Fuchsin confusion

Hi all,

From an eBay seller I bought a chemical labelled "Red Fuchsin Basic" (see attached photo). It was made by CENCO and has catalog number 39638.

I thought I was buying rosanaline hydrochloride (to perform the Schiff test for aldehydes) but now I am confused because some sources suggest that "Basic Red Fuchsin" refers to a mixture of rosanaline hcl, pararosaniline, New fuchsine, and magenta II.

I haven't been able to find any online references to the original catalog number.

The product is dark green crystals which have an amazing ability to turn a lot of water pink for even a tiny amount of powder. I accidentally spilled maybe 5mg and it took about a half hour of scrubbing with water and acetone to get the pink color off the sink and countertop.

Any idea how to tell what I have?

Thanks,
Sean

Quote: Originally posted by sbreheny

I don't know if this is a clue which might allow someone to help me out, but I did some tests which showed that when this substance is dissolved in water, the initial color is red if concentrated and pink if dilute. Further, lowering the pH to about 1 turns it a light yellow color (the pink disappears). Raising the pH to about 13 makes it go colorless. Both of these changes are reversible.

It looks like you paid $50 for a +20 year old used bottle of about 28g of something. Ouch!!

Since you have already purchased it why not try it in a Schiff test.

You could try paper chromatography it may tell you if you have a mixture but the three dyes are very similar and ideally you would want genuine samples of the three dyes for comparison.

Many triarylmethane dyes are ph indicators and have an extra colour change at extreme ph values.

from wiki: phenolphthalein colour change


[Edited on 2/16/2020 by wg48temp9]

Quote: Originally posted by wg48temp9

It looks like you paid $50 for a +20 year old used bottle of about 28g of something. Ouch!! Since you have already purchased it why not try it in a Schiff test. You could try paper chromatography it may tell you if you have a mixture but the three dyes are very similar and ideally you would want genuine samples of the three dyes for comparison. Many triarylmethane dyes are ph indicators and have an extra colour change at extreme ph values. from wiki: phenolphthalein colour change [Edited on 2/16/2020 by wg48temp9]

I am in the process of preparing Schiff's reagent using 0.25g of this dye, 0.25g of sodium bisulfite, 10mL of 1M HCl, and distilled water to make up about 60mL. The color is still quite red (although I can now see through it whereas I could not when first mixed) after about 3 hours. I will let it go overnight and see whether it mostly de-colorizes.

I also did an octanol-water partition test. It shows that the dye produces a slightly darker shade in water than in 1-octanol, but that could just be that there is only one compound and it is more soluble in water than 1-octanol. Hard to tell because even if it is a mixture, it could be that all the dyes have close to the same shade and wouldn't produce a very different color even if they separated between water and octanol.

First picture is after shaking and allowing to settle for about 1 hour. There is considerable octanol emulsified in the water phase so I then centrifuged it and the second photo shows a much more subtle difference between the two phases.

Quote: Originally posted by RadicallyStabilized

You probably need to add more (meta)bisulfite. I just tested my basic fuchsine (which I bought years ago from a reputable vendor). A spatula of this powder (weight not known, sorry, due to scale malfunction, but probably less than 100 mg) was dissolved in 10 ml of water, giving a deep red solution. I added about 25 ml of ca. 5% HCl and 475 mg of dissolved sodium metabisulfite with no color change. More solid metabisulfite was then added with stirring. The color became gradually less red. I estimate that it required about 1 g of the metabisulfite, if not more, until the color change was complete. The solution didn't become totally colorless, a slight brown tinge remained and small bits of a brown solid could be seen. A drop of acetaldehyde instantly turned the solution deep purple. A solution containing piperonal caused a pink color. Adding a little vanillin did not change the color (this is expected, as aldehydes with aromatic hydroxyls do not react with Schiff's reagent). Hope this helps, sorry for the sloppy execution. Exact proportions don't seem to be critical to me, though.

Thanks to everyone for their help. I took RadicallyStabilized's suggestion that I probably needed more reducing agent. In my case, I was using sodium bisulfite not metabisulfite, but I have seen Schiff's reagent recipes both ways.

Ultimately, I used about 250mg fuchsin, between 1 and 1.5 grams of NaHCO3, about 0.04 moles HCl, and about 100mL distilled water. After mixing until everything dissolved and it stopped becoming more colorless, I added about 1 gram of activated carbon (which I had seen as a suggestion in one recipe), stirred that for about 15 minutes, and then filtered it first through a coffee filter and then through a 220nm PES syringe-driven filter. The result was a very clear, slightly yellow-brown-tinted liquid. I'm pretty sure I had an excess of bisulfite because when poured, this solution released considerable SO2.

I tested 5mL of it with about 150uL saturated formaldehyde (H2CO) in water and got a very very deep purple color. Just to be sure, I also made a similar solution with no fuchsin (just HCl, NaHCO3, and water) with approximately the same proportions and put 150uL of formaldehyde in it with no color change. So I think that confirms that whatever I have, I am able to make a version of Schiff's reagent with it.

I also tested the original sample that I wanted to test for aldehydes (I suspect acrolein) and since I suspected that the concentration is very low, I added about 0.5mL of it to 5mL of the Schiff reagent and after shaking and letting stand for about 30 seconds, a faint purple color developed.

See the attached image.

Quote: Originally posted by RadicallyStabilized

Ok, seems to work, good. Are you sure you've got the bisulfite NaHSO3? Wikipedia says it's only stable in solution. That's why the metabisulfite (aka disulfite or pyrosulfite, NaS2O5) is commonly used.

Quote: Originally posted by RadicallyStabilized

It's in the German wikipedia: https://de.wikipedia.org/wiki/Natriumhydrogensulfit If my German doesn't fail me, "Es ist außerhalb von wässrigen Lösungen nicht stabil" means "it's not stable outside of aqueous solutions"...

Quote: Originally posted by RadicallyStabilized

Now this thread has evolved from "fuchsin confusion" to "bisulfite confusion"... I'm not really sure what to make of this. The English wikipedia is imo technically incorrect when it states that the "salt of bisulfite can be prepared by bubbling sulfur dioxide in a solution of sodium carbonate in water". In fact this makes only a solution of the salt. But it doesn't say how to isolate the salt itself which seems to be relevant here. Pubchem info is contradictory as well, some sources say it's a solid, some it's a liquid. Maybe you can test the solubility of the salt you have? NaHSO3 is 42 g/100 ml (Wikipedia, I assume at 20 °C); Na2S2O5 (I made a mistake in the formula above) is 65.3 g/100 ml at 20 °C. That should be enough of a difference to allow identification. Or maybe Sigma have now found a way to prepare solid NaHSO3... interestingly, at carlroth.com you won't find the sodium bisulfite as a solid. They only have solutions (search for "bisulphite"). I just checked my old Merck index, it says: "The bisulfite of commerce consists chiefly of sodium metabisulfite, Na2S2O5, and for all practical purposes possesses the same properties as the true bisulfite." Now this seems to be a decisive statement at least.