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Reaction with AgNO3-What happened?

  1. Jan 27, 2008 #1
    I work in a microbiology lab. I was making up some plates for growing bacteria. The only difference in plates was that they contained varying concentrations of Silver Nitrate. The media (ie. bacteria food) contained baby formula, milk powder, yeast extract and agar, a polysaccharide made from seaweed.

    After autoclaving (exposing to 112 C degree heat with high pressure) the different media, one of the plates turned a strong shade of red, maybe with a hint of orange. Only the plate with a relatively high concentration of silver turned red, so I assume the silver had something do with it. It's possible the reaction precipitated some of the silver, because I noticed more growth than I expected on some plates. But results were inconclusive.

    Can anybody guess what the reaction was? I'm mainly just curious.
    Last edited: Jan 27, 2008
  2. jcsd
  3. Jan 29, 2008 #2


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    It might have produced a red compound from the nitrate side of the compound. What color are some of the decomposition products of nitrate? Could the AgNO3 have produced diazotization conditions in the agars? Reaction of the intermediate diazo compounds with tyrosine or histidine forms red/orange and red/yellow compounds.

    In your tests with the silver, did you approximate silver concentrations which would stimulate growth and well as inhibit it? It could be a different version of Lohner's experiments with silver coins that you see.
  4. Jan 29, 2008 #3


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    Are you sure you need to sterilize your media containing silver nitrate by autoclaving.

    It is often recommended for certain media additive to be added after autoclaving. The supplementation would have to be filter sterilized.
  5. Jan 29, 2008 #4
    Based on a bit of research, I was expecting uniform growth up to a certain concentration, higher than which there would be no growth. Second guess would be progressively less growth based on increasing concentrations. I know it's not a very exciting experiment, it was just supposed to be a control for another organism that is less well studied. I googled lohner/silver coin and couldn't find anything.

    I also looked up diazotization on wikipedia, not exactly sure how NO3- fits in with that.

    Well I was making it all from scratch. I definitely realize now it was silly to autoclave after adding the AgNo3. I probably could've just used sterile technique, even without filtering, to add the silver at the end. Afterall not much is going to be living in 1 millimolar silver concentrations. I did it mostly for convenience.
  6. Jan 30, 2008 #5


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    If your silver nitrate was contaminated with significant amounts of nitrite, the result is a chemical (at low pH) called nitrous acid or HONO. This is the reagent used to produce diazo compounds from primary and secondary amines. The manufacture of silver nitrate that has no or low levels of nitrite contaminants is not straightforward and some reagent grade silver nitrate reagents are contaminated with these compounds.

    Lohner's experiment (circa 1920-something) used embedded silver coins in agar. A bacteria-free zone close to the silver coin is observed fringed by a zone of enhanced bacterial growth. Silver kills at higher levels and stimulates a lower levels. I remember doing this back in high school (no not in the 20's).
  7. Jan 31, 2008 #6
    cool, good stuff to know
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