ISO/TR 4277:2009

TECHNICAL ISO/TR
REPORT 4277
First edition
2009-11-01
Cryolite, natural and artificial —
Conventional test for evaluation of free
fluorides content
Cryolithe, naturelle et artificielle — Essai conventionnel pour l'évaluation
de la teneur en fluorures libres

Reference number
©
ISO 2009
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Foreword
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(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
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In exceptional circumstances, when a technical committee has collected data of a different kind from that
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ISO/TR 4277 was prepared by Technical Committee ISO/TC 226, Materials for the production of primary
aluminium.
This first edition cancels and replaces ISO 4277:1977, of which it constitutes a minor revision.
Introduction
This Technical Report was published in order to retain the method specified in ISO 4277:1977 in a publicly
available standard.
ISO 4277:1977 was withdrawn in 2004.

iv © ISO 2009 – All rights reserved

TECHNICAL REPORT ISO/TR 4277:2009(E)

Cryolite, natural and artificial — Conventional test for
evaluation of free fluorides content
1 Scope
This Technical Report describes a conventional test for the evaluation of the free fluorides content of natural,
artificial and recovered cryolite.
This method is applicable to products having free fluorides content greater than 0,15 % (mass fraction) of AIF
or 0,4 % (mass fraction) of NaF.
2 Principle
A test portion is sintered with a known quantity of sodium fluoride at 790 ± 20 °C for 20 minutes. Under these
conditions, aluminium fluoride in excess of that required for the stoichiometric formula AlF ·3NaF reacts with
some of the sodium fluoride to form cryolite.
The ground sintered mass is extracted with boiling water and the solution is acidified with hydrochloric acid
solution to a pH less than 3,7, followed by titration of the excess sodium fluoride with standard volumetric
thorium nitrate solution in the presence of alizarin-S as indicator.
3 Reagents
During the analysis, use only reagents of recognized analytical grade and only distilled water or water of
equivalent purity.
3.1 Sodium fluoride, anhydrous, dried at about 120 °C to constant mass in a platinum crucible and cooled
in a desiccator.
3.2 Gelatine, 3 % freshly prepared solution.
3.3 Hydrochloric acid, approximately 0,1 N solution.
3.4 Sodium fluoride, 4,20 g/l standard solution (corresponding to 0,1 N).
Weigh, to the nearest 0,001 g, 4,20 g of the sodium fluoride (3.1). Transfer quantitatively to a 1 000 ml one-
mark volumetric flask containing a little water and, after dissolution, dilute to the mark and mix. Transfer the
solution to a suitable plastics bottle.
1 ml of this solution contains 4,20 mg of NaF.
3.5 Thorium nitrate, 0,1 N standard volumetric solution.
Weigh, to the nearest 0,001 g, 13,805 g of thorium nitrate tetrahydrate [Th(NO ) ·4H O], transfer
3 4 2
quantitatively to a 1 000 ml one-mark volumetric flask, dissolve in water, dilute to the mark and mix.
3.6 Buffer solution, pH 2,7.
Neutralize a 9,5 % (mass fraction) solution of monochloroacetic acid (CH CI-COOH) with an approximately
10 N sodium hydroxide solution, in the presence of phenolphthalein. Add 50 ml of this solution to 50 ml of the
same monochloroacetic acid solution and mix.
3.7 Alizarin-S (sodium alizarinsulfonate), 0,5 g/l solution.
4 Apparatus
Ordinary laboratory apparatus and in particular the following.
4.1 Platinum crucible, diameter approximately 40 mm, height approximately 30 mm.
4.2 Electric furnace, capable of being controlled at 790 ± 20 °C.
5 Procedure
5.1 Test portion
Weigh, to the nearest 0,001 g, 4 g of the dried sample (see ISO 1619:1976, 3.3).
5.2 Preparation of the calibration graph
5.2.1 Preparation of the standard matching solutions
Into a series of seven 100 ml conical plastics flasks, place the volumes of the standard sodium fluoride
solution (3.4) shown in the following table.
Table 1 — Volumes of standard sodium fluoride solution
Standard sodium fluoride
Corresponding mass of NaF
solution (3.4)
ml mg
0 0
1,0 4,20
3,0 12,60
5,0 21,00
7,0 29,40
9,0 37,80
10,0 42,00
5.2.2 Titration
Add to each conical flask 40 ml of water, 5 ml of the buffer solution (3.6), 1 ml of the alizarin-S solution (3.7)
and 10 ml of the gelatine solution (3.2). Titrate with the standard volumetric thorium nitrate solution (3.5) until
the colour of the indicator changes to pink.
2 © ISO 2009 – All rights reserved
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