Discovering the Effect of Temperature on a Solution's pH

BY MICHAEL G. HARRIS, OD, MS, FD, BERNARD HALE, OD, DEEGAN LEE, OD & PAUL YEE, OD
July 1999

These investigators evaluated the effect of temperature on a solution's pH. Find out their results.

Due to an increase in the use of frequent replacement and disposable soft contact lenses, there has been an increase in the use of multipurpose soft contact lens solutions. Multipurpose solutions kept at standard room temperature are buffered to maintain a certain level of pH for comfort.

Contact lens solutions are comfortable on the eye when their pH is within the comfort zone. This comfort zone is between 7.8 and 6.6. However, if a solution's pH value is above or below this zone due to temperature, patients may feel some discomfort when a contact lens is placed on the eye.

There have been studies that have shown that the pH of contact lens solutions change with time. However, previous studies have not investigated if pH changes occur with a change in temperature. In different seasons and in different geographic locations, the ambient temperature may differ from the standard room temperature of 18.3šC (65šF).

We evaluated the pH of three multipurpose solutions at temperatures above and below the standard room temperature to determine if changes in temperature affect a solution's pH.

We evaluated three popular soft contact lens one-step solutions: Opti-One (Alcon, 12-oz. bottle), Complete (Allergan, 8-oz. bottle) and Renu (Bausch & Lomb, 8-oz. bottle). For statistical purposes, we used 20 factory-sealed bottles of each contact lens solution (60 bottles total per temperature) from the same production site.

The temperature measurements were made by an incubator thermometer (�0.25šC). Each weekday before measurements were taken, a 15ml sample from each bottle was placed in a vial. We used a 15ml sample size to correspond to the average daily amount of solution used by a patient for rinsing and storing soft contact lenses. Twenty samples of each of the solutions were incubated in closed air at 18.3šC (room temperature), 26.9šC, 31.1šC and 37.6šC. Twenty other samples of each of the solutions were refrigerated in closed air at 4.4šC and 8.5šC.

The pH of each sample was measured with a Fisher Scientific Accumet 950 pH/ion meter over the succeeding 24-hour period. The accuracy and repeatability of the pH meter were �0.05pH and �0.02pH units respectively. Before and between measurements, the electrode was rinsed with distilled water and calibrated using standard buffers (pH 4.01, 7.01 and 10.01). Each sample was opened for 5 minutes daily to simulate the time a bottle would be opened during lens handling by a patient.

The data for each trial was averaged and then graphed. Since storage time has been shown to change the pH of contact lens solutions, the measurements were taken over a 2-week period on weekdays from the time the bottles were opened.

The overall pH range for Renu, Complete and Opti-One was 6.84 - 7.66 during the 2-week testing period. The findings were all within the comfort range (pH 6.6 - 7.8) for the solutions stored at the five different temperatures. T-tests were done comparing the room temperature values to the values above and below room temperature. No statistically significant differences were found. Although the pH of the solutions varied with temperature, there was no apparent pattern to the pH changes.

While statistically there were no significant changes from the control, some trends were noted. Some solution brands appeared to vary more in pH than others when stored at the different temperatures. While Opti-One seemed more acidic at lower temperatures and more basic at higher temperatures, Complete showed the opposite trends and Renu showed less variability with temperature changes.

The pH values stayed within the comfort range for all three products at all temperatures. With the exception of Complete, the pH range was relatively small and insignificant. Complete showed the greatest variability (standard deviation of �0.06pH). This variability may affect the comfort of those patients who are highly sensitive to multipurpose solutions. Future studies should consider subjective differences in ocular comfort with these solutions after exposing them to the various storage temperatures.

Our findings indicate that the pH of these multipurpose solutions should remain within the ocular comfort range even if stored at temperatures other than room temperature. Thus, seasonal and geographic variables should not have a significant effect on solution pH and comfort. Contact lens practitioners should carefully select a multipurpose solution that will meet each patient's individual cleaning, disinfecting and comfort needs.

We would like thank Alcon, Allergan and Bausch & Lomb for providing the multipurpose soft contact lens solutions used in this study, the University of California, School of Chemistry for use of their pH meter, and Integrative Biology Department for technical support and the use of their laboratory. This study was supported by the Morton D. Sarver Laboratory for Contact Lens and Corneal Research and the University of California of Presidential Fellowship.

References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #50. (Be sure to have a fax number ready.)

Dr. Harris is associate dean, clinical professor and chief of the University of California School of Optometry's Contact Lens Clinic.

Drs. Hale, Lee and Yee are recent graduates of the University of California, Berkeley School of Optometry.