Silver Crystals Using Tollens’ Test Reagents

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Have you ever seen the Silver Mirror trick at a chemistry show?  That’s when silver is precipitated out of solution onto a glass surface.  The demo consists of a series of clear solutions being combined in a glass flask.  As the presenter swirls the flask, a shiny mirror forms on the inner surface of the glass.  The reaction utilizes Tollens’ reagent, which is used to determine the difference between aldehydes and ketones.  I thought it might be cool to modify the procedure slightly to be able to grow silver crystals.  This turned out to be slightly more challenging than I predicted, mostly because the Tollen’s reagent is designed to form a smooth silver film on any glass surface, so crystals are not the first structures produced.  However, with some tweaking and dilution, I was able to get a slide with some individual crystals.  Above is a picture through the Foldscope and below is a macro photo of the prepared slide.

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3 Comments Add yours

  1. Manu says:

    That’s a really beautiful preparation. Specially highlighting the fact how opaque the mini-crystals are; which is a crucial property of a mirror.

    Secondly, I am seeing diffusion limited aggregation in the crystals; a classical idea that limits the shape and size of crystals grown under solution.

    https://en.wikipedia.org/wiki/Diffusion-limited_aggregation

    I would love if you could actually share your protocol of growing silver mirror on the glass slide itself. That would be incredible useful – I am thinking about it’s use in optics itself.

    cheers
    manu

  2. Saad Bhamla says:

    @marie – In line with manu’s comment, on the macro image, i can observe a circular diffused silver object – is that where you foldscoped?
    It is very interesting how the crystals form right next to the solution interface!

    Also – Congrats on your first post and welcome to the microcosmos web community 🙂

    Saad

  3. Marie Herring says:

    @ manu Here is the procedure I followed: https://ncsu.edu/project/chemistrydemos/Organic/TollensTest.pdf
    I prepared the Tollens’ Reagent as listed in step 2 (I downsized quantities), then diluted by ~5-10x before mixing with a similarly diluted dextrose solution on the surface of a glass slide. If I didn’t dilute, the solution would quickly turn brown, then opaque, then form a silver coating on the surface, making it impossible to see through within minutes. I was originally trying for a time-lapse of the reaction, but it just looked like slowly dimming light with no crystals. And yes! I didn’t think of DLA, but I was actually trying to capitalize on diffusion to make the crystals. Optics applications are intriguing! Are there times when an uneven coating is useful? Because for smooth metal coatings I would think that chemical vapor deposition would be preferred. I want to do a similar thing with copper, which is supposed to exhibit a more “tree-like” pattern.

    @saad I imaged through the fine dark crystals just above the center silver spot. The silver spot is where I added the Tollens’ reagent, aiming for minimal mixing and the crystals are the result (I assume) of diffusion of the silver through the dextrose solution. And thanks for the welcome! 🙂
    Best,
    marie

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