Chemical Forums
Specialty Chemistry Forums => Other Sciences Question Forum => Topic started by: jag93 on October 24, 2019, 04:14:46 AM
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I am trying to find a chemical reaction that will give a yellow precipitate. I know about the "golden rain" reaction (potassium iodide + lead nitrate), but I would ideally like to find a reaction that doesn't require very hazardous/toxic chemicals (such as lead nitrate). Does anyone know of such a reaction? I am a scientist at a University, so can easily get just about any dry chemicals. Thanks.
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I found this list
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Chromates, AgBr, As2S3, AgI, PbI2, CdS, PbCrO4(s), Hg2CO3(s), Ag3PO4(s), Bi(C6H3O3)(s), Cu(CN)2(s), Ag3AsO3(s), (NH3)3[As(Mo3O10)4](s), [SbI6]3-(aq),
Yellow
https://en.wikipedia.org/wiki/Qualitative_inorganic_analysis
Yellow seems to really like nasty metals like lead, mercury, and antimony. By far the least nasty and easiest to make of these I see is the silver ones.
Simple enough, just dissolve silver nitrate, and add the appropriate sodium salt! I bet silver phosphate will be the yellowest, I've made silver halides and they usually look black, but you should try them all!
Remember silver nitrate+light will stain skin (harmlessly).
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Maybe precipitate sulfur from Thiosulfate by adding acid. It is first white and change to yellow. Some sulfur dioxide will be released as well.
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Does it have to be inorganic?
You can go all natural - take a substance with yellow coloring (turmeric works pretty well!) and steep it in 2-propanol or acetone. The yellow curcuminoids go into the polar solvent. If you drop this into a really nonpolar solvent like hexane, they will precipitate out as a yellow clumpy powder.
Or - you can use a 2,4-dinitrophenylhydrazine reaction with an aldehyde or ketone (benzaldehyde is a good one) to make the very yellow orange 2,4-dinitrophenylhydrazone. You can find procedures in almost any organic lab text.
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These are also Hazardous. Hydrazine and derivates are carcinogen. The solvents as well.
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If it's important for the solution to be colorless to begin with, I'm afraid cadmium ion exchange reactions is probably your best bet (e.g., cadmium chloride plus a sulfide source = bright yellow cadmium sulfide known as the pigment Cadmium Yellow). The good news is that the insoluble sulfides of cadmium aren't very toxic at all because they are so insoluble - which is why they are still used in paints and as pigments for plastics. Even the soluble forms aren't that bad as long as you handle them with respect.
Dispose waste according to local environmental regulations.
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In this case lead chromate can be used as well. LoL .The problem is, that the educts lead, chromate, cadmium and also sulfide are dangerous .
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I thought of CdS too, but the issue is not so much the insoluble cadmium sulfide, but probably the handling the cadmium starting solution.
Chenbier's idea it interesting, I was also trying to think of a way to precipitate sulfur from a clear solution. That method will smell some, and SO2 is a nasty lachrymator.
I stand by my choice of silver salts. It is by far the safest solution to handle of the choices presented, aside from turmeric (or any other yellow dye) but that requires starting with a yellow solution. Alas it is also probably one of the less intense yellows of all these options.
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OP didn't mention what the application is, but if lasting, strong yellow is the aim in itself, I feel that silver is a poor choice. The color isn't strong to begin with and the photoreactivity of silver ensures that what yellow color there is won't last long unless the sample is stored in the dark. They use silver salts for photograph development for a reason.
If the OP is a scientist as stated working in an equipped chemistry facility, hazards of cadmium solutions are minimal. They are not acute hazards, are not volatile, and do not involve flammable solvents. Proper disposal of waste will be the worst of it.
My opinion of course, YMMV and all that.