Human precorneal tear film pH measured by microelectrodes.

The pH value in human precorneal tear film was measured with micro- and semimicro-pH-electrodes; the mean was pH 7.6. Stimulation of tear secretion and blinking led to a decrease in the pH value. When the eyelid was open, the precorneal tear film was alkalized by equilibration with the partial pressure of the CO2 in the surrounding air. When the eyelid remained open for 60 s or longer, an equilibrium value of greater than pH 9 was attained.