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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Rutherford on June 25, 2012, 10:17:42 AM

Title: Mn2+
Post by: Rutherford on June 25, 2012, 10:17:42 AM

From the wikipedia page:
4th analytical group of cations
The fourth group of cations include Zn²⁺, Ni²⁺, Co²⁺, and Mn²⁺. This group is determined by the addition of ammonium chloride, ammonium hydroxide, and hydrogen sulfide gas to the solution of the salt. A colored precipitate indicates Mn²⁺...

Is is really neccessary to add hydrogen sulfide gas or I can add it as a liquid, because I don't know how to add it as a gas, or is there any better way to determine Mn²⁺ ions?

Title: Re: Mn2+
Post by: AWK on June 25, 2012, 10:32:17 AM

Ther best method use oxidation to HMnO₄ with NaBiO₃ (the best reagent), Pb₃O₄ or PbO₂ in nitric acid. Moreover this can be done using the whole mixture of cations without separation any of them.

Title: Re: Mn2+
Post by: Borek on June 25, 2012, 10:43:47 AM

From what I know at least since seventies instead of gaseous H₂S everyone uses thioacetamide.

Title: Re: Mn2+
Post by: Rutherford on June 25, 2012, 11:02:40 AM

I am preparing for the next level (more ions) of qualitative inorganic analisys. I need to make a chart, but I got stuck on the IVth group: Mn²⁺ and Zn²⁺. Unfortunately, I won't be allowed to use thioacetamide and as I found out, neither H₂S is allowed.

Title: Re: Mn2+
Post by: AWK on June 25, 2012, 11:12:44 AM

The fourth group of cations include Zn2+, Ni2+, Co2+, and Mn2+. This group is determined by the addition of ammonium chloride, ammonium hydroxide, and hydrogen sulfide gas to the solution of the salt. A colored precipitate indicates Mn2+...[/sub]
NiS and CoS are black. How can you see white or pale pink precipitate?

Title: Re: Mn2+
Post by: Rutherford on June 25, 2012, 11:54:34 AM

From the fourth group I only need to know the determination processes for Mn²⁺ and Zn²⁺ for the test. I need to find another way for proving those catinos because I won't be able to use H₂S_(g), so I'm stuck now.

Title: Re: Mn2+
Post by: AWK on June 26, 2012, 04:59:28 AM

I told you about ^Mn2+ test.
How this cations react with an excess of NaOH?

Title: Re: Mn2+
Post by: Rutherford on June 26, 2012, 05:59:29 AM

Thanks, good one, white precipitate soluble in acids not in excess of NaOH.
Now, I need only for the Zn²⁺ cation something good.

Edit: Found similar for Zn²⁺ only it will dissolve in  excess of NaOH. Ca²⁺ and Mg²⁺ make insoluble hidroxydes witn NaOH, too, but they won't dissolve in excess of NaOH, right?

Title: Re: Mn2+
Post by: AWK on June 26, 2012, 07:08:53 AM

http://faculty.coloradomtn.edu/jeschofnig/class/class_jeschof/ch2-lb8.htm

Title: Re: Mn2+
Post by: Rutherford on June 26, 2012, 03:13:12 PM

I found a very nice way for Mn²⁺ with HNO₃ and PbO₂. Tomorrow I should finish the whole list.

Title: Re: Mn2+
Post by: Rutherford on June 26, 2012, 03:21:56 PM

Quote from: AWK on June 26, 2012, 07:08:53 AM

http://faculty.coloradomtn.edu/jeschofnig/class/class_jeschof/ch2-lb8.htm

Is that reaction characteristic only for Zn²⁺ and not for the alkali and earth-alkali metals, because I'm planning to test their presence right after Zn?

Title: Re: Mn2+
Post by: AWK on June 27, 2012, 01:49:24 AM

Supernatant after treatment with an excess of NaOH should be used.

Title: Re: Mn2+
Post by: Rutherford on June 27, 2012, 04:38:20 AM

I will have plenty of the unkown salt so I can make a new solution. Ca, Ba, Mg, Na and K don't react with K₄[Fe(CN)₆]?

Title: Re: Mn2+
Post by: AWK on June 27, 2012, 04:55:42 AM

https://www.msu.edu/~grahamm9/General%20Solubility%20Rules%20for%20Ionic%20Cmpds.%20in%20H20.htm
There are better methods for qualitative analysis of Ca, Ba, Mg cations than using ferrocyanides.

Title: Re: Mn2+
Post by: Rutherford on June 27, 2012, 05:11:29 AM

I didn't mean to determine them with ferrocyanides (I am planning to do it on fire), but I don't understand how to be sure if there is Zn²⁺ present in the solution or one of the earth alkali metals if they all make white precipitates with ferrocyanides. A mistake could happen.

Title: Re: Mn2+
Post by: AWK on June 27, 2012, 05:34:52 AM

You should know, at least  basis of qualititative analytical chemistry.
http://fch.upol.cz/skripta/fcc_and_zvem_english/anal/anal_teorie.htm

Title: Re: Mn2+
Post by: Rutherford on June 27, 2012, 07:13:50 AM

Thanks for the link, I know the basics, but found out something new on the link. The best way to determine Zn²⁺ would be to add sulfide water, because on the link that you gave me it is said that ZnS is the only white sulfide. Only anions left.

Title: Re: Mn2+
Post by: AWK on June 27, 2012, 07:55:15 AM

There are different procedures for single cations, there are systematic procedure for mixture.
For some mixtures you can use shortcuts, but not for all. In the systematic procedure for mixture the zinc cation may be together with Al3+ and chromate only - then both sulfide and ferrocyanide works.

Title: Re: Mn2+
Post by: Rutherford on June 27, 2012, 08:00:47 AM

I will be given a salt made from one cation and one anion only, no mixtures.

Title: Re: Mn2+
Post by: Rutherford on June 27, 2012, 10:39:20 AM

On your like isn't written that Al gives a precipitate with NH₃. In my book it is said that Al gives a white precipitate with NH₄Cl and NH₃ buffer solution and Iron gives a redish-brown precipitate. What is correct?

Title: Re: Mn2+
Post by: AWK on June 27, 2012, 10:55:05 AM

Even NH₄Cl itself forms Al(OH)₃ from Al salts but in colloidal form almost invisible (solution of Al salts are often additionaly acidified - then nothing happens).
Pure ammonia or ammonia buffer (NH₃/NH₄Cl) forms visible colourless precipitates (often gelatinous).

Quote

Iron gives a redish-brown precipitate

correct

Title: Re: Mn2+
Post by: Rutherford on June 27, 2012, 11:04:10 AM

Ok, thanks.

Title: Re: Mn2+
Post by: Rutherford on June 28, 2012, 05:41:48 AM

I finished my list, but I have some doubts. Can someone check it if I write it here?