Neutral pH

 

I'm working on the next edition of my textbook. From time to time I'm going to use you (readers) as guinea pigs to try out some new ideas. This is one of those times.

The concept of pH is difficult for students. It's easy for them to memorize the definition—the negative log of the hydrogen ion concentration—but that's not the same thing as understanding what it means.

Textbooks usually tell students that the equilibrium constant (K_eq) for the ionization of water is 1.8 × 10^-16. They can then calculate the ion product for water (K_w) at 25°C knowing the concentration of pure water (55.5 M). This value (1.0 × 10^-14) happens to be a convenient round number, giving rise to the standard pH scale from 1 to 14.

The square root of the ion product for water is the concentration of hydrogen ions ([H⁺]) and the concentration of hydroxide ions ([OH^-]). This concentration is 1.0 × 10^-7 or pH = 7.0, which corresponds to neutral pH at 25°C.

It occurs to me that students would have a better understanding of the concept if they were asked to do some calculations on their own rather than just reading the derivations in the textbooks. I propose to add the following problem. How many Sandwalk readers know the answer?

Neutral pH is the pH at which the concentrations of H⁺ and OH^- are equal in aqueous solvent. This pH is 7.0 for pure water at 25°C.

What is the neutral pH in your blood? What is the neutral pH in extremeophiles growing at 0°C or 100°C? (You may have to look up the values of some parameters in the Handbook of Chemistry & Physics).

Post your answers in the comments. You can post anonymously if you want but all the best biochemists will be signing their names.

Don't look at the comments until you come up with your own answer.

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