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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Bel-p on June 16, 2006, 08:42:42 AM

Title: Tests for ions, why is acid needed?
Post by: Bel-p on June 16, 2006, 08:42:42 AM

Why is acid needed for tests for ions?

Eg nitric acid (and then silver nitrate) when testing for halide ions, and hydrochloric acid (and then barium chloride) when testing for sulphate ions?

Title: Re: Tests for ions, why is acid needed?
Post by: lavoisier on June 16, 2006, 02:31:51 PM

Don't take it as a general rule ('acid needed for tests for ions'); however, in the two specific cases you cite, the explanation is that at higher pH you can have side reactions which interfere with your test.

I give you a hint: forget about the anion you're analysing and think of what you have in solution in your reagent. In one case is NO3- and Ag+, in the other Ba2+ and Cl-.
Now, what can happen to these species if you make the pH rise (adding OH- ions)?
You should also consider that your solutions are in contact with air.

Title: Re: Tests for ions, why is acid needed?
Post by: swati on June 16, 2006, 05:31:34 PM

Quote from: lavoisier on June 16, 2006, 02:31:51 PM

Don't take it as a general rule ('acid needed for tests for ions'); however, in the two specific cases you cite, the explanation is that at higher pH you can have side reactions which interfere with your test.

I give you a hint: forget about the anion you're analysing and think of what you have in solution in your reagent. In one case is NO3- and Ag+, in the other Ba2+ and Cl-.
Now, what can happen to these species if you make the pH rise (adding OH- ions)?
You should also consider that your solutions are in contact with air.

 :D Hello everyone
In addition to this also think about the common ion effect.

Title: Re: Tests for ions, why is acid needed?
Post by: hepworth on June 16, 2006, 09:51:02 PM

so by adding acids are you neutralising the alkaline ions?
r all ions alkaline?
whats the common ion effect?

Title: Re: Tests for ions, why is acid needed?
Post by: Morphic flip on June 17, 2006, 05:55:37 AM

Common ion effect is equilibrium/Le Chatelier's principle.
AgCl _(aq)dissociates into ions Ag⁺ and Cl^- .
If you add say, NaCl_(aq), gives Cl ^-ions, the equilibrium of the solution shifts to remove Cl^- from the solution.
Therefore at the shifted equilibrium point there will be less Ag⁺_(aq) ions but more AgCl_(s).

Title: Re: Tests for ions, why is acid needed?
Post by: swati on June 17, 2006, 02:58:28 PM

Quote from: hepworth on June 16, 2006, 09:51:02 PM

whats the common ion effect?

When ther are two electrolytes in a solution & one of the electrolyte is stronger than the other & they have got a common ion between them , then the dissociation of the weak electrolyte is further suppressed.


            HCl         ----------->     H⁺        +        Cl^-  
            H₂S       ------------>   2H⁺        +         S ^2-       

 H₂S  being weaker electrolyte having common ion (  H⁺ ) with HCl , so dissociation of H₂S is suppressed & we will have less of  S ^2- in the solution.

Title: Re: Tests for ions, why is acid needed?
Post by: Bel-p on June 18, 2006, 05:43:39 PM

Thanks for the replies, I'll try and get my head around it all.