Distinguishing anions for Chemistry SPA.

• Lord dejavu

  8 May 10, 23:51

  Hi all.

  Let's say i got 3 unknown substances the first containing carbonate ions, the second containg sulfate ions, and the last one containing both sulfate and carbonate ions.

  To distinguish the 3, i can use an acid to test for efferverscence of carbon dioxide which would indiate the presence of the carbonate ions. Then to test for sulfate, i can use barium chloride and look out for the formation of barium sulfate, a white ppt.

  My question is, which acid do i use? I will choose nitric acid as sulphuric acid will interfere with the sulfate ions originally present in the mixture right?

  How do i determine which reagants to use so that the ions present in the mixtures will not interfere with each other?

  Thanks. 

   

   

• SBS261P

  9 May 10, 01:46

  if my memory serves me correctly we always use nitric acid, because if any salts are formed, they are always soluble (since all nitrates are soluble)

  you could do it the other way to be safe - add barium sulphate to isolate the one with the carbonate, then add whatever acid you want to the remaining 2

• Lord dejavu

  9 May 10, 01:51

  Thanks SBS261P. For me i cant do it by elimination sadly. I have to add both reagents that i choose to all the 3 unknowns. Btw, i plan to just use nitric acid if i am suppose to identify any acids.

  But i'm afraid:

  1)i dont have nitric acid on that day? like they purposely exclude it

  2) will nitric acid interefere with any reagants other? like say silver nitrate or barium chloride? (what a n00b qn) :(

• UltimaOnline

  9 May 10, 08:30

  Note that barium carbonate(IV) is also insoluble. Adding a mixture of nitric(V) acid and barium nitrate(V) will result in :

   

  a) carbonate(IV) ions

  effeverscence produced. CO3 2- + 2H+ --> H2CO3 --> CO2 + H2O

   

  b) sulfate(VI) ions

  white ppt produced. Ba2+ + SO4 2- --> BaSO4

   

  c) carbonate(IV) + sulfate(VI) ions

  effeverscence produced. CO3 2- + 2H+ --> H2CO3 --> CO2 + H2O

  white ppt produced. Ba2+ + SO4 2- --> BaSO4

   

   

  Since all 3 test substances will yield different observations, yes you can indeed use a mixture of nitric(V) acid and barium nitrate(V) as your test reagent.

   

  It is unlikely they will "purposely exclude" the availability of HNO3(aq) that day. If they do, using any acid other than H2SO4(aq) will be fine. Nitric(V) acid does not interefere with any of the common lab reagants.

• Lord dejavu

  9 May 10, 14:17

  Thanks UltimaOnline. Just another question, what about distinguishing Cl- ions from (SO4)^2- ions? I wanted to choose silver nitrate to look out for the white ppt of AgCl, and barium chloride for the sulfate ions but the ppt of Ba2S04 is white also.

• UltimaOnline

  9 May 10, 15:23

  Originally posted by Lord dejavu:

  Thanks UltimaOnline. Just another question, what about distinguishing Cl- ions from (SO4)^2- ions? I wanted to choose silver nitrate to look out for the white ppt of AgCl, and barium chloride for the sulfate ions but the ppt of Ba2S04 is white also.

  
  That's fine, as long as the two solutions containing the Cl- ions and SO4 2- ions are in separate test tubes. Use separate samples of each of the test solution (ie. separate test tubes) when addding acidificed silver(I) nitrate(V) test reagent, versus adding acidifed barium nitrate(V) test reagent.

   

   

• Lord dejavu

  9 May 10, 19:19

  Thanks Ultima Online. your input is always much appreciated :)

• UltimaOnline

  9 May 10, 22:37

  Originally posted by Lord dejavu:

  Thanks Ultima Online. your input is always much appreciated :)

  
  You're always much welcomed :)