# Why did ammonium chloride fumes turned pink in this synthesis?

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):

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

• Take white paint and add a bit of red. What colour will you see? – Alchimista Sep 7 at 9:30
• @Alchimista Pink. So, what red compound would have caused the fumes to turn pink? – Nilay Ghosh Sep 7 at 12:57
• 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 – Alchimista Sep 7 at 14:50
• @Alchimista that red color is possible at temperatures above 100 C. The reaction flask was at 35 C. – Nilay Ghosh Sep 8 at 3:22

$$\ce{NH3}$$ 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