To the Editor.—The article "Human Tear pH" by Carney and Hill in the May Archives (94:821-824, 1976) concludes, among other things, that tears are more acid in the morning, taken from a yet unopened eye (average pH, 7.25) than later in the day (pH, 7.45; P <.001). The authors put forward the theory that this is related to a possible increase of acid byproducts associated with the relative anaerobic conditions during sleep.
Another explanation may be that the loss of carbon dioxide (CO2) in the open eye alcalizes the tear fluid. Hind and Goyan1 showed that in an open system, the pH rose about 0.3 pH units in the course of one minute.
In calves' eyes, Pedersen2 found a tear pH of 7.44 ± 0.007 using microcapillary glass electrodes. However, corrected for CO2 (40 mm Hg), the pH was 7.29 ± 0.003.
When I estimated
Norn MS. Human Tear pH. Arch Ophthalmol. 1977;95(1):170. doi:10.1001/archopht.1977.04450010170031
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