Sidney Cam
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Ammonia test - what determines the initial colour?
I use an ammonia test kit to test the water in my fish tank.
The chemicals are supplied in two bottles:
1. polyethylene glycol and sodium salicylate (this is colourless)
2. sodium hydroxide and sodium hypochlorite (this is colourless)
This is called the salicylate method - for obvious reasons!
When these chemicals are mixed into a sample of water the initial colour is a bright yellow (before a blue dye develops in the presence of ammonia and
turns the sample green). Is anyone able to tell me what reaction is occurring to produce this initial colour?
To clarify, the initial bright yellow colour appears instantly the chemicals are mixed whether there is ammonia in the sample being tested or not.
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AJKOER
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OK, a speculation my part and perhaps, with the help of some of our organic chemists, we can provide the complete picture, or some corrections. The
possible reaction sequence:
NaOCl + NH3 --> NH2Cl + NaOH (very fast, even in the cold)
NH2Cl + NH3 + NaOH --> N2H4 + NaCl + H2O (slow or generally requires heating, perhaps sunlight?)
I have seen patents where the sodium salicylate contributes to the stability of the NaOCl.
Also, Diethylene glycol is usually used as solvent in reactions involving a base (here NaOH) and hydrazine, N2H4 (see Wolff–Kishner reduction
example discussed in Wikipedia at http://en.wikipedia.org/wiki/Wolff%E2%80%93Kishner_reduction ).
So perhaps now others can fill in the missing pieces.
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blogfast25
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http://www.chemetrics.com/ammonia
Scroll down to near bottom.
[Edited on 8-3-2013 by blogfast25]
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DraconicAcid
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That explains the colour change observed in the reaction with ammonia, but the OP is asking about the bright yellow observed in the absence of
ammonia.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Sidney Cam
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| Quote: | [quote=277279&tid=23591&author=DraconicAcid]
That explains the colour change observed in the reaction with ammonia, but the OP is asking about the bright yellow observed in the absence of
ammonia. |
Yup. I understand the reactions that are taking place for the salicylate test for ammonia quite well because there's lots of information about that.
What I'm asking is what reaction is responsible for the initial yellow colour that occurs the moment the two bottle contents are mixed - even in the
absence of ammonia.
If it's any help, in the 'standard' salicylate test there would be a catalyst involved called sodium nitroprusside, but this is toxic and so should
appear on the MSDS, but it doesn't. Now, if that was in one of the bottles I could understand the initial yellow colour because sodium nitroprusside
is yellowy/brown depending on concentration.
Like I said though the liquids in the bottles are both clear, no hint of yellow, and as the MSDS doesn't say there's any nitroprusside and the fact
that the reaction can take place without a catalyst, I can't figure out where the initial yellow colour comes from.
Is there perhaps an alternative catalyst that wouldn't need stating in the MSDS and that is initially colourless?
Anyway, thanks for the responses so far.
[Edited on 8-3-2013 by Sidney Cam]
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