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Congo Red

  • I have had a stock of Congo Red donated by a friend and recently I bought a new stock from a local Lab supplies shop. The second one is a different kind!

    In alkaline solution, it becomes purple and somewhat leaches out stain when a stained section is placed in clean KOH.


    Are there different types of Congo Red because I may buy again this time from abroad.


    Another point, it is really toxic and carcinogenic? Sometimes my fingers are stained and then I nibble my nails! : blow:



  • Hello!


    As far as i know, there's Konored SDS an Kongored NH3. SDS ist better not mixed with alkaloid media, such as KOH.
    Maybe that's the point here. When i mix Kongo SDS with KOH3%, there are lots of flakelike stains which are obnoxious under the lens. But i never saw any violet reactions.


    Concerning the Toxicity: You shouldn't drink that stuff. But opinions differ how cancerogenous it is by only skin contact. For myself, i try not to have much of it on my fingers, normally i'm using a paper towel to apply some pressure on the deckgläschen (what's that called in english? but i don't wear gloves or such.



    LG; Pablo.

  • Hello,


    pure congo red is classified as cancerogenic and toxic in high degree. Even chemists don't get it without declaration for what purpose they need it.

    The solutions I use and sell are appr. 0,5-0,7 % solutions in ammonia (NH3) or sodium dodecyl sulfate (SDS). So they are not very toxic anymore, but nevertheless the stuff is classified as cancerogenic.


    Congo red is an indicator liquid, which turns blue in acid conditions. So if you made a preparation with cotton blue (in lactic acid) on your slide, and didn't wipe it of very carefully before you make the next preparation with congo red, you will see it turn blue.

    I had several times preparations of polypores which turned my congo red solution dirty violetish in the microscopic preparation. May be these polypores contained a certain compound which was acid, I don't know.


    In which alcaline solution did you solute your congo red? Usually it should not change colour in KOH or NaOH.


    The official CIS no. of congo red is C.I. 22120 "Direct Red". So if your congo red shows C.I. 22120 it is the "true" congo red. If not it is something else.


    all the best,

    Andreas

  • Hi guys, tomorrow I show you a video of what's happening. Thank you so much for your replies. I am a bit more relaxed with regards toxicity. The other (old) congo red do form some minute crystals and I know the notorious feeling under the microscope. Sometimes it goes away by heating it in water bath at 70C until water cools down gradually

  • The one on the right is Congo Red Powder C.I. 22120 dissolved in water and filtered (I asked my friend who gave it to me) and remains red in KOH. Actually, when searching for my stuff, I just realised that I have bought a bottle and forgot about it - so at least this post was very beneficial !!! The one on the left was bought a few months ago from a commercial Lab Supplies and they provided it as a solution (1%). Well, I can make my own Congo Red solution from 22120... I guess 1% is a good concentration or a bit less is better? Since my Lab is home-based I prefer to avoid Ammonia.


    P.s. Does it really expires 🤷‍♀️😁 ?

  • Hello Steve,


    I have no idea what happened to your congo red on the left. It has a darker colour already in water solution. My congo red looks like the one on the right, and it doesn't change colour with KOH. I have congo red dissolved in water, in ammonia and in SDS - the colour is always the same.

    I also tried to create a solution of congo red in other tenside than SDS to avoid the percipitation that occures when using cngo red in SDS together with KOH. I tried Invadin, which is no longer available and may be replaced by Triton X (Octylphenol), but all this detergents did not make the solution better in my opinion. congo red in Triton X had a slightly more pinkish hue compared to the other congo red solutions.

    All these detergents (SDS, Invadin, Triton X, ammonia) have the aim to soften the tissue of your preparation, which usually is not necessary, at least not in fresh material. My favorite is still congo red in ammonia.


    all the best,

    Andreas

  • Thank you Andreas, what % is the Ammonia, around 10% maybe?


    I am preparing new solution from the powder now, that Congo Red is strange.


    I take the long cut, I dissect and put the section in congo red. Leave for a minute or so, then I absorb the extra stain with a cotton bud or kitchen roll strips and add plenty of 3%KOH and wash the dissection in it (sometimes leave it to stand for another minute if the tissue is robust/hard). Then I pick the washed section and place it in a drop of KOH+Glycerol mixture, sometimes a dissect further in smaller pieces and space them out. I place the cover slip over, press with a rubber over tissue paper and ready to go. The glycerol mixture prevents the slide from drying.


    Cheers :S

  • Hello Steve,

    I don`t know how law regulations in your contry are, but I could image that a manufacturer is obliged to print a expiry date on zhe bottle. We use silicon grease in our lab and because it`s called grease it`s covered by german food regulations and needs to have an expiry date :S Congo red is very stable and shouldn`t expire.


    Your observations regarding the differnt colours is very strange. Since we don´t know how your supplied solution was prepared it`s hard to find an explanation. One theory could be: differences in colour could originate from concentration effects. If you used higher concentrations of congo red than you get a deeper colour.


    I prepared my solution using the protocol of Erb&Mattheis: dissolve 3g of congo red in 98mL water and add 2mL concentrated ammonia solution (25%). Concentrated ammonia should be available at a pharmacy or at a do-it-yourself store.


    Regarding the hazardousness: as Andreas mentioned it`s cancerogenous. Even one single contact can induce cancer, regardless it`s on your skin, eyes or inside your body. At least, it´s watersoluble, so it cannot pass skin very well and get inside your body. I handle it in the way like Beorn does :)


    Have a good one,


    Andreas

  • Hello,

    One theory could be: differences in colour could originate from concentration effects. If you used higher concentrations of congo red than you get a deeper colour.


    I prepared my solution using the protocol of Erb&Mattheis: dissolve 3g of congo red in 98mL water and add 2mL concentrated ammonia solution (25%). Concentrated ammonia should be available at a pharmacy or at a do-it-yourself store.

    I don't agree. I prepared congo red in so many different ways and concentrations - the colour is always the same red! The only procedure where the colour chances is when you change the pH-value in direction of acid.


    The Erb/Matheis protocol is far to much concentrated. I'm wondering that a 3%-solution does not produce a fall out for being oversaturated. I made the expercience that 0,7% or even 0,5% solution is far enough and usually doesn't becomes oversaturated, not even in ammonia. I use 10% ammonia, but there is no difference with 5% and 3% ammonia.


    all the best,

    Andreas

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