Sciencemadness Discussion Board

H2S solution for Qualitative Test

RU_KLO - 21-10-2022 at 10:13


Im just stating Chemistry, and want to do qualitative test. (my experience is making some salts and mineral acids. I do understand chemistry safe issues)

H2S is needed. After a lot of reading, Im aware (theoretically) of H2S gas dangers. (I also know that there is a better substitution - Thiourea, (NH4)2CS3 - Thioacetamide II - but cannot getit)

My idea is to make some Fe2s (1 or 2 grams - 1cm - 2 cms of a test tube) + acid, then H2S bubbled into dH2O at 30° - 50°.

My thinking is that 0.01M H2S solution is "stable" at 20° (from reading) . so if bubbled at 30° (or more), it will not release H2S when uses in 15° (my house). (as solubility at 30°+ is less than at 20° , so less H2S is in solution at 20°

here are the questions:

1) Can a solution of H2S (instead of gas) be used for qualitative test? (like the solution I want to make) - or it should be bubbled gas?

2) if I want to make 0.01M (0.4 g/100 ml (at 20 °C)) with how much Fe2S should I start.

3) these quantities are dangerous?


bobm4360 - 21-10-2022 at 13:45

1) If you have to make the gas anyway, use it for the analysis. Less handling means less chance for accident.
3) All quantities of H2S are dangerous. If you don't have a hood, do it outside on a breezy day. Don't do it in the house!

Bedlasky - 21-10-2022 at 18:20

You can use Na2S and acid. It is lot more practical.

Fery - 22-10-2022 at 09:04

For some cations (all from group 2) you can make the solution acidic and then add Na2S as Bedlasky suggested (just the acid should be in excess).
read e.g. this link
group 1 is precipitated after addition of HCl (this adds the acid),
then group 2 by H2S
When I was at summer school of chemistry the professor teaching us analytical chemistry used Na2S.9H2O for us instead of H2S, we asked him why not H2S and he answered it does not matter whether Na2S or H2S. But from the group 5 we did not analyze Na+, only the rest because we artificially added Na2S instead of H2S. The Na2S.9H2O was a mixture of most of crystals and little of solution (hygroscopic ? or melted due to very low m.p. ?) over the crystals in the original commercial flask and the professor just poured a little of the solution from the bottle and diluted it with water. We appreciated how fast was his method after we have already consumed all sulfide solution for precipitations and needed more and we just asked the above question and got the qualified answer.

Fery - 24-10-2022 at 01:53

RU_KLO I have never tried the thioacetamide method, but I ordered it just now as your post hit my curiosity so I would like to try it.
I have a lot of thiourea. If any helpful member in the forum have a journey through my country I can share it for free.

RU_KLO - 24-10-2022 at 12:56

Thanks for the replies.

after checking wiki, Na2S it seems difficult for a "newly" (- non fume hood - outside) amateur to make. (sorry , no sodium or carbotermic reduction yet)

it there an easy - alternative way?

j_sum1 - 24-10-2022 at 16:04

FeS is pretty easy to make. For best results you need very fine iron. I have found that an excess of sulfur is better.
Pack the mixture into a test tube and then heat with a flame. You can remove the heat when the reaction starts.

Once you have FeS (or realistically, a mixture containing FeS), you can then generate H2S by addition of an acid. If you use a dropping funnel you can control the rate of production and bubble the gas through a NaOH solution. Obviously work outside with a breeze or fan or in a fume hood. You should end up with a solution of Na2S. Monitor the pH and do not let the solution become acidic.

Na2S is pretty deliquescent. I think it would be difficult to isolate a solid product. But I admit I have never tried.

As an alternative to using FeS + acid, there is a procedure using candle wax and sulfur to generate H2S gas. Extractions and Ire on YT uses this method.

I would consider that creating Na2S for use in a later synthesis is a lot safer than using H2S directly.

Any time you are working with H2S
  • work with small quantities
  • make sure you can control the rate of production
  • use a scrubber or trap to capture any excess gas
  • ensure you have excellent ventilation
  • prepare some test strips from filter paper and copper sulfate. When moistened, these will turn black in the presence of H2S.
  • If you smell the gas, that is warning 1. If you stop smelling it or you seem to have gotten used to it, that is warning 2. Shut everything down and get the hell out of there.
  • be conscious of neighbours / family. Or anyone else who may be down-wind. Most people have a very low tolerance for this particular odour and you will never hear the end of it.

    Another good practice is to have a spray bottle of diluted bleach that is capable of generating a fine mist. This is useful when disassembling your apparatus. It will arrest the cloud of gas coming from your flask. I would not spray directly into the flask. You don't really want to generate chlorine. For this reason, wait until the reaction has stopped completely before disassembly.

    Bedlasky - 24-10-2022 at 16:18

    Quote: Originally posted by Fery  
    RU_KLO I have never tried the thioacetamide method, but I ordered it just now as your post hit my curiosity so I would like to try it.
    I have a lot of thiourea. If any helpful member in the forum have a journey through my country I can share it for free.

    Thioacetamide release slowly H2S due to hydrolysis. It is especially useful in gravimetry, because slow release of H2S make particles of the precipitate bigger. You can similarly use urea to precipitate hydroxides.

    teodor - 24-10-2022 at 23:34

    As far as I know the classical method of systematic analysis with H2S requires with some groups adjusting pH and saturating the solution with H2S. Could it be done with Na2S?
    But there are 2 other methods of qualitative analysis: one based on chelating and second based on solvent extraction. There is no any systematic schema published but making the method of systematic analysis based on chelating and extraction could be an interesting challenge for an amateur chemist.

    Fery - 25-10-2022 at 04:04

    Hi teodor, the info is in the link I posted. The group 1 is precipitated by addition of HCl and the precipitate is filtered out. From the filtrate the group 2 precipitates with S2- in acidic environment so you can just add Na2S as the solution is already acidic by HCl (just there should be an excess of HCl). Precipitate is filtered out and then from the filtrate the group 3 precipitates again with S2- but after addition of NH3 so in alkaline environment.
    There are other methods of analysis too. Creating a chelate with some organic ligand which is soluble in some organic solvent used for extraction seems to be interesting also.
    These old school analytical methods were replaced by automated analyzers in chemistry / industry / healthcare etc.

    RU_KLO - 25-10-2022 at 04:46


    for doing the test you posted, the starting material should be dry salt + H2O Solution,
    or could be also be done from an already acid/base solution.

    For example sulphate solution (which was donde with H2SO4 + diferent metals oxides (from battery leaching))

    and from basic solution? (maybe more HCL, to acidify the solution)

    Will these starting Acid/Base solutions interfere?


    Fery - 25-10-2022 at 13:02

    Hi RU_KLO, we did the analytical chemistry summer school while I was at college. We analyzed all those cations from the link except Na+ and NH4+ as we added them during analysis. The cations were samples prepared by dissolving nitrates. At the end we had an exam when we received a sample containing a mixture of 4-5 unknown cations and had to determine what all cations were in our sample.