Sciencemadness Discussion Board - Thiocyanate turned red

Thiocyanate turned red

TmNhRhMgBrSe - 22-8-2019 at 04:58

I made some new calcium nitrate solution but its slightly yellow. I added some of it to sodium thiocyanate solution. The solution turned deep red so I think it has some iron. Is iron the only thing that can make thiocyanate turn red?

teodor - 22-8-2019 at 05:45

Not sure whether iron is the only thing to develop red color in the thiocyanate test but pretty sure that any not chemically pure sample of calcium salt contains a lot of iron. I would try to remove the iron (see "Inorganic Preparations" Henderson/Fernelius, excercise 3) and repeat the test. Also it is possible to remove iron by making complex with citric acid, once I made purification of Ca(OH)2 with this method but it requires quite a big amount of the acid.

[Edited on 22-8-2019 by teodor]

unionised - 22-8-2019 at 10:19

Iron probably isn't the only thing that makes thiocyanate turn red.
But it is the thing that is the 4th commonest element in the Earth's crust.

Rhodanide - 28-8-2019 at 12:42

I think I know what's happening. You say you "made" Ca(NO₃)₂ solution? And I'm assuming you used a Ca salt and HNO₃? The red color would make sense then, because at low concentrations KSCN reacts with most mineral acids (HNO₃ included) to form the deep red Perthiocyanic acid. With concentrated HNO3, KSCN reacts violently to form orange, insoluble Perthiocyanogen/Polythiocyanogen and readily expels LOTS of NO₂. KSCN also can react with other acids such as HCl or H₂SO₄ to form Perthiocyanic acid, which again is a cherry red color. Let me know if I'm close to correct about how you made that solution. :]

Here's a photo from a while back in which I tried making solid Perthiocyanic acid from KSCN + H₂SO₄. The chunks floating around are snow to cool the reaction mixture.

Bedlasky - 29-8-2019 at 12:29

Only one interesting thing.

Quote: Originally posted by CELamCheng

Is iron the only thing that can make thiocyanate turn red?

I recently made Mo(V)-isothiocyanate complex which is dark red too.

[Edited on 29-8-2019 by Bedlasky]

TmNhRhMgBrSe - 6-9-2019 at 07:06

Quote: Originally posted by Rhodanide

I think I know what's happening. You say you "made" Ca(NO₃)₂ solution? And I'm assuming you used a Ca salt and HNO₃? The red color would make sense then, because at low concentrations KSCN reacts with most mineral acids (HNO₃ included) to form the deep red Perthiocyanic acid. With concentrated HNO3, KSCN reacts violently to form orange, insoluble Perthiocyanogen/Polythiocyanogen and readily expels LOTS of NO₂. KSCN also can react with other acids such as HCl or H₂SO₄ to form Perthiocyanic acid, which again is a cherry red color. Let me know if I'm close to correct about how you made that solution. :]

Here's a photo from a while back in which I tried making solid Perthiocyanic acid from KSCN + H₂SO₄. The chunks floating around are snow to cool the reaction mixture.

I made Ca(NO3)2 from CaCO3 and excess HNO3. I neutralised remaining HNO3 with NaOH until some white precipitate form. I filtered to remove the precipitate.