Change of pH with increase in temp

What type of sensor and display/controller do you have? Most, if not all, of the present manufacturers today provide temperature compensation in their equipment. It's usually a separate temperature sensor that you install right next to the pH sensor. This should solve your problem without having to make up a program to compensate for the temperature.

I think there was one sensor I saw in a catalog (can't remember which) where the pH sensor had a built-in temperature compensation circuit.

Just google "pH measurement" +"temperature compensation" and you'll get a lot of hits. If you've really set your hearts on reprogramming, some of the hits provide graphs and equations.

See Cold Parmer Ph-Meter and electrodes, them measurement Temperature, pH, mV, and more

It sounds as though the pH instrument doesn't have temperature compensation. Many systems these days have a temperature sensor adjacent to the electrode, and the correct selection of equipment with temperature compensation may be the better way to go.

More factors to include: display is a foxboro 873, the probe is a foxboro 871. The probe does have a temperature sensor integrated into it. With the posts received, I may just need to ensure that the temp curve has been activated in the display. Most of our needs for pH monitoring are ambient temp so it isn't a concern. However, in this application we are heating the process quite a bit and it is altering the process.

You may want to review the datasheet for the pH probe you are using to insure that is is manufactured to function accurately within the range of temperatures it is exposed to in your process.

If you need program the process and remove the human guess factor, used a pH / ORP Control.

Your temperature are 140ºF = 60ºC all electrodes probe support (100 – 110ºC).

OAKTON pH/ORP Controller 800 Series, (it is used in the mixer) includes all the features needed for basic pH or ORP control, Hold feature maintains relay and current output status during configuration or calibration, Hysteresis feature prevent relay chatter, Automatic or manual temperature compensation (with independent process and calibration temperatures), Galvanically isolated outputs, 4 to 20 mA output graphic chard, memory store calibration data and controller configuration, programmable delay (up to 2000 sec) minimizes false alarms caused by minor fluctuations and simplify installation.

PH : -2.00 to 16.00 / mV : +/- 1000 mV / Temp : -9.9 to 125.0ºC

In line of production: use Differential pH and ORP – process Controller ORION 2001pH

For electrodes see Ingold Mettler – Toledo Industrial Combination pH

I think that the previous posters may have missed the point. There are 2 effects: the pH probe does have a temperature response, which can be corrected.

However, there is also a real change in pH at different temperatures. That is, a solution will have a different pH at one temperature, as opposed to another. This appears to be due to differing dissociation constants of the ions in solution, so the dependence would depend on the specific solution being measured. This is discussed briefly here:

http://www.newton.dep.anl.gov/askasci/chem00/chem00920.htm

If I remember correctly, pH calibration standard buffer solutions bottles actually list different standard pHs, depending on temperature. The effect is not large, but measurable.

Tad

Temperature compensation is supposed to correct the pH reading for both the sensor and the solution's temperature. An instrument that doesn't do both is a poorly designed instrument and I don't think Foxboro would be one of those. I've used the 873 and the 871 and their readings check out with buffer solutions at different temperatures.

Sorry Vulcan,

This is not correct. Temperature compensation only corrects for the sensor's response to temperature, not the actual solution's pH change at different temperatures. To "correct" for the solution's actual change in pH would be impossible (because it varies with the buffer system) and incorrect, because the pH is actually changing with temperature.

Perhaps the buffer solution you tested had very little change in pH (as would be expected for a standard). However, weakly buffered solutions can have considerable change in pH with temperature. Here is an example:

http://www.neb.com/nebecomm/tech_reference/general_data/tris_buffer.asp

Buffers used for calibration typically have a much smaller change in value with temperature. From NIST:

http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=pHBufferTips.htm

Tad

Okay, I'll buy that. The pH sensor's response is affected by temperature and the measured solution's pH also changes with temperature. I can accept that.

But the present way the technology is used seems to say that the solution temperature is being compensated for because the temperature sensors are immersed in the solution along with the pH sensor so it's not measuring the temperature of the sensor. It's measuring the temperature of the solution.

Your statement that "correcting the solution's actual change in pH is impossible" seems to say that the companies selling us these things are misleading us.

The assumption is that the temperature of the liquid and the temperature of the probe will come to equilibrium rather quickly. Many pH probes have the sensor built in, rather than a separate probe.

There is no misleading at all. The temperature correction eliminates error due to the probes response to temperature, that's all. If the solutions actual pH changes with temperature, the reading will be correct.

The probe measures actual pH. In the example I gave before, a pH 4.01 buffer has a pH of 4.00 at 0C, and 4.21 at 90C. This is the actual pH at these temperatures and so requires no correction. If your meter read 4.00 at both 0C and 90C, it would be incorrect.

Thanks Tad, it's a lot clearer now. Having worked with pH (though not as much as the other disciplines like pressure, temperature, etc) I was pretty confident in what I knew.

I knew about the pH sensor's sensitivity to temperature but not about the pH change of a solution due to temperature. Somehow, that fact never surfaced.

What you are looking for is the Nernst equation. Actually, most process meters and many pH sensors (with internal or external temperature sensor: Pt 100 / 1000) can deal with this temperature correction. What I think is quite important for your application is the "sodium error" which is quite noticeable above pH 10. This is because most of the glass pH sensors actually take the Na+ sodium ion as if it were H+ ion and compute this as a higher pH than you should have measured. Besides, the glass membrane sensors are severely affected by the high pH level so only propietary glass membranes specific to this level of pH can be reliable used (this combined with the proper electrolyte inside the pH sensor!!!). General purpose electrodes from most manufacturers will fail in a short time and the measurements will be inconsistent, leading to high replacement costs plus the lack of control of your process.

I strongly advise you to contact your Mettler-Toledo Process supplier and they will address both sources of error (temperature + alkaline errors)

http://glo.mt.com/mt/filters/industries_chemical_process-analytics-applications_process-analytics-chemical_process-ph/pH_family_browsw_0x000249740002871b0003f47b.jsp

I hope this will help you.