The term “hydrogen water” or hydrogen-rich water (HRW), refers to water that contains dissolved molecular hydrogen (H₂ gas) (Slezak et al., 2021). Common methods of providing hydrogen water include electrolysis (water ionizers), hydrogen-infusion machines that utilize proton-exchange membranes, tablets that produce H₂ gas when placed into water, H₂ gas bubblers that infuse H₂ into the water, and special pre-packaged cans and pouches as shown in Figure 1 (Henry and Chambron, 2013; LeBaron et al., 2019a). Molecular hydrogen is held together by a relatively stable nonpolar covalent bond, which means that, when it is dissolved into water, it does not chemically bind to the water molecules. In other words, hydrogen water is not a novel chemical substance (e.g., “H₄O”). When the H₂ molecules are fully dissolved in water, the solution is clear. Sometimes small gas bubbles can be seen initially in hydrogen water or it may even have a “foggy” appearance, but, unless it is dissolved, it does not contribute to the ORP or the therapeutic effects. This is because any undissolved H₂ will readily rise to the water surface and escape into the atmosphere. H₂ will not stay in solution forever. In an open container it has an approximate half-life of 2 hours (Fujita et al., 2009). Hydrogen’s high volatility combined with its small molecular size allows it to permeate most containers except for special aluminum packaging (Ohta, 2015).

Dissolved hydrogen is usually measured in molarity (mmol/L) or milligrams per liter (mg/L), which is the preferred unit of measure, instead of the equivalent unit of parts per million (ppm). The most commonly used concentrations of molecular hydrogen are 0.5 to >2 mg/L (LeBaron et al., 2019a). At standard pressure (1 atm), and ambient temperature (25°C), the saturation of hydrogen water is about 1.6 mg/L (Alwazeer et al., 2021). Increasing the pressure and/or altering the temperature results in a greater saturation level according to Henry’s Law (James et al., 2004; Harris, 2010).

As discussed, hydrogen water and ERW that have molecular hydrogen will have a negative ORP (-100 to -750 mV or more) (Shirahata et al., 2012; Chen and Wang, 2022). This negative ORP is often touted as evidence that the water has been imparted with “special properties” from the electrolysis or production process. These special qualities include claims that the water has a smaller more bioavailable molecular water cluster, or that the water itself possesses a “negative charge” due to “free electrons” entrained in the water (Mohr, 2016). Although even an elementary understanding of chemistry is sufficient to refute such claims, they continue to gain traction. These claims have added to the confusion about 1) the meaning/reason for the negative ORP, 2) the true reason of ERW’s/hydrogen water’s benefit, and 3) the proper method to compare and contrast the benefits of one water to another. It should be recognized that it is exclusively the existence of dissolved H₂ in ERW that is responsible for the negative ORP (Piskarev et al., 2010; Shirahata et al., 2012). During electrolysis, electricity produces molecular hydrogen at the cathode by converting protons (H⁺) into H₂ (g). In electrolysis units that have specialized membranes separating the cathode and anode electrodes, reduction of these hydrogen ions results in a rise in the pH of the catholyte (Henry and Chambron, 2013). The process of electrolysis does not impart special enigmatic properties to the water. Indeed, any H₂ water, including that made without the use of electricity (e.g., H₂-producing tablets or infusing H₂ into solution), will also have a negative ORP (Jackson et al., 2018). Since it is well established that molecular hydrogen is exclusively responsible for the therapeutic benefit of ERW/hydrogen water (Jackson et al., 2018), then the focus should be to determine the level of H₂, not the ORP value. A negative ORP is simply an indicator of the presence of molecular hydrogen. Moreover, the magnitude of the negative ORP reading does not provide helpful information about the level of molecular hydrogen in the water as often suggested (Okajima et al., 2022) and will be discussed in this article.

The ORP meter as seen in Figure 2 is an instrument frequently used by technical personnel in a variety of commercial and industrial sectors, such as environmental, wastewater treatment, food and beverage industry (Awulachew, 2021; Kondjoyan et al., 2022; Wu et al., 2022), pool and spa, and horticulture (Ari and Darren, 2014). A high positive ORP can indicate the effectiveness of sanitizing agents (oxidizers) used to kill various pathogens in water (Kim et al., 2000). This ORP meter operates in many ways that are similar to conventional voltmeters, with the major difference being the ORP sensor electrode. Because the metal sensor is composed of the inert metal platinum, it isn’t an active participant in redox chemistry. The ORP reading is determined by comparing the voltage produced from the platinum electrode to the internal secondary silver-silver chloride electrode that is found inside the probe housing (Ari and Darren, 2014).

There are different types of ORP meters, from consumer-grade units to more sophisticated industrial/laboratory models. The portable ones generally have the probes integrated into the housing of the meter itself. The large ORP meters have their probes located externally in a separate enclosure, which is then connected to a flexible cable that goes to the meter. When the electrode is submerged in the sample water, electrons are either accepted or donated from the probe at the platinum surface. The exchange of electrons is used by the meter’s internal electronics based on its internal reference to generate a voltage measurement. The ORP meter will then digitally display this voltage in millivolts (mV) (Harris, 2010). Depending on the type of chemical species in the water, the displayed voltage will either be negative or positive. Hydrogen gas is only one of many reducing chemical species that can give a negative ORP reading.

A water’s oxidation-reduction potential indicates its tendency to be oxidizing or reducing. Discussions about ORP generally focus on the “O” (oxidation) and the “R” (reduction) (Ari and Darren, 2014), but detailed descriptions about the “P” (potential) component do not occur as often. The ORP describes some type of electrical potential and its units are expressed in volts (V). What this voltage reading actually tells us about the water, and the source of this potential will be addressed.

First, voltage is the difference in electrical potential energy between two points (Vujević et al., 2011). This is because a single object cannot have a “voltage”; a potential difference must be measured between two points. For example, the nine-volt battery in Figure 3 does not “have a voltage of 9 V.” The nine-volt potential exists only between the positive and negative terminals. This is similar to how altitude is referenced to sea level, or temperature is referenced to the freezing point of water. Figure 2 shows these points, the ORP platinum electrode where electron transfer takes place, and the internal reference electrode.

A voltage potential is essentially the amount of work that can be done using electricity (the flow of electrons). A battery has the potential to do work because we can measure its voltage. A typical voltmeter measures the stored potential between the two terminals of the battery.

Importantly, voltage potential only refers to the tendency and directions that electrons might be transferred, not the number of electrons transferred. The larger the voltage potential, the greater the tendency. However, whether there really will be a transfer of electrons and the number, depends upon the resistance of the circuit to the flow of electrons. Similarly, the potential for electron transfer as measured by the ORP meter does not mean that electron transfer will readily occur. For example, if the wire connecting the lightbulb to the battery had high resistance due to corrosion, then, despite how good the battery was, its voltage potential may not be great enough to overcome the resistance. Similarly, chemical reactions need to be able to overcome another type of resistance (e.g., activation energy), possibly requiring energy from an outside source (e.g., heat) before they can proceed (Blanco and Blanco, 2022).

In biological systems, some less-obvious forms of work are accomplished. For example, when an antioxidant reacts with and neutralizes a reactive free radical, work is performed (Angelé-Martínez et al., 2022). A negative ORP itself does not tell us if a solution will exert antioxidant effects inside the body. This depends, not only on the identity and concentration of the chemical agent responsible for producing the negative ORP, but also as mentioned, on the ability of the reaction to overcome the “chemical” resistance, either on its own or with help (e.g., enzymatic catalysis) (Blanco and Blanco, 2022). Therefore, although the term “potential” refers specifically to voltage potential, the term also conveys a second meaning, that of “possibility.” Therefore, although a negative ORP indicates the potential for anti-oxidation, other factors must also be favorable in order for it to proceed.

The “O,” in ORP stands for “oxidation,” and the “R,” for “reduction.” This is where the term REDOX for REDuction and OXidation comes from. Redox reactions are those chemical reactions involving the transfer of electrons between two different chemical species (ions, atoms, or molecules); one species accepts electrons, while another species donates electrons (Nelson et al., 2008). Oxidation and reduction reactions must happen in conjunction with each other, i.e., when one species loses electrons, another must gain electrons. The species that loses electrons undergoes oxidation, whereas the species that gains electrons undergoes reduction (Alemanno, 2020). Configurations of the electrons dictate the chemical properties of a substance. Therefore, when the electron configurations are altered by redox reactions, the substance has a different set of chemical characteristics. Indeed, monitoring changes in redox potential can predict the oxidation of foods (e.g., lipids, meats, nutrients) (Kondjoyan et al., 2022) and food quality (Bhunia et al., 2017), as well as determine if the food packaging environment is favorable or not for bacterial growth (Roussel et al., 2022).

The redox reactions that produce O₂ and H₂ gas during the electrolysis of water are examples of oxidation and reduction reactions that occur simultaneously at the anode and cathode, respectively. During electrolysis, the cathode supplies the electrons that reduce hydrogen ions to hydrogen molecules (i.e., 2H⁺ + 2e⁻ → H₂) (Harris, 2010), as depicted in Figure 4. This reaction happens immediately (even without considering electron and proton tunneling, which also occur) (Hofbauer and Frank, 2012), meaning that the electrons from the cathode do not “swim” in the water as “free electrons” (Hofbauer and Frank, 2012).

The reaction starts with the two H⁺ ions, essentially two protons (no electron), and after the redox reaction, the H₂ molecule is formed and has two shared electrons. The disparity in electrons results in an energy potential between the two hydrogen species (i.e., H⁺ and H₂). These two chemical species represent the “potential” to do work, which, when in water, can be measured by an ORP meter. Changing the concentration of either of these species will influence the ORP.

As discussed, each reduction reaction has a corresponding simultaneous oxidation reaction. Reduction occurs at the cathode, forming H₂ (g), and oxidation occurs at the anode, forming O₂ (g). Specifically, at the anode, hydroxide ions (OH ⁻ ) are oxidized to produce O₂ (g), water (H₂O), and electrons (i.e., 4OH⁻ → O₂ + 2H₂O+ 4e⁻). The produced electrons are sent to the anode which is connected to the positive terminal of the power supply (Zeng and Zhang, 2010).

In conventional alkaline water ionizers, the anode and cathode chambers are isolated from each other with a membrane, which prevents the two types of waters from mixing (Henry and Chambron, 2013). While the water from the primary (drinking water) hose will measure a negative ORP, the water that comes from the other hose has a positive ORP (Henry and Chambron, 2013). This water will measure a positive ORP because the produced acidic water contains different redox couples (e.g., an oxygen and/or chlorine species), rather than the H⁺/H₂ redox couple (Ari and Darren, 2014). Other types of devices that do not use any membrane will produce water containing both dissolved oxygen and hydrogen gas. In this case, the water would have both oxidizing and reducing redox couples at a pH closer to neutral. Since our discussion about ORP focuses on hydrogen gas produced at the cathode, we will not spend time examining the anode reaction or its ORP/redox couples in detail. Interested readers are referred to (Aider et al., 2012).

Each chemical species has its own intrinsic tendency to give up or acquire electrons. They can be categorized according to their measured voltage potential. These voltage potentials are referred to as “standard reduction potentials” denoted as E° (Harris, 2010). Table 1 shows the standard reduction potentials for common half-cell reactions under standard conditions of concentration, pressure, and temperature. Their calculated values are relative to the standard hydrogen electrode (SHE), which has been assigned an arbitrary potential of 0.000 mV (Harris, 2010).

Those with the most negative redox potentials are the strongest reducing agents, and conversely, those that have the highest positive redox potentials are the strongest oxidizing agents. Although Table 1 gives the standard reduction potentials, ORP meters report the potential under non-standard (or formal) conditions. The magnitude of the ORP reading, as well as whether it is positive or negative, depends on the specific identity and the relative concentrations of each species in the redox couple (Harris, 2010). A positive ORP suggests that the chemical species dissolved in solution will have a tendency to act as an oxidizing agent, while a negative ORP suggests that the chemical species in the water will be more prone to act as a reducing agent.

While there are many redox couples in chemistry, a few of them noted in Table 1, we are concerned with the H⁺/H₂ redox couple. This redox couple is seen in the reaction where two hydrogen ions (H⁺) each accept an electron to make one molecule of H₂ (2H⁺ + 2e⁻ → H₂). This could also be written as one hydrogen ion accepts one electron to form atomic hydrogen (i.e., H⁺ + e⁻ → H^•). However, atomic hydrogen is a free radical that quickly reacts with another atomic hydrogen to form hydrogen gas (i.e., H^• + H^•→ H₂ gas). Multiplying half reactions by any coefficient does not change E. The hydrogen ion (H⁺) is the oxidized form of hydrogen, containing no electrons, and the H^• or H₂ are the reduced forms, containing one or two electrons, respectively (see Figure 4). The H⁺ ion does not have any electrons, but is a single proton willing and ready to accept electrons. The newly formed H₂ molecule, on the other hand, contains two electrons, which allows it (under the right conditions) to donate them, acting as a reducing (or anti-oxidizing) agent (Ohsawa et al., 2007).

It is helpful to review some primary concepts about pH since, as we will see, it has a strong influence on the ORP reading (Jackson et al., 2018). The term, pH, stands for “potential of hydrogen,” and is a measure of the concentration of the hydrogen ions (H⁺) (Nelson et al., 2008). Hydrogen (H⁺) ions are essentially the acidic constituent of water. The greater the H⁺ concentration, the more acidic the water. The pH has a direct influence on the ORP reading because one of the two species, the H⁺ ion, is in our redox couple of interest (i.e., H₂/H⁺). The pH scale is a more understandable way to express very small numbers as powers of ten (exponents), instead of using the equivalent and more cumbersome scientific notation. For example, instead of writing 1 × 10⁻⁷ or 0.0000001 for the H⁺ concentration, we can write pH 7. The pH scale is a logarithmic scale in which the concentration of H⁺ ions is expressed as a negative power of 10, and goes from 0 to 14. The equation is pH = −log [H⁺].

Importantly, because pH is a logarithmic function, every unit change in pH represents a ten-fold change in the H⁺concentration. Consequently, relatively small integer increases in pH, represent very large changes in the H⁺ concentration. Therefore, a pH of zero represents a 1 M H⁺ ion concentration, while a pH of 14 represents a very low H⁺ (1 × 10⁻¹⁴ M) concentration. The concentration of H⁺ ions determines whether the water is considered neutral (pH 7), acidic (below 7), or alkaline (above 7). Table 2 illustrates the exponential (logarithmic) relationship between the pH and the concentration of the hydrogen ions.

Importantly, pH is not associated with dissolved H₂ gas. However, some methods used to make hydrogen water can change the pH of the water. For example, ERW produced by an alkaline ionizer has an alkaline pH. This occurs due to the electrolysis of water, where hydrogen ions are reduced to H₂ at the cathode. There is an increase in pH because the H⁺ ions (acid) are being consumed, and the level of OH ⁻ ions (base) increases (Henry and Chambron, 2013). Unfortunately, the correlation between increased pH and dissolved hydrogen gas in ERW has led to confusion. This misunderstanding is exacerbated by the fact that the higher the pH the greater the negative ORP, which is often assumed to indicate more hydrogen gas. In fact, some marketers confuse the term pH and incorrectly interpret it to mean “potential of hydrogen gas,” falsely suggesting the higher the pH the higher the hydrogen gas concentration, instead of the more correct meaning “potential of hydrogen ion”, which signifies the higher the pH the lower (not higher) the hydrogen ion concentration. Bubbling H₂ gas into water will not change the water’s pH because molecular hydrogen is neither acidic nor basic, as it does not readily donate or accept H⁺ ions (Korchef, 2022).

German chemist Walther Nernst formulated what is now widely known as the Nernst equation (Cropper, 1987). The Nernst equation represents the thermodynamic relationship of the standard redox potential to the effective concentration (activity), pressure, and temperature of a chemical species undergoing a redox reaction under non-equilibrium conditions (Cropper, 1987). In the field of electrochemistry, it is perhaps the most helpful equation. Eq. 1 depicts the generalized form of the Nernst equation: E = E ° − ( R T n F ) ln ( [ r e d ] [ o x ] ) (1)

The Nernst equation can be used to determine how the contributions of dissolved hydrogen gas, pH, and temperature influence the ORP reading. In order to utilize the Nernst equation to calculate expected ORP values for ERW/hydrogen water, we need to substitute the values for the half-reaction of hydrogen: i.e., 2H⁺(aq) + 2e⁻ → H₂(g). Because E ⁰ , the standard cell potential, is zero for the hydrogen half-reaction (see Table 1), we can eliminate that term. The term “[red]” is substituted with the pressure of hydrogen gas, “PH₂”, which is the reduced form of hydrogen, and the term “[ox]” with the concentration of the oxidized form of hydrogen, “H⁺”.

The final form of the Nernst equation used in this analysis is shown in Eq. 2: E = − ( R T n F ) ln ( P H 2 [ H + ] 2 ) (2) where:

• E, Nernst potential in volts (ORP),

Figure 5 illustrates the two chemical hydrogen species from the modified Nernst equation (Eq. 2) which form our “redox couple of interest.” This term provides helpful insight into how the ORP is determined.

The oxidized and the reduced forms of hydrogen represent the two chemical species in the water whose concentrations primarily contribute to the redox potential of hydrogen water. While water usually contains multiple redox couples whose different redox potentials combine to produce the overall ORP (Ari and Darren, 2014), in our in silico calculations, we consider only the H₂/H⁺ redox couple.

By keeping pH, temperature, and pressure constant, we can determine how changes in only the H₂ concentration influence the ORP. Figure 6 shows the predicted ORP readings for an H₂ concentration of 0.5 mg/L to 50 mg/L at a pH of 7. Remember, H₂ saturation in pure water at Standard Ambient Temperature and Pressure (SATP) is approximately 1.6 mg/L, and the typical range for studies is between 0.5 and 2 mg/L.

Between the dissolved H₂ concentrations of 0.5 mg/L to 2 mg/L, the negative ORP decreases by only 18 mV. This rather small change in ORP obfuscates the fact that it is a fourfold increase in H₂ concentration, which, therapeutically, could be the difference between no effects and clinical effects. Moreover, increasing the concentration from 0.5 mg/L to 50 mg/L, the negative ORP decreases from −399 to −459 mV, only 59.16 mV more negative. Although such a high dissolved H₂ level could likely only be achieved under laboratory conditions, this, nevertheless, illustrates just how little the ORP responds to even extremely large changes in the H₂ concentration.

Moreover, the Nernst equation predicts that an H₂ concentration of only 1 × 10⁻⁵ mg/L (orders of magnitude below therapeutic levels), is all that is needed to produce a negative ORP of −260 mV. However, a dissolved H₂ concentration this low would only produce this specific negative ORP “on paper,” only based on the Nernst equation without considering any other redox couples in the water. In fact, that is more hydrogen than is naturally dissolved in water (≈8.65 × 10⁻⁷ mg/L) when at equilibrium with the 5.5 × 10⁻⁵% H₂ in the atmosphere. However, due to the presence of oxidizing redox couples at higher concentrations that are also dissolved in solution (e.g., a chlorine and/or oxygen species), regular water measures a positive ORP. Nevertheless, this example shows that even a very low concentration of H₂ is capable of producing a negative ORP. It also illustrates how little the ORP reading can actually tell us about how much H₂ the solution contains.

To evaluate the influence that changes in only the H⁺ concentration (pH) have on the ORP, we can change the pH, while maintaining a constant H₂ concentration and temperature. Figure 7 depicts the predicted ORP readings for six different values of pH, all at the same temperature (25°C) and dissolved H₂ concentration (1.57 mg/L). As can be seen, in contrast to the weak impact of H₂, the magnitude to which pH changes the ORP is very significant.

For example, as the pH increases from six to eleven, there is a significant increase in the negative ORP, from −355 mV to −651 mV. The ORP becomes more negative by 296 mV, while the temperature and H₂ concentration stay the same. Because the ORP changes almost exclusively because of changes in pH, the magnitude of the ORP reading does not really provide any helpful information regarding the actual concentration of molecular hydrogen in the water.

After examining the individual influences that differences in H₂ and pH have on the ORP, it is also informative to evaluate their combined impact. Figure 8 shows a graph with two horizontal axes that plots the lines for both ORP vs. H₂ and ORP vs. pH. Similar to the previous example, the graph of ORP vs. pH has the H₂ constant at 1.57 mg/L, and the graph of ORP vs. H₂ has the pH kept neutral. In both cases, the temperature is held constant at 25 °C.

As illustrated in Figure 8, over a common concentration range of molecular hydrogen (0.5–2 mg/L), the ORP will deviate by only 18 mV. However, over the given range in pH (6–11), the ORP will change by 296 mV. These examples clearly illustrate that the ORP reading is largely governed by the water’s pH, as the contribution from H₂ is relatively small.

It has so far been demonstrated by in silico analysis that the ORP reading changes predominantly with changes in pH and insignificantly with changes in H₂. We briefly summarize why this is true.

Most ORP meters do not have a temperature probe that adjusts the voltage reading. Compared to the very minor influence of molecular hydrogen on ORP, temperature differences are much more significant. The temperature dependence of ORP is an intrinsic part of the Nernst equation, contained within RT/nF. Since n (number of electrons) equals 2, the only term in RT/nF which is not a constant is temperature (T). This means that, even when not considering the ratio of the redox couple, every ∆T of 20°C, changes the ORP by ≈ 30 mV. This is comparable to changing the H₂ concentration by a factor of 10 (0.1 mg/L to 1 mg/L) and is essentially the same as changing the pH by ½ a unit. These changes in ORP as a function of temperature are illustrated in Figure 10, where the value of Q remains constant by keeping the H⁺ and H₂ pressure constant (pH, 7; pressure, 1 atm). As the temperature increases, the magnitude of the negative ORP increases, despite the actual concentration of H₂ decreasing. For example, at 0°C the solubility of H₂ is 1.96 mg/L, which gives −379 mV; whereas, at 100°C, the H₂ solubility is only 0.97 mg/L but the ORP is now −518 mV. According to the Nernst equation, at a pH of 7 and 25°C, the concentration of H₂ required to produce the same −518 mV is 5,500 mg/L. Therefore, as shown in Figure 10, despite the solubility of H₂ at 0°C being nearly twice as much compared to 100°C, the change in ORP would give the false impression that, instead of decreasing by twice as much, the H₂ concentration increased by nearly 3,000 times.

This can have very significant implications in real-world applications. For example, using an ORP-based method that does not include temperature compensation to estimate the H₂ concentration of cold hydrogen water (e.g., stored in the refrigerator) in which the solubility of H₂ is higher, would give you a less-negative ORP reading, underestimating the actual H₂ concentration compared to measuring it at room temperature (25 °C). Similarly, researchers using this ORP method to estimate the H₂ concentration at incubation temperature (37 °C) would overestimate the actual H₂ concentration.

As has been illustrated, the negative ORP reading cannot be directly associated with any specific concentration of aqueous molecular hydrogen. However, the question remains as to whether it is possible to compare the ORP reading from two different water samples to evaluate the relative amounts of molecular hydrogen that they contain. Theoretically, this would be possible if we knew the exact pH, temperature, and ORP of only the H₂/H⁺ redox couple. However, since ORP and ORP-based H₂-meters usually do not have this information, they must assume an exact pH and temperature, likely 7 and 25°C, which, as discussed, is rarely the case for water. Even so, on the surface, a rational supposition might be that the greater the water’s negative ORP, the higher the concentration of molecular hydrogen. However, the data in Table 3 shows why this assumption is not necessarily true.

Hypothetically-measured ORPs for hydrogen waters having different pH values, dissolved H₂ levels, and temperatures are shown in Table 3. Additionally, the predicted H₂ concentration based on the hypothetical ORP reading, without corrections for pH and temperature (assumption 7 pH and 25°C), are presented. Because the pH of sample A is one pH unit higher than B’s, it has a more negative ORP, even though B has twice the concentration of H₂. Similarly, sample C at the higher temperature appears to have more H₂ than sample B despite having 38% less H_2. Moreover, there could be many permutations of slightly increased pH and elevated temperature that individually may not seem significant, but collectively have consequential effects. For example, sample D has a slightly acidic pH, like reverse osmosis/distilled water, from absorbing CO₂, and a lower temperature (15°C). Despite a high actual H₂ concentration, its lower pH and temperature result in an ORP reading that is significantly less negative than all the other samples. Using the Nernst equation, the estimated H₂ concentration from this ORP reading, assuming 25°C and pH 7, would give a large underestimation of the true H₂ concentration (0.01 mg/L vs. 3 mg/L).

These scenarios demonstrate why it is wrong to presume that, if one water has a more negative ORP reading than another, then it also has a greater concentration of molecular hydrogen. Some marketers may employ the technique of reducing the flow rate of an alkaline water ionizer to a “trickle” or increasing the temperature before measuring the ORP. However, as illustrated, a greater negative ORP does not necessarily signify a higher concentration of molecular hydrogen. Indeed, the water with the more negative ORP may easily contain a lower concentration of molecular hydrogen.

Theoretically, an ORP-based H₂ meter would work if the measured ORP was truly an accurate ORP value. But, in order to achieve an accuracy in H₂ concentration of ≈ ± 0.1 mg/L, such a meter would need to be able to determine the exact differential or potential between only the H⁺ and H₂ species within approximately ±1 mV. However, normal ORP meters cannot do this and, in hydrogen water, ORP readings often have an error range of ±100 mV, depending on a variety of factors (e.g., pH, temperature, H₂ concentration, minerals, etc.). This precludes us from obtaining an accurate E. Therefore, the only way to determine the H₂ concentration using the redox potential (E) would be to first determine the concentration of each redox species of interest (i.e., H₂/H⁺). Once the concentration of each is known, then the equation can be solved for E, and then finally for PH₂. This value is then converted to concentration of H₂ (C) as shown in Eq 3, which is a rearrangement of the Nernst equation (Eq. 2) combined with Henry’s Law (i.e., C=P/Kh). C = e − ( E n F R T ) ∙ [ H + ] 2 K h (3)

However, if it is required that we know the H₂ concentration in order to use the ORP method (i.e., E value) to determine the H₂ concentration, then there is no point in determining the ORP at all, when one could simply stop after directly obtaining the H₂ concentration.

All measurements require a certain level of accuracy (proximity to the true value) and precision (reproducibility of results) to be valid. The degree of accuracy and precision are intrinsically linked to the measuring instrument and, along with systemic errors, contribute to the uncertainty of the results. When measuring a calibration solution, ORP meters typically have a range of ±5 mV. While this may not seem like a lot, this 10-mV range corresponds to an error of H₂ estimation of ≈ ± 1 mg/L. This alone represents a significant problem since the normal range of H₂ often used by the researcher and consumer is between 0.5 and 1.5 mg/L. However, the situation may be even less favorable with hydrogen water because, as discussed, the error range of an ORP meter when measuring hydrogen water may be as large as ± 100 mV. This is significant, as it corresponds to an error in H₂ estimation of ±500 mg/L. Therefore, the ORP meter’s contribution to uncertainty might be greater than the combined influence of pH and temperature. Table 4 shows the different sources of error from typical consumer-grade (not lab-grade) sensors for ORP, pH, and temperature devices in measuring neutral-pH water with a hydrogen concentration of 1.57 mg/L. For example, the ORP may deviate by 100 mV from its true value of -414 mV. This corresponds to either a significant underestimation (0.0006 mg/L) or an egregious overestimation (3,750 mg/L) of the H₂ concentration. Similarly, even if we tried to correct for pH or temperature by measuring their values, these instruments still contain inherent error ranges that likewise result in significantly under or overestimating the true H₂ concentration. Indeed, the rather minor ±0.1 error in pH corresponds to an error that spans a 1.50 mg/L range (0.99 mg/L to 2.49 mg/L).

In order to have the lowest practical resolution of 0.1 mg/L for H₂ concentration, the ORP meter would have to be accurate within about 0.8 mV, which, as expected, is not even attainable when using normal calibration solutions. The possible ranges of error for each instrument are essentially greater than the tolerance required for a meaningful ORP reading. These errors are further compounded when used to estimate the H₂ concentration, which precludes the use of ORP for this purpose.

Any redox reaction that involves hydrogen ions is subject to the overwhelming influence that pH has on the ORP reading. Most redox reactions that occur in aqueous solutions either involve hydrogen ions directly as part of the reaction or are influenced by the changes in pH. This has led to the attempt to remove the influence of pH by mathematically transforming the ORP reading. This is referred to as the relative hydrogen score (rH or rH₂). This method was first proposed by W.M. Clark in 1923 for evaluating the redox capacity of a system (Clark and Gibbs, 1925). This rH index is based on the Nernst equation and Gibbs free energy using the Boltzmann constant to make it pH-independent relative to the standard hydrogen electrode (SHE). This is then used to estimate the solution’s antioxidant capacity (Henry and Chambron, 2013). The rH score ranges from 0 to 42, where 28 is considered neutral. Although this method may have some benefits in some areas (Clark and Gibbs, 1925), it also has fundamental problems in biochemistry, nutrition, and ecology (Garban, 2008). Moreover, since the rH score is derived from the ORP and pH, it maintains the same inherent errors and limitations as previously delineated for all ORP-based measurements. Additionally, like ORP, the rH score doesn’t consider the identity or the concentration of chemical specie(s) responsible for the rH score, or the conditions to determine if a redox reaction would occur at a meaningful timescale. These intrinsic problems of ORP are obfuscated by the mathematical transformation to the rH score. Accordingly, the use of rH score in evaluating the physicochemical properties of hydrogen water is not recommended.

Over the past decade novel hydrogen water technologies have emerged. Irrespective of the method used to make hydrogen water, the relationships among H₂, pH, temperature, and the predicted ORP remain the same. Unfortunately, it is likely that the ORP meter will continue to be used by marketers, researchers, and those in the food and beverage industry as tool to promote or evaluate a hydrogen water product. However, due to the disproportionate impact of pH on ORP, neutral-pH devices will produce water with less-negative ORP measurement compared to alkaline water. This is even despite the fact that the neutral-pH water may have the same or even a much greater concentration of H₂. For example, the Nernst ORP prediction for H₂ water at a pH of 7 and a concentration of 1.57 mg/L is −414 mV. However, water at 9.5 pH with tenfold less H₂ (0.16 mg/L), a concentration lower than what is used in clinical studies, will measure −533 mV. Similarly, H₂-producing tablets often used in clinical studies (LeBaron et al., 2019b; Korovljev et al., 2019; LeBaron et al., 2020) can provide a high concentration of H₂, >7 mg/L, but due to their acidic pH (≈4), the measured ORP is predicted to only be around −256 mV. Those promoting and selling alkaline water devices might knowingly or unknowingly exploit the ORP difference (due solely to a difference in pH and/or temperature), and use it to convince potential customers that, based on its more negative ORP, the alkaline water has a higher level of dissolved H₂ and/or has more therapeutic benefit than the neutral or lower pH water of their competitors.

Importantly, because ORP meters are not specific to the H₂ molecule, other redox couples including antioxidants (e.g., vitamin C at high pH) can give water a negative ORP. This can be interpreted by the ORP-based H₂-meter as either a higher concentration of H₂ than truly exists, or as even the presence of H₂ when none exists. Moreover, a negative ORP does not always signify a therapeutic benefit. Some toxic substances can also produce a negative ORP (e.g., β-Mercaptoethanol). It is critical to know what the chemical species in the water is that responsible for the negative ORP, as well as the concentration of that substance. Figure 11 shows a flow chart decision tree of how to navigate water claimed to have a negative ORP and/or contain molecular hydrogen. If the water truly does have hydrogen, it will have a negative ORP, meaning we can reject water purported to be hydrogen water if its ORP is positive. However, as mentioned, just because water has a negative ORP does not mean that it contains hydrogen, as other substances, including toxic ones, could be responsible for the negative ORP. It should first be confirmed what the identity of the chemical species is. Finally, since the ORP meter does not indicate the concentration of the chemical species, the concentration should be specifically determined as it may not have any biological activity when ingested.

With an understanding of the concepts presented here, researchers and consumers can understand why the magnitude of the “negative ORP measurement” does not indicate either the concentration of molecular hydrogen or the potential therapeutic benefits of the water. The main value of the ORP meter might be that it does demonstrate a difference between hydrogen water and other waters. Hydrogen water at a concentration of 0.05 mg/L to >5 mg/L will measure a several hundred negative millivolts reading. Therefore, although a high negative ORP does not mean that there is a sufficient concentration of hydrogen, it does mean that, if the hydrogen water measures a positive ORP or only a slightly negative ORP, then it most likely does not contain molecular hydrogen, or at least not enough for a therapeutic effect. Thus, a rapid ORP test might be useful in some scenarios but could be problematic when not interpreted correctly.

The analyses and calculations were performed only with the variables found in the equations. We did not calculate the activity coefficients or discuss instrument limitations (e.g., drift in junction potential, sodium, acid error, etc.). Moreover, we did not consider equilibration time or reaction kinetics with the many other potential redox-active chemical species in water. Lastly, we made general assumptions based on the Nernst equation about ORP meters and ORP-based H₂ meters and how they would respond to changes in pH, H₂, and temperature. However, there could be additional algorithms and trade-secret computations that adjust for some of these errors. For example, our in silico calculations may suggest an error much greater than what is normally seen in practice. This may be particularly true for a small range of hydrogen water concentrations (e.g., 0.5 mg/L to 1.5 mg/L), at or near neutral pH and room temperature. On the other hand, the deviation from the actual concentration may be more than what is predicted by the calculations. Indeed, not all redox-active species were considered (e.g., oxygen, chlorine, minerals). Additionally, at the extreme ends of pH, temperature, and H₂ concentrations, the error might be compounded due to the meter’s lack of sensitivity. Experimental analysis in real-world situations on these issues is required before drawing definitive conclusions. However, this in silico analysis of the significance of pH and temperature provides a foundational framework for ORP/ORP-based H₂ meters that should be considered by consumers and researchers.

Because of the shortcomings of ORP measurements, the International Hydrogen Standards Association (IHSA) has specifically discouraged the use of ORP sensors and ORP-based hydrogen meters by researchers, consumers, and companies. Instead, the gold standard for hydrogen measurements is gas chromatography performed by a qualified expert. IHSA also approves of certain types of hydrogen meters/sensors that selectively measure the H₂ molecule. This is often based on polarographic, voltammetric, and other intrinsic properties of the H₂ molecule (Boshagh and Rostami, 2020). With all of these measurement techniques, a standard calibration curve is created using known concentrations of molecular hydrogen, and then the concentration of the unknown sample is determined by comparing it to the calibration curve. To the authors’ knowledge, the only simple and relatively accurate method of measuring dissolved H₂ concentration is with a redox titration reagent (e.g., MiZ, Japan; and H₂Blue ^® , United States of America). The reagent comprises methylene blue and platinum nanoparticles, which act as a catalyst to facilitate the reduction of methylene blue (blue color) to leucomethylene blue (clear) (Seo et al., 2012). For these products, the user simply adds a 6-ml aliquot of hydrogen water to a beaker, followed by stepwise drops of the reagent until the titration endpoint is reached. Typically, one drop represents 0.1 mg/L of dissolved H₂, thus, if 16 drops are required to reach the titration endpoint, the concentration of H₂ is 1.6 mg/L. However, this also has its limitations (e.g., not specific to H₂, catalyst can be poisoned, etc.).