THE PH IN THE PRECORNEAL TEAR FILM AND UNDER A CONTACT-LENS MEASURED WITH A FLUORESCENT-PROBE

The reaction of the precorneal tear film of the human eye was determined non-invasively by instilling pyranine, a pH-sensitive fluorescent dye. The mean value was 7·83 (s.d.±0·10) and it takes up this value immediately on opening the eye after the lids had been kept closed. The HCO3 system seems to be responsible for only a portion of the buffering power of the tear film. When a drop buffered to pH 6·4 with 0·075 m PO4 was instilled, the tears returned to their normal value in about 7 min, consistent with the washout time of solutes in the conjunctival sac. A pH of 7·3 was established in the tear fluid behind contact lenses, either gas permeable or impermeable, probably as a result of their restricting the loss of CO2 from the eye. The rabbit pre-corneal tear film is more alkaline, at about pH 8·2. © 1990.