aqueous solutions between pH = 4.2 and pH = 9.8. Due to the non-selective nature of Oxine as a reagent
many other metals are also precipitated within the same range. To achieve a separation of aluminium
from these other elements pH control may be employed. Thus an acetic acid solution, buffered to pH =
5 by sodium acetate, will provide a medium for the separation of aluminium from troublesome
contaminants such as Be, Mg, Ca, Sr and Ba. Ammoniacal solution (pH = 9) will enable a separation
from As, B, F and P to be made, and if used in conjunction with H 2 O 2 from Cr, Mo, Nb, Ta and V as
well. Selectivity is further enhanced by the use of masking agents such as cyanide, tartrate or EDTA.
By careful control of these parameters aluminium may be separated, and few interferences are observed
if the precipitation is carried out from an ammoniacal-cyanide-EDTA solution. When large amounts of
Ca or Mg are present the homogeneous precipitation procedure (Table 5.18) is usefully employed at pH
= 5. The precipitate is readily filtered and may be weighed after drying at 150°C as the anhydrous
compound.
Applications of Gravimetry
Gravimetric methods provide precise and accurate results and have found a wide utility in chemical
analysis for many years. They are best suited to the determination of major constituents in samples
because of the practical limitations in accurately weighing quantities of less than 0.1 g. The analysis of
rocks, ores, soils, metallurgical and other inorganic samples for their major components has depended
very much on gravimetric methods. Gravimetric procedures are, however, usually time consuming and
demanding, with the result that there is a steady trend towards the use of quicker, instrumentally based
methods. Notable for its impact in recent years on these traditional areas has been X-ray fluorescence
analysis (Chapter 8). Nevertheless gravimetric methods are still very much needed to calibrate these
newer procedures. Table 5.14 and 5.15 give a good indication of the scope of gravimetry.
Problems
(1) Suggest suitable colour change indicators for the titration of (a) acetic acid (0.100 M) with sodium
hydroxide (0.100 M) and (b) nitric acid (0.0100 M) with sodium hydroxide (0.0100 M). Give the
reasons for your choice.
(2) Sketch a titration curve (pH vs volume of titrant) for phosphoric acid titrated with sodium
hydroxide. Comment upon the curve shape and the reasons for it.
(3) On pp. 204–5 the problem of titrating Fe(II) in the presence of chloride ions is discussed and the use
of Zimmerman–Rheinhardt reagent suggested. An alternative is to use a different oxidizing agent.
Suggest one, and give the reasons for your choice.
(4) Hydroxylamine (NH 2 OH) is oxidized by ferric iron in boiling sulphuric acid – an oxide of nitrogen
being amongst the products. 25.00 cm^3 of a solution of hydroxylamine (2.00 g dm–^3 ) were boiled with
an excess of ferric chloride in dilute sulphuric acid. 30.30 cm^3 of potassium permanganate solution
(0.0200 M) were required to reoxidize the ferrous ions produced. Deduce the identity of the oxide of
nitrogen.
(5) Use the data in Tables 5.6 and 5.7 to deduce a pH suitable for the quantitative titration of Cu2+ in the
presence of a small amount of Ag+.