Effect of pH on ppm readings of Ozone by ORP? 1

[glennpan]

I know that when you measure the ppm of Chlorine in water by ORP that pH is an important variable. There are many charts that show this relationship.

When ORP is used to measure Ozone, does anybody know if these same type of charts exist? Is the measurement reading of ORP in mV tied to pH in a similar manner for Ozone readings?

Thanks,

Glenn

 

[kenvlach]

All ORP values for oxidation/reduction reactions of which I am aware vary with pH: The ORP decreases as pH increases. This is because each reactant has an equilibrium involving H⁺, and ORP measurements are relative to the hydrogen electrode,

2 H⁺ + 2 e^- = H₂

for which the ORP is taken as 0.000 mV when [H⁺] = 1 Molar & P_H2 = 1 atm.

For ozone, the pH dependent relationship is
O₃ + 2 H⁺ + 2 e^- = O₂ + H₂O
ORP = 2.87 V at standard conditions. Sorry, don’t have an [O₃] curve as f(ORP,pH) but presume one exists somewhere.

Table 19.2 in The NALCO Water Handbook, 2nd edn. (or Table 7.1 in The Industrial Wastwater Systems Handbook) gives some standard ORP values at 25^oC. For hypochlorite, the ORP is 1.49 V in acid and 0.90 V in basic solution. Similarly, hydrogen peroxide has an ORP of 1.76 in acid and 0.87 V in basic solution.

The same behavior (ORP decrease with pH increase) occurs for reducing agents. I have observed sodium hydrosulfite (reducing agent for Cr⁺⁶) additions that appear to increase, rather than decrease, the ORP. The effect is because the addition lowered the pH.