Direct from Hagen's website - FAQ
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My pH High tests do not show a color on the chart. Why?
Please find our report regarding "purple coloration" from the Liquids Division technicians. This phenomenon is sometimes seen during analysis with our pH High range test kit (A-7812). The problem is linked to the light source. It occurs when the source of artificial light is yellow.
There have been some reports concerning the pH High range test kit (A-7812) that produces a color, purple, that does not correspond to any color on the chart.
Upon close observation it has been seen that yellow light produced by such sources as High Pressure Sodium lamps and other incandescent light bulbs can influence the coloration, by adding "red" into the color. When one adds red to blue, the result is purple.
To remedy to this, either change the room or simply walk outside where there is true natural light.
A simple way to assess the severity of this problem on site is to perform the test normally. Then, observe the color with room light and a white sheet of paper behind the tube. Afterwards take the test outside under normal sunlight, again ensure a white piece of paper is placed directly behind the tube. Finally, place the tube under a Day Glo basking lamp (PT-2140) with a white paper on its back. This lamp does produce a spectrum well into the yellow red interval. You will clearly see the purple coloration.
