Buffering in human tears: pH responses to acid and base challenge.

The buffering capacity of tears collected from six young, healthy subjects was assessed using a microtitration technique. Each subject provided, on six separate occasions, about 100 microliter of tears, collected in small amounts and with minimal mechanical stimulation over several hours. The pH of the total stirred pool of tears from each subject was determined at the outset. This pool of tears was then divided into two equal volume aliquots, the pH of each being determined following each titration step of one of them with acid, and of the other with base. In all, 28 titration steps across the acid-base spectrum were completed for each patient pool collected. A total of 1044 tear pH measurements were made, all being done in a closed, temperature stabilized (36 degrees C) microelectrode chamber having an accuracy of within 0.04 pH units. For a comparative reference, an identical titration procedure was used on degassed, demineralized distilled water (348 pH determinations). Buffering capacity was found to show considerable intersubject variations, but in all cases the effect was more pronounced and more uniform following acid titration. Local zones of enhanced buffering across the pH spectrum could be identified, presumably reflecting the multiple buffering components (bicarbonate, protein and others) present in tear fluid.