# Why does HgCl2 react with HCO3^- but not with CO3^2-?

I have started learning Qualitative Salt analysis recently. When I was reading about tests to distinguish $$\ce{CO3^2-}$$ and $$\ce{HCO3-}$$, I came across a line in a book which states

$$\ce{HgCl2}$$ forms a reddish-brown precipitate of $$\ce{3HgO.HgCO3}$$ with $$\ce{HCO3-}$$ but no reaction is reported in case of $$\ce{CO3^2-}$$.

I am not able to understand why there will be no reaction of $$\ce{HgCl2}$$ with $$\ce{CO3^2-}$$.

It would be better if I can get a sound logic for this so that I can remember it better.

• The problem is $\ce{CO3^{2-}}$ only exists in very basic solution where the predominate reaction is: $$\ce{Hg^{2+} + 2OH- <=>[aq] HgO + H2O}$$ – MaxW May 8 at 18:56
• Mercuric compounds do react with carbonate ions, but the precipitate does not contain any carbonate anion. The equation is _$$\ce{3Hg^{2+} + 2 Cl^- + 2 CO_3^{2-} -> 2 CO_2 + Hg_3O_2CO_3}$$ The precipitate $\ce{Hg_3O_2CO_3}$ is reddish brown. – Maurice May 8 at 19:00
• @Maurice I am confused, how does the precipitate contain no carbonate ion and then you render it $\ce{Hg3O2CO3}$? – Oscar Lanzi May 8 at 21:17
• @MaxW As far as what I understand from your answer,all reactions of CO32- is accompanied by a competitive reaction with OH-.Then,why do certain reactions of CO32- occur even though they exist in very basic solution For example: I have learnt that CO32- can react with AgNO3 to form a white precipitate of Ag2CO3.Here,why isn't Ag2O reported as the major product?Why is there a difference in product observed even though both Ag2O and HgO are both oxides that lie above the ΔG=0 reference line in the Ellingham Diagram? – Chem-Learner May 9 at 3:17
• @Chem-Learner. Usually compounds like $\ce{Hg_3O_2CO_3}$ are not considered as double slt, because in the past oxides were not considered as salts. That is why they are simply described as basic salt. In the past a salt was defined as the result of a reaction between an acid and an oxide (or a hydroxide). Today a salt is often the result of an anion joined with a cation. In this cas the basic mercury carbonate may be considered as a double salt. – Maurice May 10 at 19:22

Anirudh, which book is teaching this stuff? Sorry, I can only find this discussion in 1890s and the latest in my favorite Mellor's Treatise from 1930s.

Coming to 2020:

After doing a simple acid test for carbonates, a nice safe & sound method to distinguish carbonate vs. bicarbonate salt is

(a) to react it with a neutral barium chloride solution. If the solution is turbid, carbonate is present, otherwise bicarbonate. Of course the methods fail with a mixture of the two.

(b) Sodium Carbonate-Phenolphthalein Test: phenolphthalein is turned pink by soluble carbonates and but remains colourless by soluble bicarbonates

Your textbook is not completely correct when it states that bicarbonate does not react with $$\ce{HgCl2}$$. For your reference from A treatise on chemistry, Volume 2 by Henry Enfield Roscoe, Carl Schorlemmer, 1913, pg 699

• Is there a way to carry out test for HCO3- and CO32- when they are present as a mixture?Will CaCl2 followed by NH3 addition work? – Chem-Learner May 9 at 18:35
• Yes, you will have to do a acid-base titration with methyl orange as indicator. Which textbook are you using? – M. Farooq May 9 at 18:37
• Oh Okay,I was given that if the white precipitate of CaCO3 is formed,then it contains HCO3- along with CO32- in my lecture notes.Can you elaborate on why the further acid-base titration is required? I am using Concise Inorganic Chemistry authored by JD Lee 4th Edition along with my notes as my main reference.Can you suggest me any book where I can learn Inorganic chemistry with concrete logic and not just filled with reactions? – Chem-Learner May 9 at 18:46
• For qualitative analysis, A. I. Vogel A Textbook of Qualitative Analysis is the best amazon.com/Vogels-Qualitative-Inorganic-Analysis-7th/dp/…. There is more more German book Lehrbuch der analytischen und präparativen anorganischen Chemie which has a lot of good schemes for qualitative analysis. One can read relevant sections with the help of machine translation. – M. Farooq May 9 at 20:20
• If you are willing to spend some money and find a lot of details, one of best books is Holleman and Wiberg "Inorganic Chemistry" in English. It will help you as a handy reference for life. More than > 2000 pages and highly highly detailed. It is the only book which has survived for 100 of years as a textbook from Germany. – M. Farooq May 9 at 20:26