How to prepare buffers of constant ionic strength and calibrate optical pH sensors?

The most basic version is the application of a PBS buffer system with the following recipe for two solutions: Please put in a beaker glass the acid solution (A) together with an optical pH sensor mounted according to the respective manual and a calibrated pH electrode. Make a note of the pH (of the electrode) and the phase value (displayed at the pH transmitters). By adding basic alkali solution (B) you will set a new pH value and therefore produce subsequent the required different pH-phase results. Make sure that the achieved pH values cover the pH range of interest for your application.