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I was watching the synthesis of tetrasulfur tetranitride performed by Tom on his Youtube channel, Explosion & fire. He followed the basic route: adding ammonia gas to sulfur chlorides ($\ce{SCl2}$ and $\ce{S2Cl2}$). At first, everything was going normal, purple-black precipitate on the flask with white $\ce{NH4Cl}$ fumes emitting as expected and white $\ce{HCl}$ fumes resulting from hydrolysis of sulfur chlorides. But suddenly a problem occurred. The fumes turned pink. The following is a sequence in chronological order (snapshots from his videos; temperature of ppt. in brackets):

enter image description here

  1. white $\ce{NH4Cl + HCl}$ fumes on top of precipitate ($\pu{T = 25 ^\circ C}$)

  2. fumes turned pink ($\pu{T = 27-28 ^\circ C}$)

  3. fumes entering the condenser from flask turning it red ($\pu{T = 35 ^\circ C}$)

  4. fumes escaping the condenser ($\pu{T > 35 ^\circ C}$)

  5. condenser changed, $\ce{NH4Cl}$ pellets observed above it, fuming doesn't stop ($\pu{T > 35 ^\circ C}$)

Eventually, after so much fuming and snowing, the end-product was achieved (when fumes turned golden brown, the original color of $\ce{S4N4}$). So, why did ammonium chloride fumes turned pink (when it was evolving)? Is it due to the compounds in the precipitate? If so, what species could have caused that color?

I did some research and found out that $\ce{S4N4}$ is thermochromic(changes color according to temperature). From britannica:

Tetrasulfur tetranitride forms thermochromic crystals, which are crystals that change colour with temperature. They are red at temperatures above 100 °C (210 °F), orange at 25 °C (80 °F), and colorless at −190 °C (−310 °F).

There is no mention of $\ce{S4N4}$ turning pink, so I proposed pink to be an intermediate color (orange -> pink -> red) and since it is a volatile compound, it was turning the fumes pink. But if the compound was pink, why did it later turn golden-brown, the original color of $\ce{S4N4}$? The precipitate was a purple-brown-black sludge, so it was definite not the end product. Thus my proposal contradicts.

prepchem says:

Tetrasulfur tetranitride is prepared by dissolving 1 volume of sulfur dichloride in 8 or 10 volumes of carbon disulfide, cooling, and passing in dry ammonia gas until the dark brown powdery pre­cipitate which forms at first has dissolved and an orange-yellow solution results which contains light-colored flocks of ammonium chloride.

Here, there is no mention of pink. What species could have caused fumes to turn pink in color?

I recommend to watch the synthesis in details on his second channel, Extraction & Ire: Part 1 and Part 2

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    $\begingroup$ Take white paint and add a bit of red. What colour will you see? $\endgroup$
    – Alchimista
    Commented Sep 7, 2020 at 9:30
  • $\begingroup$ @Alchimista Pink. So, what red compound would have caused the fumes to turn pink? $\endgroup$
    – Nilay Ghosh
    Commented Sep 7, 2020 at 12:57
  • $\begingroup$ Doesn't your text mention that due to thermochromism of tetrasulphur tetranitride red can be a possibile color? I mean the part in the box $\endgroup$
    – Alchimista
    Commented Sep 7, 2020 at 14:50
  • $\begingroup$ @Alchimista that red color is possible at temperatures above 100 C. The reaction flask was at 35 C. $\endgroup$
    – Nilay Ghosh
    Commented Sep 8, 2020 at 3:22

1 Answer 1

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$\ce{NH3}$ and $\ce{SCl2}$ form $\ce{NH4Cl}$ which evolves as white fumes. $\ce{NH4Cl}$, when passed through $\ce{SCl2}$ (cherry-red liquid), gives the appearance of dense-pink fumes

$$\ce{\underset{(cherry red)}{6SCl2} + 16 NH3 -> S4N4 +2S + \underset{(white)}{12NH4Cl}}$$

That is why the fumes of $\ce{NH4Cl}$ turned pink

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  • $\begingroup$ For formatting, See here and here. For a more detailed MathJax guide, look here, minor other details $\endgroup$
    – Safdar Faisal
    Commented Sep 9, 2020 at 15:39
  • $\begingroup$ While it explains the color change, (1) why did it take some time for the color to change? Also, the change was sudden and not gradual. (2) what was the reason for the temperature spike? The color changed as the temperature was rising, suggesting thermochromosity(?) (3) the precipitate was a purple-black sludge indicating that it was a mixture of products, by products and side products not just red sulfur chloride. $\endgroup$
    – Nilay Ghosh
    Commented Sep 9, 2020 at 15:47

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