pH & Temperature

The temperature compensation thing actually applies to the per unit volume of water here. pH standards are based on specific standard temperature.

Water specific volume changes along with temperature (see thermodynamic pv diagram)

Recall that pH is just like, concentration mg of suspended solids over a liter of Water (mg/L). Consider this what if say temperature at initial state is 15 deg C then at final process stage is at 18 deg C. The specific volume of water changes there, that's why concentration will also change at 18 deg C.

The standards we used say for pH or that kind of concentration, is given let us say for example at 15 dec C. But your actual process medium monitored is at different temperature, therefore, this is needed to be adjusted to that standard value for consistency and convenience.

Otherwise, you will have to revise or make your own standard at certain temperature just like the psychometric chart at 1 atm, variable trends behaves pretty different at 0.5 atm unlike at 1 atm.

Ronie

Philippines

Sorry, but I disagree. While pH is based on concentration, it is expressed as a fraction of molar volumes, and so pH does not change because the water expanded.

Temperature causes a shift in certain equilibrium. This changes the pH. The effect is small, but varies from solution to solution.

Hi Tad,

I support your comment that pH doesn't change when temperature changes. Let say, we measure at 15*C and at 20*C with the same pH meter. What's the difference? Nothing or the equipment cannot read the difference! If your product has a solubility very different at those temperatures, I can accept some differences in pH but name me ordinairy chemicals we use does the dissimilarity in those temperature onditions. Wait for answer to solve the problem of the mentioned pH meters, Gil.

Hi Gil,

No, my point is that pH does change with temperature, just not for the reason you had given.

I have a buffer solution here. The manufacturer states that the ph is 6.88 at 20C, but 6.86 at 25C. Using a properly temperature compensated pH instrument, you would read these values. Thus, pH does change with temperature (this buffer goes from pH 6.98 at 0C to 6.89 at 100C). Without going into details, the actual change in pH depends on the system measured. It is not a function of solubility, but rather of equilibriums.

If you have an un-temperature compensated instrument, there will be error, as the instrument's response changes with temperature. Thus, even if you had a solution that did not change pH with temperature, you would not read the same pH

Hi Tad,

Again, I agree with your comment. However, who will complaining about 0.01*C variation of pH at the same temperature, statistically?

Nothing wrong with opinions, sometimes different on the same subject but after 54 years I used pH meters to make waterbourne coatings and other chemical mixtures, no one complained about a variation of 0.01 to 0.1 pH variation in any product.

What's the bruhaha about this "soooo" high precision pH reading? Tell me, Gil.

Hi Tad,

I don't want to argue with anyone but the insistancwe about this extreme precision of pH measurements, fascinating!

After my statistical experience, if you make pH measurements of your buffer at the same temperature at different time or just repeat it 20 or 30 times, and you get a surprising variations between them.

I made reactions dependent on pH but I was fine with, for example pH 7.8 at the end of the reaction, and another time I get 7.9 or 7.6. The percentage of yield was never different.

I just not underdstand why we have secret works for the bloggers around the world but we are not open to tell the reality. Fine?!

Mistery is the answer? I can accept that too, Gil.

I think the OP probably got his answer and has moved on. I can think of many processes which would require measuring a pH to within 0.1 pH units. For example, monitoring a titration without a sharp endpoint.

The reference given by Mikerho is very good.

In very simple terms, the buffer must be at the same temperature as the measurement. Measuring at say 60 deg Celsius will give you a different pH to measuring the same solution at say 20 deg Celsius.

You cannot measure at 20 deg C and turn the temperature compensation to 60 and measure again; you will get the wrong result.

If the sample is moving (flowing or stirred), the buffer must be under the same conditions

Tony

Mikerho provided some good information. Let me condense:

There are two issues related to temperature measurement:

Solutions actually change pH with temperature. This effect is slight, but is different for each solution. Most standard buffers come with a chart showing the pH at various temperatures. You would have to determine this characteristic of your sample experimentally.

Temperature effects the response slope of the electrode. This is theoretically well understood, and can be compensated for.

Theoretically, you could calibrate the pH electrode at 1 temperature, and measure the samples at different temperatures, as long as you apply the Nernst correction. However, I would not be comfortable doing this if very precise values are needed.

So, you may have a problem that the solution pH may actually change with temperature. Additional error may occur when temperature compensation is applied, simply because actual doesn't always exactly match theoretical.

http://www.reagecon.com/techpapers/effectsoftemperatureonphv4.pdf bioramani

Hi Jitesh,

Please, could you explain what's the difference in your question between: "exact practical relation" and "pH measurement very crucial"? To answer to your another question: "I mean, what really happens with pH when there is temperature is differing" we have to know what's the purpose of "sooo" high precision in pH measurement?

Please, take time and let us know what you really want, Gil.

Hi Gil,

In my present plant measuerment of pH is crucial since the after reaction pH decides the completion of reaction.

Thank You All who have posted/replied this thread. I shall definitely go through the link for pH & Temp effect..

Thanks again....

Jitesh

Hi Jitesh,

Thanks for the NO ANSWER! However, why did you ask for help? I am not after you or someone else, I just want to know why the so big insistence for the "sooo" high precision in pH measurement?

Good luch, Gil.