Increasing The Oxidation Reduction Potential In Tap Water

[nvpacchi]

Hi everyone,

I am having a hell of a time trying to achieve an ideal ORP for a test I am about to run in our laboratory. A bit of background about the project:

We are required to fill two 75 gallon tanks with tap water at 90 degrees F. Once tank needs to be at an ORP of 650 mV while the other needs to be around 900 mV. An acceptable pH would be 6-8.

So when I began filling the tanks to run so preliminary tests on the water, I was achieving a pH around 7.3, which seemed fine, BUT my ORP was less than 20. At first I thought that the meter and electrode I had was faulty, but we just had it calibrated, and the pH was reading plausible numbers. I really hope that my meter is just lousy, which is causing the low ORPs but I have not yet been able to obtain another to confirm my initial readings.

Yet, I went forward assuming that my readings were accurate, and i went to the pool store to buy some chlorine in hopes of increasing the ORP. In a 5 gallon test bucket, I incrementally added the chlorine and was slowly able to increase the ORP to 250 mV. However the pH it this point was less than 4.5. (understandable, but a deadend for me at this point).

Next I started asking municipal water facilities, and calling other municipalities all around the country to determine what their ORP levels need to be. I discovered that water companies are not required to maintain a certain ORP, so I have not been able to establish a baseline ORP to work off of.

Since ORP is important for hot tubs, commercial pools, and such, I have been calling local pool stores and spas asking for advice. However I kept getting the standard (pH, alk, Cl, hardness) mandates which they are required to maintain. (I have also left messages with several spas in Atlantic City (I live 10 miles south) asking how they read their pool and spa ORPs, and I have yet to get a response from any of their management.

SO I guess this brings me here. If anyone has any suggestions on how to raise the ORP of tapwater without significantly decreasing the pH, it would be MUCH appreciated!

Thanks in advance for all your help.

[chem geek]

Your ORP meter is most definitely broken. The dissolved oxygen in water would have the ORP be much higher than 20 mV and having chlorine in the water would have the ORP be much much higher than 250 mV. You said the pH dropped when you added chlorine, but what were you adding that did this? It sounds like it might have been Trihclor, probably granular. If you were to add chlorine bleach and then lower the pH (if needed) with some acid, your ORP should be able to get very high.

ORP isn't as standard as everyone says, but you only need around 0.1 ppm FC with no CYA in the water to get to 650 mV or higher at a pH of around 7.5. To get to 900 mV, however, you'd have to get to around 50 ppm FC at 7.5 pH (around 30 ppm FC at 7.0 pH). Again, this is with no CYA in the water so do not use Dichlor or Trichlor as they both add CYA.

Also, you can raise the ORP with other oxidizers and a very strong one like potassium permanganate would get the ORP up high rather easily. I don't know what experiment you are trying to do but remember that ORP is measuring a thermodynamic quantity and in the case of chlorine it doesn't exactly follow the Nernst equation. If you want to measure reaction rates with high ORP water, then that is a completely different story. One can have a strong oxidizer in the water, but if it is a selective oxidizer then the reaction rate will be fast for some chemicals and slow for others. Thermodynamics says that the organics in our bodies should all burn up (oxidize), but this doesn't happen very quickly because the reaction rate is slow due to a high activation energy. So be careful about attributing too much meaning to ORP for your experiment.

[nvpacchi]

Thanks for the answer chem geek. I'm am gonna try using those numbers to get some ballpark figures for the time being.

I tried 2 brand new electrodes and another millovolt reader from another company and was still yield the same low mV results. I feel there must be multiple mV readings that can be measured with different equipment, since the local pool is running at 815 mV and all 3 meters I used to test it were negative.

For now I am just going to do some estimates based on charts I found, and buy a meter solely dedicated to ORP reading.