# pH scale at various temperatures

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?

• 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. – Poutnik Oct 20 '20 at 12:47
• 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. – 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.

• 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. – Karl Oct 20 '20 at 18:14