The Biggest Misconception in pH Measurement

The pH probe is the most ubiquitous analytical sensor on earth. Even people who never took a science class in college have heard of pH (though few truly understand what it means). In our industry nearly every person I talk to understands that water is “neutral” at and only at pH 7. Very few understand what a value of 7 means but they know it is smack dab in the middle of the acid-base continuum. It’s the pH of pure water and it is a sacred number in the same way that one hour is always 60 minutes. Except that it’s not sacred at all.

This blog arose from a frustrated user’s attempts to reconcile a maddening problem. His probe calibrated perfectly in his two pH calibration solutions but, when immersed in his nickel plating solution it read low by almost half a pH unit. The probe always read low no matter how many times he calibrated. The temperature of the bath was 140 F and he suspected that temperature was the problem. But he also knew that our differential probe is temperature compensated. Surely temperature compensation would restore the true reading of the probe to that which he would read if the solution were at room temperature.

But he was mistaken. In fact, the probe worked exactly as it should.

To understand just why we need to back up just a few yards: The pH value is a measure of the concentration of hydrogen ions in water. The water molecules in a container of water are largely intact but a tiny fraction of them dissociate into H+ hydrogen ions and OH- (hydroxide ions). In pure water the two fractions must be equal to each other. That fraction is 10^-7 moles/liter. If you don’t know what a mole is you can push this gap in your knowledge aside or Google it. That 1/10th of one one millionth of a mole is equivalent to 1.8 parts per thousand. That “-7” in the exponent gives us a pH of 7, which is the pH of pure, neutral water. (Notice that we ignored the minus sign in front of the 7.) Add acid and you increase the concentration of H+ ions and decrease the concentration of OH- ions. Add base and the opposite happens.

But I left out one caveat. Only at 25 C (77 F) are there 10^-7 moles/liter of H+ ions in water. When water heats up some of the additional energy goes into breaking up more water molecules into more H+ and OH- ions. At 40 C there are 10^-6.6 moles/liter of both H+ and OH- ions. So the pH of water at this temperature is 6.6. The water is still neutral because the concentration of OH- ions is equal to the concentration of H+ ions. Only at 25 C is the pH of pure water 7.0.

The drop in pH of his process was very real. Temperature compensation corrects for the voltage that H+ ions surrounding the process electrode produce but that varies with temperature. The Nernst Equation tells us what that voltage is and how temperature affects its value. A pH probe measures voltage and uses the Nernst Equation to turn that voltage into a pH value. Had our user used a probe without temperature compensation he would have read a pH of 6.0. With temperature compensation the true pH is not 7.0 but 6.6.

So the big lesson: There is nothing sacred about the number 7. If you’re in the business of measuring pH remember the temperature matters.