Using the Nernst equation to predict ORP values​

In chemistry, the well-known Nernst equation is used to predict reduction potentials (ORP) in a variety of systems. The form of the Nernst equation used for dissolved hydrogen gas is (Emv= ORP):

Why do some waters containing dissolved H2 have an alkaline pH, while others are closer to neutral?​

Electrolyzers (alkaline ionizers) produce H2 gas at the negative cathode by reducing (adding electrons to) H+ ions to H2 gas using electricity. Because H+ ions are the acid component in water, their consumption during the production of H2 gas elevates the pH of the drinking water into the alkaline range ( above 7pH) as the hydroxide level (OH-) rises.​ The following equation describes the reduction of H+ ions to H2 gas:

Viewing the relative contributions in this way, it can be seen that, over a typical range in H2 concentration, 0.5 to 2mg/L, the ORP will only change by a total of 18mV. Conversely, over a range of 3 pH units, the ORP will change 178mV. Therefore, over their respective typical ranges, the pH contributes approximately 90% to the ORP reading, while H2 contributes only about 10%.

The information about H2 and ORP on this page is a summary from the article 

"Analyzing the Influence of Dissolved Hydrogen Gas, pH, and Temperature on the ORP of Water Using the Nernst Equation".

The complete article in PDF format is available here:

The bottom term, H+, is the ion whose concentration determines how acidic water is, and is the "H" in pH (potential of hydrogen). This is significant, as it tells us that the ORP measurement is influenced by the pH of the water being tested.

(Note: although pH means "potential of hydrogen", pH represents the concentration of the hydrogen ion, H+, not H2 gas)

Both species of hydrogen contribute to the final ORP reading, producing a small corresponding voltage potential at the electrode (based upon their relative concentrations and # of electrons) contained within the meter's probe assembly. The meter amplifies and compares the voltage measurement against an internal reference, and displays a digital reading (usually in "millivolts"). However, while the dissolved H2 gas (a reducing agent) is responsible for the negative ORP reading, the magnitude of the reading does not directly correlate with the amount of dissolved H2 gas in the water, and cannot be used as an accurate "measurement" of the dissolved H2 concentration. It is also

important to note that, just because water has a negative ORP, this does not necessarily mean the water will have any therapeutic benefit. There are many other compounds (some toxic) which, when dissolved in water, can produce a negative ORP reading. In order to know whether or not there may be therapeutic benefit, the particular compound producing the negative ORP (redox couple) must be specifically identified. Based upon published research, we know that H2 gas, the source of the negative ORP, is a therapeutic agent.

Because of H2's ability to reduce the level of oxidative stress in biological systems, combined with the knowledge that H2 can scavenge free-radicals (under non-biological conditions, when no other competing antioxidants are present), the assumption has often been that the reduction in free radicals seen in research studies is being accomplished by free-radical scavenging. While it is possible for H2 to scavenge an extremely small number of hydroxyl radicals (perhaps one in a billion), the scavenging mechanism cannot account for the reductions in the levels of reactive oxygen species (ROS) seen by researchers. Because conditions for the scavenging of hydroxyl radicals by H2 are so kinetically unfavorable, it is more likely that the reduction in free radicals/oxidative stress is due, instead, to an indirect mechanism, H2's ability to act as a signal modulator, reducing the production of free radicals rather than scavenging them. While the end result (the reduction in both free radicals and oxidative stress) may be the same, it is more accurate to refer to H2's ability to reduce oxidative stress as "antioxidant-like".

Where does the negative ORP reading come from and what does it mean?​

The negative ORP is an indication of the presence of a reducing agent, in our case dissolved hydrogen gas (H2). An ORP meter does not "measure" dissolved hydrogen in the water; rather, it measures the relative contributions to the water's redox potential from two species of hydrogen in the water, hydrogen gas (H2) and hydrogen ions (H+). Together, these two species are called a "redox couple". 

Is H2 a free-radical scavenger (conventional antioxidant)?

​Water containing dissolved H2 gas is often called "antioxidant water". And, in fact, research confirms that, in biological systems (in vivo/live cell, animal and human), ingestion of H2 can result in reduced levels of dangerous free radicals (reactive oxygen species, ROS), resulting in an overall reduction in levels of oxidative stress. Research also shows that, under non-biological conditions (test-tube, cell-free), H2 can act as a conventional antioxidant, and scavenge free radicals, including the dangerous hydroxyl radical (*OH).  Conventional antioxidants protect our bodies by donating electrons to free radicals, thereby neutralizing them. The balanced equation that describes the direct scavenging of the hydroxyl radical by H2 is:

What is ORP?

ORP, or "oxidation-reduction potential" (also called "redox potential"), is a measurement of water's tendency to act as either a reducing agent (electron donor), or oxidizing agent (electron acceptor). A positive ORP indicates the presence of potential oxidizers, while a negative ORP indicates the presence of potential reducers. The measurement is done using an ORP meter, which includes a probe designed to be inserted into the water being tested. The ORP of water produced by ionizers, hydrogen infusion machines (HIM's), and other hydrogen water technologies (e.g. H2 tablets, sticks, cartridges, etc.), will typically measure some negative value (-200 to -750mV, depending on pH). Keep in mind that this "measured potential" only indicates the water's tendency to act as a reducer/oxidizer, and does not guarantee that any particular oxidation/reduction reaction will occur, or indicate the strength or speed of the reaction if it does occur. In order to know whether or not any oxidation or reduction reaction will occur inside the body, the chemistry associated with the reaction in question (kinetics/thermodynamics) must be thoroughly investigated and understood. 

Another type of hydrogen water device, the hydrogen infusion machine (HIM), utilizes a proton-exchange membrane (PEM) to make H2 water, and does not remove H+ ions from the drinking water. Instead, it produces hydrogen gas using H+ ions from a separate water chamber, and then infuses the filtered drinking water with the gas, typically using a special device called a "dissolver". As a consequence, an HIM does not alter the pH of the original source water. Therefore, depending on the pH of the hydrogen water being tested, ORP measurements will produce very different (and often confusing) results, making it impossible to use the ORP reading for measuring dissolved H2 levels or for evaluating the therapeutic benefit of the water.

Neutral-pH devices will produce water having a less-negative ORP reading    than alkaline devices, even if the water contains higher levels of dissolved H2

ORP-type hydrogen meters

​While there are meters that do measure dissolved H2 gas, these meters, which utilize sophisticated hydrogen gas probes, are very expensive and can cost thousands of dollars. Relatively inexpensive meters currently available (500 dollars or less), claiming to measure dissolved hydrogen, DO NOT use hydrogen gas probes, but are, instead, based on the ORP-meter platform, and use ORP-type electrodes. These meters attempt to approximate the level of dissolved hydrogen using the water's ORP measurement. Because of their sensitivity to pH, these meters are recommended by the manufacturer to be used only to test neutral-pH H2 water (not alkaline-ionized water), and must assume that the pH of the water being tested is exactly 7. But, because of the large contribution of pH to the ORP measurement described earlier, even a small variance in the pH from neutral 7 (which is inevitable and difficult to either measure accurately or control) will cause the measured H2 reading to vary significantly.

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As an alternative, the H2Blue hydrogen test reagent, which reacts directly with the dissolved H2 in the water, provides an accurate, easy and affordable way (about 50 cents per test) to test your dissolved H2 levels over a wide range of pH.

For more information on ORP, please view the following article: 

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https://www.frontiersin.org/articles/10.3389/frfst.2022.1007001/full

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Hydrogen Redox Couple

Notice that the H2/H+ redox couple mentioned earlier (circled in red) is contained within the equation (the brackets signify "concentrations").  Just like the ORP meter, the Nernst equation must "know" the relative concentrations of each species of hydrogen to predict the ORP. The equation also includes other important chemistry values and variables (water temperature, gas constant, number of electrons transferred, and Faraday's constant). Using the Nernst equation, along with selected values for pH and H2 levels, we can predict the effect that changes in each will have on the resultant ORP measurement. These predictions can then be used to analyze and understand the relationships between the pH (concentration of H+), H2 concentration, and the ORP measurement. Note that, in order to simplify this explanation, the presence of other oxidizing redox couples, which also contribute to the final ORP reading, have not been considered.

The graph above of ORP (vertical axis) vs both "pH" and "H2 concentration" (horizontal axis) shows that, while the ORP reading is extremely sensitive to changes in pH (red line), it is relatively "insensitive" to changes in H2 (blue line). Because of this, even small variations in the water's pH can result in large changes in the ORP reading, masking the relatively small contribution made to the ORP by the dissolved H2.  Below is another way of viewing the relative contributions to the ORP value made by pH and H2 in terms of their percentages:

Does a "more negative" ORP mean more dissolved H2?​

As discussed earlier, the ORP measurement is very sensitive to changes in the water's pH (H+), but insensitive to changes in dissolved hydrogen gas. Because ORP is the result of the relative concentrations of two species of hydrogen in the water (H+/H2), a change in the concentration of either species will change the ORP measurement. Therefore, H2 water from a neutral-pH hydrogen water device with its much lower average pH, will measure a lower-magnitude negative ORP reading than alkaline water, even if it contains twice as much dissolved H2!