cnidocyte - 1-3-2011 at 11:31
I got a nitrite test kit from the pet shop and tested it out on an NaNO2/NaCl solution. The test reagent came in 2 bottles A and B, the manual that
came with it states that the reagent contains 4-aminobenzene sulfonic acid. Says on bottle A "contains HCl" and on bottle B it says "contains acetic
acid". Anyhow I added 5 drops of each reagent to my NaNO3 solution and when I added the 5 drops from bottle B the solution initially turned pink but I
left it for a few minutes and when I came back it was yellow. The chart they provide only contains different shades of pink they never said anything
about yellow. Any idea why the solution turned yellow?
The manual claims that the test works because the sulfanilic acid undergoes diazotisation in the presence of nitrite and an acid. Since they only list
sulfanilic acid the product must be the azo coupling product of 2 sulfainilic acid molecules. Anyone know the name of this compound? I know methyl
orange is yellow in basic solution and red in acidic solution but the product here isn't methyl orange.
[Edited on 1-3-2011 by cnidocyte]
matei - 1-3-2011 at 12:08
The kit is based on the Griess test for nitrites. Solution A contains sulfanilic acid and HCl, and solution B contains an aromatic tertiary amine in
acetic acid. The nitrites will react with the HCl and give nitrous acid, which will diazotise the sulfanilic acid. The diazonium salt of sulfanilic
acid will react with the aromatic amine in solution B and form an azo dye. As far as the change in colour is concerned, I guess you have to compare
the colour a specified time (1 minute maybe) after you've mixed the solutions, because the dye may not be so stable...
cnidocyte - 1-3-2011 at 12:50
Ah right so theres an unlisted amine in solution B. The instructions say to wait a minute but the solution turns yellow after only 30 seconds. Once
the drop of solution B hits the surface of the mixture it turns it blood red but when this red dye gets dispersed in the solution it becomes yellow.
The solution I have should be saturated with NaNO2 so I don't think yellow is an indication of low concentration.
DJF90 - 1-3-2011 at 13:20
Providing its used to test aquariums, your concentration will be well out of the range the manufacturer will be expecting it to be used for. Try a
much more dilute solution and see how that fares.
matei - 1-3-2011 at 13:23
I think you should use a concentration of nitrite in the range the kit is supposed to measure. A large excess of nitrous acid can lead to side
reactions, such as nitrosation of the aromatic ring of a tertiary aromatic amine, etc.
Sedit - 2-3-2011 at 07:42
As well as what DJF90 stated about possibly being out of range, arn't diazonium salts unstable undergoing reactions simular to the sandmeyer reaction
when warm substituting the closest nucleophile? Do the instructions state anything about keeping the samples cold or provide possibly some sort of
stablizer?