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

"Analysis of the Relationship between Dissolved Molecular Hydrogen Gas, pH and ORP 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 indicates 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) contained within the meter's probe. The meter amplifies and compares the voltage measurement with an internal reference and displays a digital reading (usually in "millivolts"). However, while the dissolved H2 gas 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 a "measurement" of the dissolved H2 concentration.

It is also important to note that, just because water has a negative ORP, it does not necessarily mean the water will have 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 is a therapeutic benefit, the particular compound producing the negative ORP (redox couple) must be specifically identified. Based upon research, we know that H2 gas is a therapeutic agent.

​​​Other types of hydrogen water devices, hydrogen infusion machines (HIM's), utilize a proton-exchange membrane (PEM) to make H2 water, and do not perform electrolysis directly on the drinking water. Instead, they produce H2 gas in a small electrolysis chamber, and then mix the gas with the filtered drinking water in a special chamber called a "dissolver". 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 evaluating the therapeutic benefit of the water.

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 oxidizers, while a negative ORP indicates the presence of 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 (-100 to -750mV). 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  (e.g. the scavenging of free-radicals), the chemistry of the reaction in question (kinetics/thermodynamics) must be thoroughly investigated and understood. 

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 ranges, the pH contributes about 90% of the ORP reading, while H2 contributes only about 10%.

Accurate - Easy - Affordable 

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:

Notice that the H2/H+ redox couple (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 (e.g. water temperature and gas 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 ORP. These predictions can then be used to analyze and understand the relationships between the concentrations of H+ and Hand the ORP measurement.

Neutral-pH devices will produce water having a lower  average negative ORP reading than alkaline devices, even with more H2

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 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 instead due to an indirect mechanism, H2's ability to act as a signal modulator, reducing the production of free radicals in the first place, rather than scavenging them. While the end result (the reduction in 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 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". 

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 (H2).  Because the ORP measurement is produced by 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, a neutral-pH hydrogen water device, with its lower average pH, will produce a lower negative ORP reading than alkaline water, even if it contains twice as much dissolved H2!

Hydrogen Redox Couple

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

Electrolyzers (alkaline ionizers) produce Hgas 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:

ORP-type hydrogen meters

While there are meters which do measure dissolved H2 gas, these meters, which utilize sophisticated hydrogen gas probes, are very expensive, costing 10,000 dollars or more. Relatively inexpensive meters (500 dollars or less), claiming to measure dissolved hydrogen, DO NOT use hydrogen gas probes, but are, instead, based on the ORP-meter platform. 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 assume that the pH of the water being tested is exactly 7. But, even a small variance in the pH from neutral 7 (which is inevitable and difficult to control) will cause the measured H2 reading to vary significantly.

As an alternative, the H2Blue hydrogen test reagent provides an accurate, easy and affordable way (about 75 cents per test) to test your dissolved H2 levels.

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 a reduction in oxidative stress. Research also shows that, under non-biological conditions (test-tube, cell-free), H2 can act as a conventional antioxidant, and scavenge dangerous hydroxyl radicals, *OH.  Conventional antioxidants protect our bodies by donating electrons to free radicals, thereby neutralizing them. The balanced equation which describes the direct scavenging of the hydroxyl radical by H2 is:

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