Preparation of Mercurous nitrite

I read that [Prafulla Chandra Ray](https://en.wikipedia.org/wiki/Prafulla_Chandra_Ray) prepared Mercurous nitrite for the [first time](https://en.wikipedia.org/wiki/Prafulla_Chandra_Ray#cite_note-PetitjeanJami1992-4) and published a paper.


Both Mercurous ions and nitrite ions are quite stable. So they should readily form the compound.


1. Why is it difficult to prepare it, and how to prepare it?
2. Can it be obtained by simply mixing $\ce {Hg2Cl2}$ and $\ce{AgNO2}$, where $\ce{AgCl}$ will be precipitated?





> 
> 1.Why is it difficult to prepare it, and how to prepare it?
> 
> 
> 


Prafulla Chandra Ray prepared mercurous nitrite, $\ce{Hg2(NO2)2}$ by reacting metallic mercury with cold, dilute nitrous and nitric acid. According to his paper(1):



> 
> Mercurous nitrite is the product of the combined action of nitrous
> and nitric acids on mercury :
> $$\ce{2Hg + NO2.H + HO.NO2 -> Hg2(NO2)2 + H2O}$$
> Some of the nitrite is decomposed by the nitric acid, the quantity
> of nitrous acid thus rapidly growing, until mercurous nitrite and nitrate
> are accumulated in molecular proportions. After some, the nitrous acid remains constant in the solution. It has become the catalytic agent between the nitric acid and the mercury,which now interact in the following manner:
> $$\ce{4Hg + 4HNO3 -> Hg2(NO2)2 + Hg2(NO3)2 + 2HO}$$
> 
> 
> 


It is difficult to prepare the compound because many mixed species are created in the reaction mix like and it becomes difficult to separate the target product i.e. mercurous nitrite. Also to note that both mercurous nitrite and mercurous nitrate can react with elemental mercury creating hemi- species.


$$\ce{2Hg2(NO3)2 + 2Hg + Hg2(NO3)2 + H2O -> 4HO.Hg2NO3 + 2NO}$$



> 
> 2.Can it be obtained by simply mixing $\ce{Hg2Cl2}$ and $\ce{AgNO2}$, where $\ce{AgCl}$ will be precipitated?
> 
> 
> 


Although P.C.Ray used elemental mercury and dilute nitrous and nitric acid, nobody tried this reaction route because A) Dealing with mercury salts is already dangerous B) P.C.Ray's sythesis is already famous so nobody has bothered trying it.


**References**


1. XX.—Theory of the production of mercurous nitrite and of its conversion into various mercury nitrates by Prafulla Chandra Ray, J. Chem. Soc., Trans., **1905**,87, 171-177, DOI: [10.1039/CT9058700171](https://pubs.rsc.org/en/content/articlelanding/1905/ct/ct9058700171/unauth)
2. THE CHEMICAL RESEARCHES OF ACHARYA PRAFULLA CHANDRA RAY by ANIMESH CHAKRAVORTY, Indian Journal of History of Science, 49.4 (**2014**) 361-370 - [PDF](https://www.insa.nic.in/writereaddata/UpLoadedFiles/IJHS/Vol49_4_3_AChakravorty.pdf)
3. Samanta, S. & Goswami, Sudeep & Chakravorty, A.. (**2011**). On mercurous nitrite and a basic mercurous nitrate derivative. Indian Journal of Chemistry - Section A Inorganic, Physical, Theoretical and Analytical Chemistry. 50. 137-140 - [Abstract](https://www.researchgate.net/publication/289285311_On_mercurous_nitrite_and_a_basic_mercurous_nitrate_derivative)
4. Das, A.K. Thermodynamic and Kinetic Aspects of the Stability of Sir P. C. Ray’s Mercurous Nitrite Compound. Reson 25, 787–799 (**2020**). DOI: [10.1007/s12045-020-0996-9](https://link.springer.com/article/10.1007/s12045-020-0996-9)
5. [Molecule of the Month - Mercurous Nitrite by Rajarshi Ghosh](https://www.ias.ac.in/article/fulltext/reso/019/10/0958-0960)
6. *CXCVIII -The double nitrates of mercury and alkali metals* by P.C. Ray
7. *LXVI - Dimercuryammonium nitrite and its haloid derivative* by P.C.Ray
8. *190. Theory of the production of mercurous nitrite and its conversion to various mercury nitrate* by P.C.Ray


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