The Vitamin C test strip uses 2,6-dichlorphenolindophenol as a red-ox indicator.  This indicator changes from blue to colorless as the amount of ascorbic acid (Vitamin C) increases. Unfortunately, this indicator also detects other acids by changing from blue to red.

To help offset this effect, we include buffer salts in the formulation. This works for many applications, but it doesn’t work as well when the other acid is present in large amounts (such as in some fruit). In this situation, the starting blue color becomes more lavender.

This is why we have constructed a color scheme showing the effect of citric acid on the Vitamin C strip. Citric acid was chosen because of the likelihood that it would be present in some of the same solutions being tested for Vitamin C. The mechanism for the color change is based on the effect of pH on the indicator.