Sciencemadness Discussion Board

Problems with precipitation of first analytical Group of Cations

RU_KLO - 17-10-2023 at 06:43

Im having problems with the precipitation of first group. (insoluble Chlorides Pb+, Hg+ Ag+)

(Ag+ still not learned, because of lack of silver...., but will enventually.)

Pb+ did not produce any problems (0.1gr of lead nitrate) was disolved in 5ml H2O, then adding 1 drop of 6M HCl gave the white precitate. Pb+ confirmed then with Potassium Cromate (Yellow precipitate)

the problem arises with Hg+ (or Hg2+).

First a solution of mercury nitrate was made:
2.40 gr of mercury (from a broken termometer) was disolved in 20ml nitric acid (do not know concetration exactly but is around 60% (- 63% is indicated in the label, its a year old, but it does not put latex gloves on fire....)
Mercury disolved completly, and a lot of NO2 was generated - solution was orange/green + erlenmeyer full of NO2 gas.
this was heated (40ºC tops) + stiring . In 30 minutes a clear solution was obtained.

When testing this solution with HCl no precipitate was formed.

My thoughts is, that was very acidic (from nitric acid), so there must be some chemistry that did not allowed precipitation.

Then added ammonia till slightly alkaline. then when adding 1 drop of 6M HCl, white precipitate was formed.

(Note: if ammonia is added in excess, a white precipitate is formed before adding the HCl)


Now the problem is, that the confirmation test for Hg+ ask for
"filter the precipitate and add 2 or 3 drops of ammonia (diluted) to the filter.

Precipitate should "turn black".

I saw pictures in Internet of this "turning black", but mine did not. (white precipitate with ammonia smell is left)

Could someone point if doing something wrong?

(as writing this post, learned from wiki that I could use potassium chromate or potassium iodide as confirmation test, but want to know what Im doing wrong.)

thanks










Rainwater - 17-10-2023 at 13:59

I try to stay away from the really toxic stuff. Stay safe

Quote: Originally posted by RU_KLO  
My thoughts is, that was very acidic (from nitric acid), so there must be some chemistry that did not allowed precipitation.


Never tried this reaction, but sounds like a good hypothesis.
Quote: Originally posted by Polverone  

The mercury is added to and dissolved in an excess of nitric acid density 1.42, and the mercuric nitrate solution produced is evaporated to dryness,


They used thermal decomposition in a similar reaction.
Keeping the temperature at 120c may prevent decomposition while removing the nitric acid.
Quick wiki search .....
Conflicting literature... some say 400c+ some 265c+ so 120c should be a nice middle ground. Vacuum distillation may be better

B(a)P - 17-10-2023 at 14:23

Quote: Originally posted by RU_KLO  
Im having problems with the precipitation of first group. (insoluble Chlorides Pb+, Hg+ Ag+)


the problem arises with Hg+ (or Hg2+).



If you used dilute nitric acid you will have mercury (I), if you used concentrated nitric acid you will have mercury (II).

Quote: Originally posted by RU_KLO  


First a solution of mercury nitrate was made:
2.40 gr of mercury (from a broken termometer) was disolved in 20ml nitric acid (do not know concetration exactly but is around 60% (- 63% is indicated in the label, its a year old, but it does not put latex gloves on fire....)
Mercury disolved completly, and a lot of NO2 was generated - solution was orange/green + erlenmeyer full of NO2 gas.
this was heated (40ºC tops) + stiring . In 30 minutes a clear solution was obtained.

When testing this solution with HCl no precipitate was formed.

My thoughts is, that was very acidic (from nitric acid), so there must be some chemistry that did not allowed precipitation.



Mercury chloride is soluble in both water and nitric acid and mercury salts are also volatile. You may have lost some of your mercury to the air (hopefully not your lungs) and the rest is likely still in solution.

Quote: Originally posted by RU_KLO  



Then added ammonia till slightly alkaline. then when adding 1 drop of 6M HCl, white precipitate was formed.

(Note: if ammonia is added in excess, a white precipitate is formed before adding the HCl)


Now the problem is, that the confirmation test for Hg+ ask for
"filter the precipitate and add 2 or 3 drops of ammonia (diluted) to the filter.

Precipitate should "turn black".

I saw pictures in Internet of this "turning black", but mine did not. (white precipitate with ammonia smell is left)

Could someone point if doing something wrong?

(as writing this post, learned from wiki that I could use potassium chromate or potassium iodide as confirmation test, but want to know what Im doing wrong.)




You likely have mercury amidochloride. It is sparingly soluble in cold water and quite soluble in hot water and will decompose in very hot water. So, you could try dissolving your solid product in hot water and see what happens.