Reactivity 3.14 The pH scale

why the pH scale should be logarithmic

because majority of acids are weak and therefore produce H+ concentrations with large negative exponents

inventor of pH scale

Soren Sorensen, 1909

definition of pH scale

pH= -log10[H+]

features of pH scale

• usually positive

• has no units

• change of one pH=10 fold change in H+ concentration

relationship pH and H+ ion concentration

measuring pH

• universal indicator paper/ solution

  • colour estimation shows pH

more exact:

• pH meter

  • has to be calibrated prior to each measurement

  • temperature must be standardised

pH calculations

relationship between H+ and OH- is inverse in aqueous solutions, so low H+ means high OH-