Uploaded September 2016 | Updated September 2026, 1 week ago
A yellow (hard to see on camera) Potassium Ferrocyanide solution (left) and a very dilute yellow Potassium Chromate solution (right) are prepared. Some drops of a Thallium(I) Nitrate solution are added to both of them.
Although both compounds are yellow for Potassium and the other Alkali metals (Thallium(I) can under certain circumstances be treated as an alkali metal-ion) the Ferrocyanide one is white.
We discovered this effect when we tried to prepare a bunch of Thallium compounds. It appears that many Ferrocyanides especially with d-Metals as counterions show completely different colors.
We are still not sure how exactly the counterion takes part in the coloration of complexes and will discuss this in a further video on Ferrocyanides. One idea is, as we visualized some structure data for Tl4[Fe(CN)6] that the Tl has trigonal prismatic and octahedral coordination (N-coordinated by the Cyanide's backside).
So this has either to do with the Lewis-Acidity (electrophilic) of the counterion, pulling some of the ligand's electron density away from the coordinated metal (Fe) thereby lowering the ligand field splitting and causing a red-shift.
Or for d-Metals as counterions (we checked their crystal structure as well) there are octahedra (N-coordinated) around the counterions as well. So we might have a second ligand field splitting here causing a mix of two colors.
We will try to find out more about the actual mechanism of this (hopefully) soon.
Still we found this effect quite interesting, as Thallium compounds usually are not colored (much like Alkali Metals or Silver) and the color here comes from the Chromate and Ferrocyanide that the products although the complex part has the same color end up in different colors.
A yellow (hard to see on camera) Potassium Ferrocyanide solution (left) and a very dilute yellow Potassium Chromate solution (right) are prepared. Some drops of a Thallium(I) Nitrate solution are added to both of them.
Although both compounds are yellow for Potassium and the other Alkali metals (Thallium(I) can under certain circumstances be treated as an alkali metal-ion) the Ferrocyanide one is white.
We discovered this effect when we tried to prepare a bunch of Thallium compounds. It appears that many Ferrocyanides especially with d-Metals as counterions show completely different colors.
We are still not sure how exactly the counterion takes part in the coloration of complexes and will discuss this in a further video on Ferrocyanides. One idea is, as we visualized some structure data for Tl4[Fe(CN)6] that the Tl has trigonal prismatic and octahedral coordination (N-coordinated by the Cyanide's backside).
So this has either to do with the Lewis-Acidity (electrophilic) of the counterion, pulling some of the ligand's electron density away from the coordinated metal (Fe) thereby lowering the ligand field splitting and causing a red-shift.
Or for d-Metals as counterions (we checked their crystal structure as well) there are octahedra (N-coordinated) around the counterions as well. So we might have a second ligand field splitting here causing a mix of two colors.
We will try to find out more about the actual mechanism of this (hopefully) soon.
Still we found this effect quite interesting, as Thallium compounds usually are not colored (much like Alkali Metals or Silver) and the color here comes from the Chromate and Ferrocyanide that the products although the complex part has the same color end up in different colors.
![Mercury Chemistry: Co[Hg(SCN)4]](https://i.ytimg.com/vi/jUqjZziourY/mqdefault.jpg "Mercury Chemistry: Co[Hg(SCN)4]
In this video, we make an interesting compound Co[Hg(SCN)4]. It is prepared by mixing a Hg solution with NH4SCN and adding some CoCl2. The compound does not form right away but after a while, tiny blue parts can be seen all over the solution. They quickly fall to the bottom where after a night bigger flakes had formed as well.
As you can the solution is clear in the end, so all the CoCl2 has reacted to form the insoluble Co[Hg(SCN)4]. As they grow so slowly often bigger crystals can be made.
Now you might say this could also be Co(SCN)x. True, they have a similar color. But we made the Thiocyanatocobaltate(II) about 2-3 years ago and since then left it on the shelf and it never precipitated. While here we have a much lower conc. of Co(II). Hg(SCN)2, on the other hand, is not colored because Hg(II) is a d10-system.")
![Mercury Chemistry: Hg3[AsO3]2 and Hg3[AsO4]2](https://i.ytimg.com/vi/kCN6V7e5-Eo/mqdefault.jpg "Mercury Chemistry: Hg3[AsO3]2 and Hg3[AsO4]2
In this video, we make and compare a Mercury(II)-Arsenite and -Arsenate by mixing HgCl2 with NaAsO2 and NaHAsO4. Now usually we do not make every possible compound just to increase the number of videos but for this, we wanted to test something. The Arsenate is, according to literature yellow, while the Arsenite is colorless, but develops a yellow color which is believed to be due to oxidation. We wanted to test this and made both solutions.
At first, you cannot see any precipitate (right jar = Arsenate) but after a while, and a second addition of more Arsenate you can see a yellow turbid color. For the Arsenite, it happens much fast but the solution turns yellow as well, much as expected. If you compare the two after a while you can see that they have the same type of yellow in their solution. 24h later the Arsenate had precipitated as yellow compound while the Arsenite formed a colorless one.
This proves, that the compounds differ in color and only a small fraction, the parts in solution are yellow, probably due to oxygen in the water and the stirring process and that it probably is some oxidation of the Arsenite.
We also tested both up to 350°C for thermochromism but nothing happened.")
