These tests use 'ion specific' electrodes that measure potential difference (voltage) in direct proportion to the concentration of the ion in solution.
pH
pH is commonly measured using an electrode specific for H+ ions. The electrode is incorporated into a meter or pen.
Fluoride
An electrode used to measure Fluoride ion (F-), has a small crystal of Lanthanum Fluoride (LaF3) at the end of an electrode.
NH4+(L3)
An electrode specific to ammonia is used. A solution of concentrated caustic soda and EDTA is added prior to measurement to convert the ions to ammonia and 'tie up' metal ions which may affect the result.
Electrodes are very sensitive and must be handled and stored very carefully. Background factors will affect the voltage produced and electrodes will need to be calibrated using known solutions prior to each use.
The calibration may be a 1-point calibration with a standard close to the expected value or a 2-point calibration (preferred) using solutions either side of the expected range.