Calculate pH and hydrogen-ion concentration

Use the log key for pH = -log[H⁺] and the 10ˣ key to go back from pH to concentration.

pH is the negative base-10 logarithm of the hydrogen-ion concentration: pH = -log[H⁺]. To go the other way, [H⁺] = 10^(-pH).

If [H⁺] = 2.5 × 10⁻⁴ mol/L, then pH = 3.602. A pH of 3.6 gives [H⁺] = 10^(-3.6) = 2.51 × 10⁻⁴.

Classic Graphing 84 · Concentration to pH

Start on the main screen: Press 2nd then mode (QUIT) to get back to the home screen. Pressing clear on a menu also backs out of it.

Press the negative key, then log. Type the concentration with the exponent key (2nd, comma): 2.5ᴇ-4. Close the bracket.
(−)log2.52ndEE,(−)4)
Press enter. The pH is 3.602.
enter

You should see3.602059991

Classic Graphing 84 key reference

Classic Graphing 84 · pH to concentration

Start on the main screen: Press 2nd then mode (QUIT) to get back to the home screen. Pressing clear on a menu also backs out of it.

Press 2nd and log (10ˣ), then type -3.6 and close the bracket. Press enter.
2nd10ˣlog(−)3.6)enter

You should see2.511886432E⁻4

Classic Graphing 84 key reference

Natural Scientific 991

Start on the main screen: Press MENU, then 1 (Calculate) to return to the main calculation screen. AC clears the line you are on.

Press the minus key, then log, and type the concentration 2.5 ×10ˣ (-) 4. Close the bracket and press = .
−log2.5×10ˣ(−)4)=

You should see3.602059991

Natural Scientific 991 key reference

RPN Scientific 15

Start on the main screen: There is no menu to leave. Press the back arrow to clear a number you are typing, or CLx (g then back arrow) to clear the X register.

Key in 2.5, EEX, 4, then CHS to make the exponent negative.
2.5EEX4CHS
Press g and LOG (the 10ˣ key), then CHS to flip the sign. The pH is 3.6021.
gLOG10ˣCHS

You should see3.6021

RPN Scientific 15 key reference

Tips

A change of one pH unit is a tenfold change in concentration.
log is base 10. The ln key is a different logarithm (base e) and gives a wrong pH.