There are relatively inexpensive consumer-grade digital hydrogen meters on the market for around $500. Although these meters are widely used and accepted, they do not specifically/selectively measure dissolved hydrogen gas. Their design is based on ORP technology and uses non-selective redox electrodes to measure the reduction potential of water instead of expensive hydrogen probes.
However, these meters should not be confused with more complex technologies for measuring hydrogen. These use gas chromatography-mass spectrometer (GC-MS) technology, which can easily cost $100.000 or more.
There are also digital hydrogen meters that contain highly sensitive hydrogen gas probes (polarographic/voltammetric), but these devices can still cost $10.000 or more.
These technologies, which are primarily designed for scientific and industrial use, are very expensive and unaffordable for the average consumer.
In this section, based on our previous discussions on the relationship between H2, pH, and ORP, we will evaluate the ability of an ORP-based hydrogen meter to accurately measure dissolved H2.
ORP-based hydrogen meters attempt to determine the dissolved H2 content by measuring the redox potential of the water.
As we have seen, this results primarily from the contribution of the H+ species of the H+/H2 redox couple (represented by pH) and also to a relatively insignificant degree from the H2 species of the redox couple.
Such a meter attempts to determine the H2 concentration from the ORP measurement using the following process:
1. The Nernst predictions for the ORP of H2 water at pH 7 are factory programmed into the meter's computer firmware;
2. The redox potential of the sample water (in mV) is determined by the ORP and reference electrodes of the measuring device and processed by the electronics;
3. The dissolved H2 content of the sample is calculated by comparing the redox potential of the sample (whose pH is exactly 7) with the Nernst predictions for the ORP of hydrogen-enriched water (pH 7).
4. Display of the H2 value in PPB/PPM on the LCD screen of the meter. For example, from the graph in Figure 14 we can see that in water with a pH of exactly 7 at two different H2 concentrations of 0,1 mg/l and 2 mg/l by the Nernst equation ORP values of -379 mV and -417 mV respectively (2 ppm is the typical upper limit for this type of meter).
Therefore, it seems logical that if our water sample has a pH of exactly 7 and its ORP measures -379 mV, it should actually have a dissolved H2 level of 0,1 mg/L. On the other hand, if the ORP measures -417 mV, it should have a dissolved H2 content of 2 mg/L. If the measured ORP is somewhere in between, it is a simple matter for a computer to interpolate the reading and display the appropriately calculated H2 level.
But: Please note the following points:
1) Water will rarely have a pH of exactly 7, and any deviation, even as small as half a pH unit, will change the ORP reading by almost the entire contribution of dissolved H2 over the range of 0,1 to 2 mg/L (30 mV vs 38 mV).
Because the meter does not measure pH (there would be no practical method of correcting the pH of the water to exactly 7 if it were measured), this inevitable deviation in the pH of the water prevents the use of the redox potential to produce H2 to measure accurately.
2) In our discussion of the Nernst predictions for ORP, we said that we do not consider other redox couples in water that also contribute to the measured ORP (another form of the Nernst equation can predict the redox potential for multiple redox couples).
However, water usually contains other redox couples that contribute to a positive redox potential and counteract the negative ORP of dissolved hydrogen.
Although we are aware of their presence, there is no easy way to measure and subtract their contributions to the overall ORP. Therefore, they influence the ORP measurement in unpredictable ways and distort the H2 measurement.
Excerpt from Randy Sharpe’s book: “The Relationship Between Dissolved H2, pH and Redox Potential”
