Micro-chemical test for nitrate
Author: Christine Bläuer
back to Micro-chemical testing
For more information on the required equipment and materials see the Micro-chemistry article.
Procedure
Evaporate the sample test drop to complete dryness. Place 1 or 2 crystals of diphenylamine[1] on the residue and add 1 drop of concentrated sulfuric acid (H2SO4 approx. 98%).
If NO3- is present the solution instantly turns blue to dark violet or purple.
Substances that may inhibit the test or affect its sensitivity:
The reaction is very sensitive but not very specific, as nitrites and other oxidizing reagents like chlorates or organic contaminants can produce the same color reaction. In our context, and especially when the micro-chemical test has the aim of confirming the nature of microscopically determined salt phases present, it can be considered that a positive reaction is due to the presence of nitrates.
Important notice on waste disposal
Diphenylamine is poisonous. The test drops have to be collected in a separate waste container and disposed of accordingly (not down the drain!). For each of these tests only one or two milliliters of waste water are produced, that can be collected and periodically disposed off. It should be no problem to find a bigger laboratory or company that will accept such small amounts of waste and dispose of them appropriately.