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What is the test for negative non-metal ions? Test for positive metal ions using sodium hydroxide solution?


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#1 rb.uk

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Posted 20 December 2016 - 11:46 AM

Hi all,

I was wondering what the test is for negative non-metal ions? Also, what is the test that uses sodium hydroxide solution for positive metal ions? Is it a flame test?

 

Thanks in advance for any replies 


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#2 _Rick_

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Posted 20 December 2016 - 12:10 PM

What negative ion are you testing for? Just any anion?

 

Also the sodium hydroxide test for metal ions will produce different coloured solutions or precipitates with different transition metals. You'll have to look those up.


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#3 Sensei

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Posted 21 December 2016 - 06:41 AM

Methyl orange
https://en.wikipedia...i/Methyl_orange
Methyl red
https://en.wikipedia...wiki/Methyl_red
can be used as pH indicators.

"Methyl red is a pH indicator; it is red in pH under 4.4, yellow in pH over 6.2, and orange in between, with a pKa of 5.1"

Phenolphthalein
https://en.wikipedia...Phenolphthalein
"Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions."
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#4 Sriman Dutta

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Posted 21 December 2016 - 06:48 AM

What negative ion are you testing for? Just any anion?
 
Also the sodium hydroxide test for metal ions will produce different coloured solutions or precipitates with different transition metals. You'll have to look those up.

Yes, NaOH is generally used in analytical chemistry to determine the cations present in a solution.
Another reagent used commonly is ammonium hydroxide (NH4OH).
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#5 _Rick_

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Posted 21 December 2016 - 01:44 PM

Methyl orange
https://en.wikipedia...i/Methyl_orange
Methyl red
https://en.wikipedia...wiki/Methyl_red
can be used as pH indicators.

"Methyl red is a pH indicator; it is red in pH under 4.4, yellow in pH over 6.2, and orange in between, with a pKa of 5.1"

Phenolphthalein
https://en.wikipedia...Phenolphthalein
"Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions."

 

You cant use those indicators for anything other than H+ and OH-...


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#6 Sensei

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Posted 21 December 2016 - 03:18 PM

You cant use those indicators for anything other than H+ and OH-...

 

If you have salt of strong-acid with weak-base (acidic salt), or strong-base with weak-acid (alkali salt), pH indicators will also change color accordingly.

https://en.wikipedia.../wiki/Acid_salt

https://en.wikipedia...iki/Alkali_salt

 

f.e. saturated solution of sodium sulfite has pH 9 according to wikipedia

https://en.wikipedia.../Sodium_sulfite


Edited by Sensei, 21 December 2016 - 03:21 PM.

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#7 _Rick_

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Posted 21 December 2016 - 04:44 PM

 

If you have salt of strong-acid with weak-base (acidic salt), or strong-base with weak-acid (alkali salt), pH indicators will also change color accordingly.

https://en.wikipedia.../wiki/Acid_salt

https://en.wikipedia...iki/Alkali_salt

 

f.e. saturated solution of sodium sulfite has pH 9 according to wikipedia

https://en.wikipedia.../Sodium_sulfite

 

They change the pH because they form hydronium ions just like any other acid. You're still only measuring the acidity of solution with any PH indicator, if you put an indicator into any of those solutions you're still only detecting hydronium/hydroxide molecules, it's not like you're detecting the Na+ or Sulphide ions.

 

For the OP, unless you give more info we just cant help you with the Anions part.


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#8 Sensei

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Posted 21 December 2016 - 05:09 PM

 

They change the pH because they form hydronium ions just like any other acid.

 

Where do you have hydronium ions in f.e. NH4NO3... ? NH4Cl.. ?

(which is salt of weak-base NH4OH with strong-acid HNO3 / HCl)

Just an example..


Edited by Sensei, 21 December 2016 - 05:18 PM.

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#9 John Cuthber

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Posted 21 December 2016 - 05:20 PM

 

If you have salt of strong-acid with weak-base (acidic salt), or strong-base with weak-acid (alkali salt), pH indicators will also change color accordingly.

https://en.wikipedia.../wiki/Acid_salt

https://en.wikipedia...iki/Alkali_salt

 

f.e. saturated solution of sodium sulfite has pH 9 according to wikipedia

https://en.wikipedia.../Sodium_sulfite

So what?

You can get a solution with pH near 9 with sulphite, carbonate, phosphate, borate and a stack of other ions.

So a pH indicator is ( as was pointed out) useless.

 

You get hydronium ions from ammonium ions: there's an equilibrium

NH4+ +H2O  <----> NH3 + H3O+

 

If you don't know that you probably shouldn't be posting an answer.

It's pretty much irrelevant since the question is about anions, rather than cations.

 

Also "NH4OH" (so far as it exists) is a very strong base. It is essentially entirely dissociated in water.


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#10 Sensei

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Posted 21 December 2016 - 06:06 PM

John Cuthber, OP asked question (if you didn't notice):
"I was wondering what the test is for negative non-metal ions?"
It's not equivalent of question (at least not for me):
"I was wondering how to check whether negative ions are carbonate, phosphate, sulfate, (....).. ?"
 
My post #3 was answering question "how to check whether there are some negative ions in solution", but without getting into details of their kind.
 
ps. Again, and again. Stop nitpicking posts of people who try to answer (whether they are answering correctly or not).
And concentrate on helping authors of threads. You have serious problem with this. Your post is absolute no help to the OP of this thread.
 

Also "NH4OH" (so far as it exists) is a very strong base. It is essentially entirely dissociated in water.

 
Then how come if we search for "NH4OH" on Google there is so many answers it's weak base?
Even ammonia is listed on wikipedia as weak base in examples section https://en.wikipedia.../wiki/Weak_base
 

You get hydronium ions from ammonium ions: there's an equilibrium
NH4+ +H2O <----> NH3 + H3O+


You can't be serious with this equation...
To get NH4+
in the first place you had to have:
NH4+ OH-
Now answer yourself, what will happen with OH- and H3O+... ?

Edited by Sensei, 21 December 2016 - 06:35 PM.

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#11 John Cuthber

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Posted 21 December 2016 - 06:47 PM

"John Cuthber, OP asked question (if you didn't notice):

"I was wondering what the test is for negative non-metal ions?""

I noticed. 

I also noticed that you didn't answer it. You told him how to check the concentration of H+

 

My post #3 was answering question "how to check whether there are some negative ions in solution", but without getting into details of their kind.

And, as was pointed out by Rick, your suggestion just doesn't work.

For example a solution of sodium chloride has plenty of negative ions in it, but barely affects any pH indicator

Did you not realise that?

 

 

" Stop nitpicking posts of people who try to answer (whether they are answering correctly or not).

And concentrate on helping authors of threads. You have serious problem with this. Your post is absolute no help to the OP to this thread."

I was helping the OP. I was explaining why following your suggestion was a waste of time.

 

Perhaps you should "concentrate on helping authors of threads." by not replying to things with stuff that doesn't help them at all, but distracts from the issue.

 

Even ammonia is listed on wikipedia as weak base in examples section https://en.wikipedia.../wiki/Weak_base

Yes, it is.
and nobody said otherwise.

What I said was " "NH4OH" (so far as it exists) is a very strong base. It is essentially entirely dissociated in water."

 

Do you not understand that there is a difference between NH3 and NH4OH 
Not least- one of them barely exists.

As for "You can't be serious with this equation...

To get NH4+
in the first place you had to have:
NH4+ OH-"

it's just not true.

I can start with NH4Cl for example.

I can dissolve it in water 

and the equilibrium I indicated earlier will be established

NH4+ +H2O <----> NH3 + H3O+

 

So to some extent the solution will produce hydronium ions

And, if you put a pH meter or indicator in there, you will find that the solution is, indeed, acid.

 

But the big issue here is you saying 

" Stop nitpicking posts of people who try to answer (whether they are answering correctly or not)."

 

It's not "nitpicking"- you were totally wrong, unhelpful and irrelevant.

Not only that, but when this was pointed out you tried to argue that you were right.

Why not accept that you get stuff wrong- it's not as if it's the first time is it?
 

 


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#12 hypervalent_iodine

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Posted 21 December 2016 - 08:34 PM

Sensei, as I have mentioned to you before (numerous times as a result of your posts across numerous threads), you have a habit of posting answers in chemistry threads that are either completely wrong, off topic, or so convoluted as to be completely useless. As has been outlined by John Cuthber, your posts here are an example of this, and pointing that out is not nitpicking.

I would suggest we return to the OP before this gets too out of hand, however it is a bit vague, and they don't seem to have returned since their initial post. Perhaps it is best left here, and any further questions regarding the specific exchange directly prior to this be moved to a new thread if further discussion is desired.
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#13 John Cuthber

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Posted 21 December 2016 - 09:00 PM

I wonder who marked Hypervalent down for that post. :-)

It's difficult to think of a single test for anions (well- there's ion chromatography but I doubt that's what the OP asked for).

The best I can come up with off hand is the addition of silver nitrate which gives different coloured ppt with three of the halogens (after the addition of dilute nitric acid to dissolve silver salts of weak acids.

It also gives some interesting results with other ions; for example the ppt with sulphite turns black on boiling. But that's not characteristic, the same is true of formate and tartrate.


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#14 Sensei

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Posted 21 December 2016 - 09:42 PM

Edit: off-topic removed:

Edited by Sensei, 22 December 2016 - 07:20 AM.

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#15 _Rick_

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Posted 21 December 2016 - 10:46 PM

 

Where do you have hydronium ions in f.e. NH4NO3... ? NH4Cl.. ?

(which is salt of weak-base NH4OH with strong-acid HNO3 / HCl)

Just an example..

 

In this scenario the ammonium ions go into solution and interact with with hydronium ions already in solution due to the spontaneous dissociation of water. It's an equilibrium remember.

 

H2O + NH3 <----> OH- + NH4+

So when that goes into solution it reduces the concentration of OH- ions. (Le chatiers' principle)

 

Then because of the recuced OH- concentration the dissosciation of H3O changes and more are left in solution for your pH indicator to detect. 

 

All of that was stupid by the way as you'd be detecting a Cation which wasn't what the OP asked about.

 

How do you suggest detecting NaCl with a pH indicator.


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#16 Sensei

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Posted 21 December 2016 - 11:07 PM

How do you suggest detecting NaCl with a pH indicator.


I didn't suggest it any of my posts.
I was suggesting how to detect weak/strong acid/base solutions, acidic/alkali salts, ignoring neutral salts.

Edited by Sensei, 21 December 2016 - 11:08 PM.

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#17 _Rick_

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Posted 21 December 2016 - 11:19 PM

I didn't suggest it any of my posts.
I was suggesting how to detect weak/strong acid/base solutions, acidic/alkali salts, ignoring neutral salts.

 

Then don't post that in a thread about a test for anions. Because as someone already pointed out its off topic and annoying


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#18 Sensei

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Posted 22 December 2016 - 07:19 AM

Then don't post that in a thread about a test for anions. Because as someone already pointed out its off topic and annoying


Then be so kind and come up with any answer to the first OP question,
which was "I was wondering what the test is for negative non-metal ions?"
You, nor anybody in this thread, answered this, and you complain on me..

Edited by Sensei, 22 December 2016 - 07:27 AM.

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#19 hypervalent_iodine

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Posted 22 December 2016 - 07:29 AM

Then be so kind and come up with any answer to the first OP question,which was "I was wondering what the test is for negative non-metal ions?"You, nor anybody in this thread, answered this, and you complain on me..


I wonder who marked Hypervalent down for that post. :-)

It's difficult to think of a single test for anions (well- there's ion chromatography but I doubt that's what the OP asked for).
The best I can come up with off hand is the addition of silver nitrate which gives different coloured ppt with three of the halogens (after the addition of dilute nitric acid to dissolve silver salts of weak acids.
It also gives some interesting results with other ions; for example the ppt with sulphite turns black on boiling. But that's not characteristic, the same is true of formate and tartrate.


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#20 John Cuthber

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Posted 22 December 2016 - 11:41 AM

Then be so kind and come up with any answer to the first OP question,
which was "I was wondering what the test is for negative non-metal ions?"
You, nor anybody in this thread, answered this, and you complain on me..

Two points.

First: I did.

Second: what you did was worse than useless.

 

By cluttering up this thread with irrelevant dross about pH indicators and then with complaints and denials when it was pointed out that your stuff was useless, you have not merely failed to help the OP, but you have actively interfered with others' attempts to do so.

 

How do you dare to call yourself "Teacher"? (That's a rhetorical question; please don't clutter the tread any further by answering it here. 


I didn't suggest it any of my posts.
I was suggesting how to detect weak/strong acid/base solutions, acidic/alkali salts, ignoring neutral salts.

Do you understand that posting a knitting pattern would also have been the wrong thing to do?


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