I am an electrician apprentice in charge of a large number of pH-sensors/transmitters at a power plant and chemical plant.

I do all the pH-instrumentation related maintenance, calibration, adjustments etc.

By studying pH measurements I have learned about the Nernst equation. The transmitter is able to (via an inbuild Pt1000 sensor) automatically keep the temperature variable in the Nernst equation constantly updated.

However, I have also learned that pH bear no meaning unless a temperature of the measured medium is specified. The transmitter is ALSO able to compensate for this variability, however. This is done by heating up a sample of the process medium and noting down the pH difference at various temperatures. This information can be plotted into a matrix in the transmitter.

HOWEVER, since the process medium composition is ever changing at the plant I work at (waste incineration) this matrix can't be made.

Seeing that many of the proces temperatures at the plant are anywhere between 40 and 80 degree celcius, how large of a discrepancy am I looking at? I know that both chemical composition and temperatures dertermine how much, but realistically, what are we talking about? 0,1 difference in pH? 0,5 difference? More? Worst case? Best case?

Thanks in advance.

  • $\begingroup$ Note that one thing is how pH changes with T, another things is, how pH-meter value changes with T. Optimal would be measuring at 25 deg C. The second best would be probably correlation curve between measured pH at T versus measured pH at 25 deg C. The point is, pH buffers calibrating pH meters assume T=25 deg C. $\endgroup$ – Poutnik Oct 20 '20 at 12:47
  • $\begingroup$ A Factor is composition may change with temperature . For example ,on a gas well , some components of interest are water, salts, CO2 ,and H2S, The relative proportions in the water phase change as temperatures change from 250 F to 350 F. $\endgroup$ – blacksmith37 Oct 20 '20 at 15:09

The pH of acidic solutions does not change much with increasing temperature. The pH of basic solutions does change more significantly with increasing temperature.

Ex. : The pH of $0.05$ M Potassium tetroxalate is $1.67$ at $0$°C to $1.81$ at $95$°C

The pH of $0.05$ M potassium hydrogenophtalate is $4.012$ at $0$°C and $4.23$ at $95$°C.

The pH of pure water is $7.47$ at $0$°C and $6.13$ at $100$°C

The pH of $0.1$ M borax is $9.46$ at $0$°C to $8,83$ at $95$°C.

The pH of $0.02$ M calcium hydroxyde is $13.42$ at $0$°C, and $11.45$ at $75$°C

Ref. : D. R. Lide, Handbook of Chemistry and Physics, CRC Press, 43rd Ed. 1961, and 83rd Ed., 2002.

  • $\begingroup$ The pH of an acidic solution is not very temperature dependant, neither is the pOH of a basic solution. What's temperature dependant is the pKs. $\endgroup$ – Karl Oct 20 '20 at 18:14

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