Notes on qualitative analysis.

  • Fenton, Henry John Horstman, 1854-1929.
Date:
1906
Catalogue details

Licence: In copyright

Credit: Notes on qualitative analysis. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

    95/226 (page 83)
    of ferric chloride, it proves that the phosphates of Ca, Ba, Sr or Mg were present. The ferric chloride is added in slight excess (indicated by the red colour of ferric acetate), and the excess of iron is then removed by boiling the solution; ferric acetate then undergoes partial hydrolysis, giving an insoluble basic salt xFelCIlsCO^)^ yYefi^. zK^O, e.g.:— Fe(CH3C02)3 + 2H,0 = Fe(OH)2(CHsC02) + 2CH3COOH. (Ferric phosphate is somewhat soluble in solutions of ferric chloride or acetate; hence only a slight excess of ferric chloride is added, and all the excess of iron is then precipi- tated as basic acetate.) By the above process the phosphoric acid is entirely eliminated, and the calcium, barium, strontium and magnesium, which were originally present as phosphates, now remain in solution as soluble salts, which can be analysed in the usual way, i.e. by precipitation in their proper groups (V. and VI.). Many of the changes involved in the operations here described can be more satis- factorily explained in terms of the ionisation hypothesis; the older views have been given however, for the sake of comparison. Thus the action of sodium acetate on hydrochloric acid may, as far as the essential change is concerned, be represented as H' + CHsCOg' ^ CH3CO2H, the sodium and chlorine ions taking practically no part. The concentration of the hydrogen ions (from the hydrochloric acid) is consequently greatly diminished, i.e. the solution becomes much less strongly acid ; if excess of sodium acetate is employed the hydrogen ions are still further 'suppressed,' as explained on page 65. 'Insoluble' salts of weak acids dissolve in strong acids owing, as previously stated, to the tendency to foim the less ionised weak acid* at the expense of the hydrogen ions of the strong acid. The concentration of the anions of the weak acid will therefore be smaller as that of the hydrogen ions is greater. The solubility in acids of the phosphates, oxalates, &c. with which we are here concerned, will depend on the degree of ionisation of the free acids, and on the solubilities of the particular salts in water—other things being supposed equal. The phosphates of Fe, Cr, Al are less soluble in water than those of Ca, Ba, Sr, Mg, and a smaller concentration of the anions PO4' is therefore required in order that the solubility product of the former may be reached; the concentration of H * which remains after addition of sodium acetate is sufficient to reduce that of PO4' below the critical value in case of the latter, but is insufficient in case of the former. Similar explanations apply to the oxalates and fluorides. * Or its more stable ions, e.g. ^O^m + H- HPO4/'.