ISO 925:2019

INTERNATIONAL ISO
STANDARD 925
Fourth edition
2019-07
Solid mineral fuels — Determination
of carbonate carbon content —
Gravimetric method
Combustibles minéraux solides — Dosage du carbone sous forme de
carbonate — Méthode gravimétrique
Reference number
©
ISO 2019
© ISO 2019
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ii © ISO 2019 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents . 1
6 Apparatus . 2
7 Preparation of the test sample . 3
8 Procedure. 4
8.1 Check test . 4
8.2 Determination . 5
8.2.1 Preparation . 5
8.2.2 Conditioning . 5
8.2.3 Reaction and completion . 5
9 Expression of results . 5
10 Precision . 6
10.1 Repeatability limit . 6
10.2 Reproducibility critical difference . 6
11 Test report . 7
Annex A (informative) Derivation of factors used in calculations in this document .8
Foreword
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This document was prepared by Technical Committee ISO/TC 27, Solid mineral fuels, Subcommittee
SC 5, Methods of analysis.
This fourth edition cancels and replaces the third edition (ISO 925:1997), of which it constitutes a minor
revision. The changes compared to the previous edition are as follows:
— the normative references have been updated and the dates removed;
— the references in Clause 7 have been updated.
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iv © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 925:2019(E)
Solid mineral fuels — Determination of carbonate carbon
content — Gravimetric method
1 Scope
This document specifies a gravimetric method of determining the carbon in the mineral carbonates
associated with solid mineral fuels.
NOTE The result obtained will include any carbon from atmospheric carbon dioxide absorbed by the fuel.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 687, Solid mineral fuels — Coke — Determination of moisture in the general analysis test sample
ISO 1170, Coal and coke — Calculation of analyses to different bases
ISO 5068-2, Brown coals and lignites — Determination of moisture content — Part 2: Indirect gravimetric
method for moisture in the analysis sample
ISO 11722, Solid mineral fuels — Hard coal — Determination of moisture in the general analysis test
sample by drying in nitrogen
ISO 13909-4, Hard coal and coke — Mechanical sampling — Part 4: Coal — Preparation of test samples
ISO 13909-6, Hard coal and coke — Mechanical sampling — Part 6: Coke — Preparation of test samples
ISO 18283, Hard coal and coke — Manual sampling
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
4 Principle
A known mass of sample is treated with hydrochloric acid, which reacts with the carbonates present
to liberate carbon dioxide. The carbon dioxide resulting from the decomposition of the carbonates is
absorbed and weighed.
5 Reagents
WARNING — Care should be exercised when handling reagents, many of which are toxic and
corrosive.
During the analysis, unless otherwise stated, use only reagents of recognized analytical grade and only
distilled water, or water of equivalent purity.
NOTE Distilled water can be freed from carbon dioxide by boiling gently for 15 min.
5.1 Hydrochloric acid, approximately 3 mol/l.
5.2 Hydrogen sulfide absorbent, any of the following:
a) copper(II) phosphate, granular, particle size 1,2 mm to 0,7 mm;
NOTE 1 Copper(II) phosphate granules can be prepared as follows.
Mix copper(II) phosphate powdered reagent to a stiff paste with 1 % starch solution. Press through a sheet
of metal, perforated with apertures of approximately 1 mm diameter. Dry the extruded material at 110 °C.
Sieve to recover the desired size fraction.
b) copper(II) sulfate, deposited on a supporting base of ground pumice;
NOTE 2 A suitable absorbent, based on copper(II) sulfate, can be prepared as follows.
Prepare pumice by crushing and sieving to obtain the 2,8 mm to 0,7 mm fraction. Transfer approximately
60 g of the prepared pumice to an evaporating basin, covering with a saturated solution of copper(II) sulfate,
evaporate to dryness with constant stirring, and heat at 150 °C to 160 °C for 3 h to 4 h. Cool in a desiccator
and store in a glass-stoppered bottle.
c) silver sulfate, granular.
5.3 Magnesium perchlorate, anhydrous, particle size 1,2 mm to 0,7 mm.
WARNING — Due regard shall be taken of local regulations when disposing of exhausted
magnesium perchlorate. It is essential that regeneration of magnesium perchlorate is not
attempted, owing to the risk of explosion.
5.4 Sodium hydroxide, on an inert base, preferably of coarse grading, for example 1,7 mm to 1,2 mm,
and preferably of the self-indicating type.
5.5 Wetting agent, suitable for use in acid solution.
NOTE A liquid wetting agent at a concentration of 100 ml/l or ethanol (a volume fraction of 95 %) are
suitable.
5.6 Check test reagent, either of the following:
a) anhydrous sodium carbonate;
b) anhydrous calcium carbonate.
6 Apparatus
6.1 Analytical balance, capable of weighing to the nearest 0,1 mg.
6.2 Graduated glassware, conforming to the requirements for Grade A in the International Standards
prepared by ISO/TC 48, Laboratory glassware and related apparatus.
6.3 Purification tube, consisting of an absorption tube containing sodium hydroxide on an inert base
(5.4). Absorption tubes may be U-tubes or Midvale tubes (which reduce back-pressure and, hence, risk of
2 © ISO 2019 – All rights reserved

leakage). The tops of the reagent columns should be covered with a layer of glass wool to guard against
entrainment of any fine particles in the circulating air.
6.4 Reaction flask assembly, comprising a 300 ml round-bottomed flask fitted with a tap funnel, a
double-surface condenser and a bulbed tube.
6.5 Absorption train, consisting of three absorption tubes a), b) and c) packed, respectively, as follows.
a) Magnesium perchlorate (5.3) to dry the gas.
b) Hydrogen sulfide absorbent (5.2) followed by a protective layer of magnesium perchlorate (5.3).
The connection from this tube to tube c) shall be fitted at its outlet end with a stopcock or other
means of closure.
c) Sodium hydroxide on an inert base (5.4), to absorb carbon dioxide generated in the reaction flask,
followed by a protective layer of magnesium perchlorate (5.3) to absorb water produced in the
reaction between carbon dioxide and sodium hydroxide. This tube shall be fitted with stopcocks or
other means of closure at the inlet and outlet ends.
6.6 Air circulation equipment, a suction pump, capable of drawing air at a rate of 50 ml/min through
the apparatus, connected through a flowmeter to a tee-piece fitted with a stopcock (the air vent).
6.7 Heating source, for the reaction flask. Either an electric heating mantle to accommodate a 300 ml
flask or a small gas burner. An example of a suitable assembly of the apparatus is illustrated in Figure 1.
7 Preparation of the test sample
The test sample shall be the analysis sample, prepared to a nominal top size of 212 μm. Sample
preparation procedures shall be in accordance with ISO 13909-4, ISO 13909-6 or ISO 18283, as
appropriate. The moisture content of the sample shall be equilibrated with the laboratory atmosphere
by exposure in a thin layer on a tray. Exposure time shall be kept to a minimum.
Before commencing the determination, thoroughly mix the equilibrated test sample for at least one
minute, preferably by mechanical means.
If the results are to be calculated other than on an “air-dried” basis (see Clause 9), then, after weighing
the test portion (see 8.2), determine the moisture content using a further portion of the test sample by
the method described in ISO 687, ISO 5068-2 or ISO 11722, a
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