The pH test strips and papers are based on acidobasic indicators, which change their colors in a particular pH range. Acidobasic indicators are generally very stable compounds, and we have chosen the best indicators based on their properties for use in our test strips. They are so stable that special packaging (such as air-tight stoppers, unpermeable material for packing, dessicant, etc) is not necessary.
When stored, keep the strips out of direct sunlight. Sunlight can fade the color with time, like any other colored materials exposed to light for too long. The biggest issue in this situation is fading (bleaching) of the color scale on the label, which serves as a calibration for pH reading. Fading of the color scale on the label can occur faster than the fading of strips themselves.
Similarly, exposure to high humidity for a long period of time can also cause changes in the color reaction system. When the moisture dries on the paper, it causes fading or color changes.
Generally, the minimum expiration date for pH test strips and papers is 3 years when stored at 10 – 30C in a dry place, away from chemical fumes in laboratories.