pressure Po, or:
  rH = -log Po

rH yields a hydrogen proton-unbiased measure of absolute reducing potential of a substance; eliminating effects due to pH.

rH is log1/hydrogen pressure; a 1 unit change in rH = 10X change. E.g., a shift from 27 to 26 = 10X increase in reducing power.

rH scale runs from 0 to 42; 28 is mid-point (balance), below 28 is reducing, above 28 oxidizing; rH 42 indicates 1 ATM O; rH 0 = 1 ATM H.

In plain English, an rH score of 0 would indicate maximum possible reducing (antioxidant) power, while a score of 42 would indicate the maximum possible oxidizing environment; a score of 28 is mid-point.

rH 28 = H partial pressure (pp) of 10^-28 atmospheres; an rH 11 = H pp is 10^-11 atmospheres. 0 rH is rH value of pure H2 at STP.
 

Care, Storage, Use and Cleaning of ORP and pH Electrodes (Probes)

Here are some fairly complete instructions for care, use and cleaning of pH and ORP electrode probes; they should also work for conductivity probes as well. 

Caveat
I make and offer no guarantees as to the accuracy of usability of this information, although it has worked well for me.  I take no responsibility for any results or lack or results, or any harm or injury from any caustic substance used in these procedures. Frankly and bluntly, if you are a stupid-head, you should should not be using pH or ORP probes in the first place, much less trying to clean them!

Origins of Instructions and Procedures
These procedures seem to be fairly standardized in the various lab equipment fields, although many equipment manufacturers will not call the cleaning compounds or solutions by their simple names, but rather will want to sell you the same stuff under their own brand name for a thousand times the price of what you could purchase it for under it's generic name. Your choice...
I have listed sources for all materials at the end of the post.

Storage
First, however, let us talk about how to store pH and ORP probes (and conductivity) between uses, since that is even more critical than occasional cleaning.  Storage and maintenance of probes after each use:

1) After each use, probes should be dipped in clean tap water and agitated to remove any contaminants or stray substances.

2) Then, the probes should always be stored wet, that is in water, and the water must never be distilled or (fully) de-ionized water (both of which are too "hungry" and will steal critical ions from the channels of the pH and ORP probes), but rather, only in plain tap water.  Even better is to add a few percent (about 5% to 8%) of hydrochloric acid (HCL) to the storage soak water (keep away from kids and pets.)  In any case, probe soak water should be emptied and refreshed occasionally, perhaps once per week or two or three. 

3) Unless you have a specialized probe for which the manufacturer has suggested otherwise, ORP and pH probes should never be stored dry.

1) Make sure probe has been wetted with tap water.  Looking at probe area, clean the probe surfaces lightly with a soft or medium-soft brush; a small, soft toothbrush will do.  Then rinse.  Sometimes this step alone will remove enough of visible and invisible contaminants to restore probe to "good" (usable) use. However, at least once per month or two or moderate to heavy use, it will be necessary to go on to one or more of the next steps as well:

2)  Soak probe in a 25% to 80% solution of chlorine bleach (aka Clorox) in water (keep soak water away from kids or pets) for at least 15 minutes. Agitate or swirl probe at times to help circulate cleaning solution and remove contaminants.  Sometimes this step and the preceding step alone will remove enough of visible and invisible contaminants to restore probe to "good" (usable) use.  (For ORP probes, a good sign is if and when the ORP reading [of the solution] goes above +800, or better, even higher.) 
Rinse in tap water when finished.
If readings are still funky or slow, it will be necessary to go on to one or more of the next steps as well:

3) Soak probe in a 10% to 30% solution of HCL in water for at least 5 to 15 minutes. Be sure to observe the usual precautions about mixing these substances, and please wear eye protection when mixing.  BE SURE to keep any such acids or pre-mixed storage bottles of water/acid solution covered and away from kids and pets.  (For pH probes, a good sign of cleanliness is if and when the pH reading [of the solution] goes down to about 1.1 or less, or better, even lower.) Sometimes this step and the preceding steps alone will remove enough of visible and invisible contaminants to restore probe to "good" (usable) use. 
Rinse in tap water when finished.
If readings are still funky or slow, or "stuck" in one area, or if pH readings now seem too stuck in the acid range, it will be necessary to go on to the next step as well:

4) Soak probe in a 10% to 30% solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH) in water for about 2 to 4 minutes.  Be sure to observe the usual precautions about mixing these substances, and please wear eye protection when mixing.  BE SURE to keep any such alkalis or pre-mixed storage bottles of water/alkali solution covered and away from kids and pets. 
Rinse in tap water when finished.
This will usually take care of any remaining gunk or contaminants. 

Use of pH Probes
Please note that for greatest accuracy, a pH probe should remain in the substance to be tested for about 3 minutes or longer, and even gently swirled or swished in the beginning of test. However, for pH readings, even the readings after 30 seconds will be fairly accurate unless the probe has some contaminant layers.  Then, longer will be necessary.

Use of ORP Probes
Please note that for greatest accuracy, an ORP probe should remain in the substance to be tested for at least 15 to 30 minutes or longer, and even gently swirled or swished at the beginning of test.  For ORP readings, even the readings after 3 minutes will be fairly accurate unless the probe has some contaminant layers (and most do!).  Then, the full 15 to 30 minute soak will be necessary. See Warning About ORP Probes below!

Warning About ORP Probes
Please realize that almost all ORP probes which have been in use for more than a few hours without fanatical cleaning will usually have a thin layer of oxidized material on the electrode surfaces. Almost invariably, this thin contaminant layer will usually result in ORP readings which "pull' or regress, the readings toward the moderate oxidized range, usually toward the +300 to +450 mv. range. Further, these deposits will also slow settling time, making it take longer for the probe to yield a fairly stable reading (which may still be too high, or pulled toward the positive range.  For several reasons, this is often not much of a problem when measuring oxidized samples in the range of +150 mv and above. However, for liquids with an ORP below +150, and particularly those in the reduced range, with an ORP below about -050, this phenomenon can result in very serious distortion or pulling of resultant ORP readings toward the positive or oxidized ranges. 

Sources:

Chlorine bleach: supermarket. Be careful handling and storing it.
Hydrochloric acid: most hardware stores and home improvement stores. Often sold for scrubbing scale from toilets or for scrubbing concrete. Be careful handling and storing it.
Potassium hydroxide: lab supply house. Be careful handling and storing it. Usually easier to use Sodium hydroxide, below.
Sodium hydroxide: hardware store, often sold as lye crystals or pellets. Read label to be sure it is pure NaOH. Be careful handling and storing it.

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