Reagent testing is not 100% accurate. There are a lot of things that come in to play when testing your samples. Some of them include:
- age and state of the reagent
- lightning conditions
- amount of sample reacted
- purity of the sample
Age and state of the reagent
Age and state of the reagent are very important. Reagents have an expiration date and you can usually tell they have gone bad by the change in color. To increase their shelf life you should keep them in an upright position (so the liquid doesnt touch the plastic parts), dark and refrigerated place. Even then most reagents last mostly couple of months. You can learn more about reagents here. If using reagents that have gone bad one might risk getting false results and thus making wrong conclusions. This is specially pronounced when testing substances that give a clear reaction.
Example: a tiny amount of substance X reacts yellow. If you add a bigger chunk it may react so hard that it kind of saturates the area and the color can turn beige/brown. So you can try reacting 2 different amounts. The same goes for reactions that produce darker colors.
If you are reacting a substance which doesn't give a color reaction and that sample contains impurities (they can also be colorless) the impurities itself may give off a false reaction.
Example: slightly yellow colored, lab verified 4-FA crystal turns yellow with marquis. Pure 4-FA doesnt give a color reaction with marquis.
Amount of reacted sample
Color obtained is also dependent on the amount of sample reacted. If you are expecting a dark reaction, try to react the smallest amount possible (less than 1 mg). If you are expecting a light colored reaction you can try reacting a little bit more of the sample.
Example:If you react 5 mg of substance X which produces dark blue color, you might get intense black instead of dark blue. To help with this situation try reacting 1 mg or even less and improve lightning conditions.
Test your samples on a white ceramic plate or something similar and under good lightning conditions. Clear white light is optimal. Observe the reaction closely from the beggining. Some substances may have the same final color, but can be different in early stages of the reaction!