Magnesium (II), Sodium (I), Potassium (I), Ammonium (I)

Transcription

1 s (The Soluble Group) Magnesium (II), Sodium (I), Potassium (I), Ammonium (I) The salts of the cations of group V, with few exceptions, are quite soluble, this accounts for the fact that there is no common precipitating reagent for the group. - This group is called "Soluble Group" because its cations remain soluble throughout the whole scheme of separation. They are not precipitated as chlorides, as sulphides in acid medium, as hydroxides, as sulphides in alkaline medium or as carbonates under the conditions for precipitation of the preceding groups of cations. 146Page

2 - Ammonium is included in this group since its compounds resemble those of the alkali metals particularly those of potassium because the ionic radii of these two ions are almost identical. - Magnesium, although present in the same periodic group as the alkaline earth elements, is incorporated in group V because its carbonate is not precipitated by ammonium carbonate solution in the presence of ammonium chloride. Also, its chromate and sulphate are highly soluble, therefore, the same reagents cannot be used for the precipitation and separation of magnesium as for the alkaline earth metal ions. Separation of group V cations - The test for the ammonium ion is always carried out on the original solution (the first test to be done in the scheme) because ammonium compounds are introduced throughout the procedure of analysis, so the solution remaining after group IV analysis cannot be used for the ammonium test. - Ammonium salts interfere with the tests for potassium, so they must be removed. Boiling with concentrated nitric acid and evaporation to dryness destroys ammonium salts. 147Page

3 NH NO3 - N 2 O + 2H 2 O Nitric acid is used to form ammonium nitrate which decomposes at a low temperature giving N 2 O. - After removing ammonium ion from the mixture, the solution is divided into 3 portions to test for Na +, K + and Mg 2+. I- Magnesium, Mg The atom of magnesium has two valence electrons which are easily lost to form a cation with an oxidation state of +2. Magnesium displays similar reactions to those of group IV cations, however, magnesium carbonate, in the presence of ammonium salts is soluble and therefore magnesium does not precipitate with group IV. Reactions of magnesium (II) ions, Mg Ammonia and sodium hydroxide solutions: Alkali hydroxides and ammonium hydroxide precipitate white gelatinous magnesium hydroxide 148Page

4 Mg OH - Mg(OH) 2 Mg NH 3 + 2H 2 O Mg(OH) 2 + 2NH 4 + The precipitate is not soluble in an excess of any of these reagents because it is not amphoteric and does not form complex ammines. In the presence of ammonium salts, no precipitation occurs (by common ion effect). 2- Ammonium carbonate solution: In the absence of ammonium salts, a white precipitate of basic magnesium carbonate is formed 5Mg CO H 2 O 4MgCO 3. Mg(OH) 2. H 2 O + 2HCO 3 - In the presence of ammonium salts no precipitation occurs because the equilibrium : NH CO 3 2- NH 3 + HCO 3 - is shifted towards the formation of hydrogen carbonate ions (by common ion effect) and the concentration of carbonate ion necessary to produce a precipitate is not attained. 149Page

5 3- Sodium carbonate solution: A white precipitate of basic magnesium carbonate soluble in acids and in solutions of ammonium salts is formed. 4- Disodium hydrogen phosphate solution: Magnesium phosphate precipitates in a variety of forms depending on the conditions of precipitation: a) A white precipitate of magnesium hydrogen phosphate is produced in neutral solutions. Mg 2+ + Na 2 HPO 4 MgHPO 4 b) In strongly alkaline solutions Mg 3 (PO 4 ) 2 precipitates c) If the solution contains NH 4 Cl/NH 4 OH a white crystalline precipitate of MgNH 4 PO 4 is formed. Mg 2+ + NH 3 + HPO 4 2- Mg(NH 4 ) PO 4 5- para-nitrobenzene-azoresorcinol (or Magneson I): This test depends on the adsorption of the reagent which is a dyestuff upon Mg(OH) 2 in alkaline solution to produce a blue "lake" (blue precipitate). The formation of this "lake" is specific for Mg 2+ in the presence of other cations of group V. 150Page

6 O 2 N N N OH HO II- Potassium (L. Kalium), K - The atom of potassium has a single valence electron which is easily lost giving a cation with an oxidation number of Potassium salts are usually soluble and form colorless solutions unless the anion is colored. Reactions of potassium (I) ions, K + 1- Sodium cobaltinitrite solution: Na 3 [Co(NO 2 ) 6 ] A yellow precipitate of potassium cobaltinitrite is formed 3K + + Na 3 [Co(NO 2 ) 6 ] K 3 [Co(NO 2 ) 6 ] The precipitate is insoluble in dilute acetic acid. Interfering substances include : 151Page

7 Ammonium salts give a similar precipitate and must be removed before testing for K +. In alkaline solutions a brown or black precipitate of cobaltic hydroxide Co(OH) 3 is obtained. Strong acids, oxidizing and reducing agents destroy the reagent and should be removed before applying the test. 2- Perchloric acid solution:(hclo 4 ) A white precipitate of potassium perchlorate is formed. K + + ClO 4 - KClO 4 salts. This reaction is unaffected by the presence of ammonium 3- Flame test:(dry test) Potassium salts, preferably the chloride, color the non luminous Bunsen flame violet (lilac). The yellow flame produced by small quantities of sodium obscures the violet color, but by viewing the flame through an optical filter consisting of two thicknesses of cobalt blue glass, the yellow sodium rays are absorbed and the reddish violet potassium flame becomes visible. 152Page

8 III- Sodium (L. Natrium), Na Sodium is a silver-white soft metal. It reacts violently with water forming sodium hydroxide and hydrogen 2Na + 2H 2 O 2Na + + 2OH - + H 2 Sodium atom has a single valence electron which is easily lost giving a cation with an oxidation number of +1. The salts of sodium form colorless solutions unless the anion is colored. Almost all sodium salts are soluble in water. Reactions of sodium (I) ions, Na + 1- Potassium antimonate solution: [KSb(OH) 6 ] A white crystalline precipitate of sodium antimonate NaSb(OH) 6 is formed after scratching the walls of the vessel with a glass rod. Na + + KSb(OH) 6 NaSb(OH) 6 153Page

9 2- Flame test: (Dry test) Sodium salts color the non-luminous Bunsen flame yellow. The color is not visible when viewed through two thicknesses of cobalt blue glass. IV- Ammonium ion, NH The ammonium ion carries a single positive charge and most of its salts are soluble in water. - The ammonium ion formed by the addition of a proton to the NH 3 molecule has an ionic radius comparable to that of potassium, that's why its reactions are in general similar to those of potassium. - The ammonium ion differs from the alkali metal ions in that it forms a relatively weak base, NH 4 OH, and that its salts decompose on heating to ammonia and the appropriate acid. NH 4 Cl NH 3 + HCl NH 4 NO 3 N 2 O + 2H 2 O Ammonium salts can be completely removed from a mixture with other salts by heating for a time at a temperature just below 154Page

10 red heat. This fact is of great analytical importance in that it provides a method for eliminating interference by the ammonium ion in a number of tests. Reactions of ammonium (I), NH Sodium hydroxide solution: Ammonia gas is evolved on warming. NH OH - NH 3 + H 2 O This may be identified by : a) Its odour (pungent odour) b) The formation of white fumes of ammonium chloride when a glass rod moistened with concentrated hydrochloric acid is held in the vapor. c) Its turning moistened red litmus paper blue. d) Its ability to turn a strip of filter paper moistened with mercurous nitrate solution black due to the formation of finely divided mercury metal (black precipitate) and mercuric aminonitrate (white precipitate). 2NH 3 + Hg NO 3 - Hg(NH 2 )NO 3 + Hg 0 + NH Page

11 2-Nessler's reagent: [Alkaline solution of potassium tetraiodo- mercurate (II)] Orange-brown precipitate or brown or yellow coloration is produced according to the amount of ammonia or ammonium ions present. The test is an extremely delicate one and will detect traces of ammonia present in drinking water. All metals, except sodium or potassium must be absent NH 3 + 2HgI OH - NH 2. Hg 2 I 3 + 5I - + H 2 O 3- Sodium cobaltinitrite solution : Na 3 [Co(NO 2 ) 6 ] A yellow precipitate of ammonium cobaltinitrite (NH 4 ) 3 [Co(NO 2 ) 6 ], similar to that produced by potassium ions. 4- Perchloric acid or sodium perchlorate solution: No precipitate is formed (distinction from potassium). Reference 1. Vogel. A.l. Macro and Semimicro Qualitative Analysis Longman, fourth edition. London, ' V 2. Abdine H, and Amer MM Qualitative inorganic analysis" The scientific Book Center. Cairo Amer MM "Qualitative inorganic analysis" The scientific Book, Center, Cairo= Gin S. Bajpai DN and Pandey OP Practical Chemistry" SQ Chaud & Company LTD. New Delhi I Page