Qualitative analysis / by William Briggs and R.W. Stewart.

  • William Briggs
Date:
1913
Catalogue details

Licence: Public Domain Mark

Credit: Qualitative analysis / by William Briggs and R.W. Stewart. Source: Wellcome Collection.

    135/180 (page 119)
    NOTES ON THE SEPARATION OF THE METALS OF GROUP VI. The detection and separation of these metals is based on— (a) The decomposition of ammonium salts and volati¬ lization of ammonia by heating them in presence of caustic alkalies: NH4C1 + NaOH = NaCl + NH3 + H20. (b) The insolubility of magnesium ammonium phos¬ phate (MgNH4P04) in a neutral or alkaline solution : MgCl2 + NH4OH + Na2HP04 = MgNH4P04 + 2NaCl + H20 As the precipitate is only formed very slowly in dilute solutions, the filtrate from Group V. should always be concentrated before adding NH4OH and Na2HP04. Even then the precipitate only forms slowly unless the solution is very strong, and its formation should be promoted by warming gently and stirring. Neglect of these precautions often results in magnesium being missed altogether. Note.—Oa, Sr and Ba /often escape precipitation by (NH4)2C03, and are precipitated by Na2HP04, being mis¬ taken for Mg: Whereas however, MgNH4P04 always settles completely on standing, leaving the supernatant liquid quite clear, this is not the case when the precipitate is composed of phosphate of Ca, Sr or Ba. Note.—There is no really satisfactory confirmatory test for magnesium. (e) The separation of magnesium before testing for sodium and potassium. On adding baryta-water magnesium hydrate, Mg(OH)2, is precipitated : MgCl2 + Ba(OH)2 = Mg(OH)2 + BaCl2.