In the last section, we noted the very small influence of changes in H2 concentration on the ORP value while maintaining the pH at exactly 7. To examine the influence that changes have on the pH of the ORP, we will now gradually change only the pH while maintaining the H2 concentration at 1,6 mg/L.
Since the ORP becomes more positive as the pH decreases, a dissolved H2 concentration of 1,6 mg/L at a pH of Zero an ORP of zero millivolts.
As you will see, the degree to which pH influences ORP compared to the influence of H2 will provide even more compelling evidence to show why the ORP reading cannot be used to measure H2.
Previously, we observed the small effects of changes in H2 concentration when entering values into the Nernst calculator from a typical range of H2 concentrations (0,5 to 2 mg/L).
Since, as previously stated, the concentrations of both H2 and H+ species contribute to the redox potential, a change in either factor will cause a change in the ORP measurement.
An important question we need to ask ourselves is: “Do each factor contribute equally to the change in ORP?”
To answer this question, let's now look at how changes in pH affect ORP. In order to make a valid comparison, we will use a typical range of pH values (as we did with H2).
The graph in Figure 9 shows the predicted ORP readings (in millivolts) for four different pH values, all at the same dissolved H2 concentration of 1,6 mg/L.
The graph shows how an increase in pH alone (while maintaining a constant dissolved H2 level) affects the ORP measurement. At a pH of 7, hydrogen water with a concentration of 1,6 mg/L measures an ORP value of -414 mV. As the pH increases from 7 to 10, you notice the sharp increase in the negative redox potential from -414 mV to -592 mV.
Over this range of just three pH units, the predicted ORP becomes 178 millivolts more negative! Remember that the increase in negative ORP is only the result of the increase in pH (decrease in H+), while the H2 concentration remains unchanged at 1,6 mg/L.
As pH increases, H+ concentration decreases, increasing the negative redox potential created by the H+/H2 redox couple.
As a consequence, the ORP reading becomes more negative, not because the dissolved H2 concentration has increased, but because the H+ concentration has decreased; in other words, not because something was added to the water, but because something was removed from the water!
This sharp increase in the negative redox potential occurs without a change in the H2 concentration. Since the ORP responds almost exclusively to changes in pH, the size of the ORP value does not provide any useful information about the actual dissolved H2 content in the water.
Excerpt from Randy Sharpe’s book: “The Relationship Between Dissolved H2, pH and Redox Potential”
