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All Message Boards : Nitrogen test
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 Message 1 of 5 in Discussion 
From: MSN NicknameGuy_SoCa  (Original Message)Sent: 5/7/2008 5:19 PM
Why is KF (Potassium Fluoride) used for a nitrogen test of an unknown organic molecule.
 
Thanx in advanced


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 Message 2 of 5 in Discussion 
From: MSN NicknameGuy_SoCaSent: 5/7/2008 6:18 PM
The question probably didn't make any sense..
it's a nitrogen test with the unknown organic molecule, Ferrous Sulfate, Potassium Fluoride and then Sulfuric acid to for a complex ion, NaFe[Fe(CN)6] (ppt)  
and what role does the KF play in this reaction?
 
sorry and thanx in advanced

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 Message 3 of 5 in Discussion 
From: MSN Nickname·Steve·Sent: 5/8/2008 7:24 AM
Well, I'm not sure!  The standard reactions involves adding ferric ion in the presence of fluoride ion in order to get the dark-blue color.  Here are the steps:

(1)  Sodium fusion
                                                                                       high temperature
      Organic sample containing C, H, O, N, S, Cl   +   Na    –––––––––––�?gt;    NaCN, NaCl, Na2S, NaOH, etc.

(2)  Reaction of cyanide with ferrous sulfate (alkaline solution)

      2 NaCN (aq)  +  FeSO4 (aq)   ––�?gt;   Fe(CN)2 (aq)  +  Na2SO4 (aq)

      4 NaCN (aq)  +  Fe(CN)2 (aq)    ––�?gt;    Na4[Fe(CN)6] (aq)
                                                              sodium ferrocyanide

(3)  Reaction with ferric ion (in acidic solution to avoid precipitation of Fe(OH)3)

      Na4[Fe(CN)6] (aq)  +  FeCl3 (aq)   ––�?gt;   NaFe[Fe(CN)6] (s)  +  3 NaCl (aq)
                                                                    Prussian blue

To get the dark blue color, you need a mixture of Fe2+ and Fe3+ in the Prussian blue complex commonly formulated as MFe[Fe(CN)6] where M+ is usually Na+ or K+.  Here are the reactions of ferrocyanide ion with Fe2+ and Fe3+ ion:

(a)   2 Fe2+ (aq)   +   Na4[Fe(CN)6] (aq)    ––�?gt;    Fe2[Fe(CN)6] (s)   +   4 Na+ (aq)
                             sodium ferrocyanide           "ferrous ferrocyanide"
                                                                           (blue-white)

(b)   Fe3+ (aq)   +   Na4[Fe(CN)6] (aq)    ––�?gt;    NaFe[Fe(CN)6] (s)   +   3 Na+ (aq)
                           sodium ferrocyanide             "ferric ferrocyanide"
                                                                          (dark blue)

The only mention of the purpose of adding fluoride ion that I could find was in
http://www.sciencemadness.org/library/books/Semimicro_and_Macro_Organic_Chemistry_by_Cheronis_1942.pdf

which says that the fluoride ion "intensifies" the blue color.  It may be that fluoride "ties up" Fe2+ ions more than Fe3+ ions, favoring reaction (b) over reaction (a) above.

Good question!

Steve

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 Message 4 of 5 in Discussion 
From: MSN Nickname·Steve·Sent: 5/8/2008 4:03 PM
Another possibility is that fluoride enhances formation of larger particles of the Prussian blue precipitate by absorption and attraction of more material on the surface of the initially microscopic particles.  The larger the particles, the darker the color.
 
This is supposed to be how fluoride ion prevents tooth decay.  Absorption of fluoride on the surface of tooth enamel fosters "re-mineralization" of the tooth surface.
 
"Second, fluoride promotes the remineralization of enamel. It is not entirely clear how this phenomenon occurs. It has been shown, however, that fluoride increases the concentration of calcium in plaque fluid. This retards enamel demineralization through the common ion effect but also promotes the formation of calcium-phosphate-fluoride salts which precipitate onto the enamel surface."  http://www.accessscience.com/studycenter.aspx?main=13&questionID=4858
 

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 Message 5 of 5 in Discussion 
From: MSN Nickname·Steve·Sent: 5/14/2008 11:15 PM
BTW, a good discussion of the precipitation process of ionic compounds is at http://www.csudh.edu/oliver/che230/textbook/Grav03.htm.  The role of the electric double layer in precipitation is covered.  The process is covered a little more briefly at http://www.newi.ac.uk/buckleyc/gravi.htm.
 


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