INTERNATIONAL ISO

STANDARD 351
Third edition
1996-02-15

Solid mineral fuels - Determination of

total Sulfur - High temperature
combustion method
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Combustibles mineraux solides - Dosage du soufre
par combustion 2 haute temperature
total - Methode

ISO 351:1996
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cb124182a6f9/iso-351-1996

Reference number
ISO 351:1996(E)
ISO 351:1996(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide
federation of national Standards bodies (ISO member bodies). The work
of preparing International Standards is normally carried out through ISO
technical committees. Esch member body interested in a subject for
which a technical committee has been established has the right to be
represented on that committee. International organizations, governmental
and non-governmental, in liaison with ISO, also take part in the work. ISO
collaborates closely with the International Electrotechnical Commission
(1EC) on all matters of electrotechnical standardization.

Draft International Standards adopted by the technical committees are

circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting
a vote. iTeh STANDARD PREVIEW
International Standard ISO (standards.iteh.ai)
351 was prepared by Technical Committee
lSO/TC 27, Solid mineral fuels, Subcommittee SC 5, Methods of
analysis. ISO 351:1996
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This third edition cancels and replaces cb124182a6f9/iso-351-1996
the second edition
(ISO 351 :1984) 1 which has been technically revised.

0 ISO 1996
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced
or utilized in any form or by any means, electronie or mechanical, including photocopying and
microfilm, without Permission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-1 211 Geneve 20 l Switzerland
Printed in Switzerland
 0 ISO ISO 351:1996(E)

lntroduction
An alternative method to that specified in this International Standard is
given in ISO 334:1992, Solid mineral fuels - Determination o f total Sulfur
- Eschka method.

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ISO 351:1996
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ISO 351:1996
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INTERNATIONAL STANDARD 0 ISO ISO 351:1996(E)

Solid mineral fuels - Determination of total Sulfur -

High temperature combustion method

1 Scope ISO 2309:1980, Coke - Sampling.

This International Standard specifies a method of de- ISO 5068: 1983, Brown coals and lignites - Determi-
termining the total Sulfur content of hard coal, brown nation of moisture content - Indirect gravimetric
coal and lignite, and coke by high temperature com- me thod.
bustion.
iTeh STANDARDciplesPREVIEW
ISO 5069-2:1983, Brown coals and lignites - Prin-
of sampling - Part 2: Sample preparation for
(standards.iteh.ai)
determina tion of moisture con ten t and for general
analysis.
2 Normative references
ISO 351:1996
The following https://standards.iteh.ai/catalog/standards/sist/25ba2256-c4bc-4c29-8604-
Standards contain provisions which, 3 Principle
through reference in this text, constitute cb124182a6f9/iso-351-1996
provisions
of this International Standard. At the time of publi- A known mass of a Sample of coal or coke is burnt in
cation, the editions indicated were valid. All Standards a stream of Oxygen, in a tube furnace at a tempera-
are subject to revision, and Parties to agreements ture of 1 350 “C. The acid gases formed (chlorine and
based on this International Standard are encouraged oxides of Sulfur) are absorbed in hydrogen peroxide
to investigate the possibility of applying the most re- and subsequently determined titrimetrically. A cor-
cent editions of the Standards indicated below. rection is made to take account of any chlorine liber-
Members of IEC and ISO maintain registers of cur- ated and a suitable combustion additive prevents the
rently valid International Standards. retention of Sulfur in the ash.

ISO 3313983, Coal - Determination of moisture in

the analysis Sample - Direct gravimetric method. 4 Reactions

ISO 587:1981, Solid mineral fuels - Determination 4.1 SO, + H,O, -+ H,SO,
of chlorine using Eschka mixture.
4.2 Cl, + H,O, --) 2HCI + 0,
ISO 687:1974, Coke - Determination of mois ture in
the analysis Sample.
4.3 Na,B,O, + 2HCI + 5H,O -) 2NaCI + 4H,BO,
ISO 1015: 1992, Brown coals and lignites - Determi-
nation of moisture content - Direct volumetric 4.4 Na,B,O, + H,SO, + 5H,O + Na,SO, +
me thod. 4H,BO,

ISO 1170:1977, Coal and coke - Calculation of

4.5 NaCI + Hg(OH)CN -+ NaOH + HgCICN
analyses to different bases.

ISO 1988:1975, Hard coal - Sampling. 4.6 2NaOH + H,SO, --) Na,SO, + 2H,O

1
ISO 351:1996(E) 0 ISO

5 Reagents 5.6.3 Mixed solution

WARNING - Care should be exercised when Mix equal volumes of Solution A and Solution B. Store
handling reagents, many of which are toxic and in a dark bottle. Discard the mixed Solution after
1 week.
corrosive.

During the analysis, unless otherwise stated, use only 5.7 Oxygen, laboratory grade, of purity at least
reagents of recognized analytical grade and only dis- 99,6 % (ndm).
tilled water or water of equivalent purity.
5.8 Sodium hydroxide on an inert base, preferably
of a coarse grading, for example 1,2 mm to 1,7 mm,
5.1 Aluminium Oxide (alumina), finely divided.
and preferably of the self-indicating type.

5.2 Hydrogen Peroxide, approximately 3 % (m/m)

6 Apparatus
neutral Solution.

Dilute 100 ml of 30 % (m/m) hydrogen peroxide sol- 6.1 Analytical balance, capable of weighing to the
ution to 1 litre with water and neutralize with the nearest 0,l mg.
sodium tetraborate Solution (5.4) using the mixed in-
dicator (5.6). 6.2 Graduated glassware, conforming to the re-
quirements for class A in the International Standards
5.3 Mercury(ll) oxycyanide, saturated Solution at prepared by ISO/TC 48, Laboratory glassware and re-
Ia ted appara tus.
20 “C, approximately 45 g/l.

Saturate a suitable volume of water with mercury(ll) 6.3 Furnace, capable of heating a tube, of approxi-
iTeh STANDARDmately PREVIEW
oxycyanide by prolonged agitation; filter and neutralize 28 mm external diameter over a length of
125 mm to 150 mm, to a temperature of 1 350 “C.
the filtrate with the sulfuric acid (5.5) using
(standards.iteh.ai)
bromothymol blue as an external indicator. Store in a
The furnace may be heated electrically, using either
dark glass bottle; do not keep for more than 4 d.
Silicon carbide resistance rods (controlled by a variable
ISO 351:1996
transformer) or a resistance wire (controlled by a
5.4 Sodium tetraborate, https://standards.iteh.ai/catalog/standards/sist/25ba2256-c4bc-4c29-8604-
Standard volumetric sol- variable resistance).
ution, c(Na,B,O,) = 0,025 mol/l. cb124182a6f9/iso-351-1996

Dissolve 9,534 2 g of sodium tetraborate decahydrate 6.4 Combustion tube, of approximately 28 mm ex-
in water and dilute to 1 Iitre. Mix thoroughly. ternal diameter, 3 mm wall thickness and 650 mm
length, made of aluminous porcelain which is
impervious to gases up to the working temperature
5.5 Sulfuric acid, Standard volumetric Solution, (see figure 1).
c(H*SO,) = 0,012 5 mol/l.
A straight tube is most convenient and may be used
5.6 Mixed indicator solution. in conjunction with an adapter of fused silica having
a hell-shaped end, which gives a narrow clearance
5.6.1 Solution A from the inner wall of the heated tube, and a heat-
resistant stopper (acrylonitrile or chloroprene is suit-
Dissolve 0,125 g of 2-(4-dimethylaminophenyl- able). Alternatively, the tube may have, a beak end,
azo)benzoic acid, sodium salt (methyl red) in 100 ml at the exit, with a tubulure to enable condensation
of water. products to be washed out after a test; or a straight
tube of aluminous porcelain may be used in conjunc-
5.6.2 Solution B tion with a borosilicate glass adapter, having a cap-
shaped end which fits onto the outer wall of the tube.
Dissolve 0,083 g of 3,7-bis(dimethylamino)pheno- In this case it will be necessary to lag the end of the
thiazine-5-ylium chloride (methylene blue) in 100 ml combustion tube with a suitable heat-resistant mineral
of water. Store in a dark bottle. fibre to prevent condensation in the tube.
 ISO 351:1996(E)

Lt
to 2

tü
a0
+-
IJ-l
c
c
m
+-
.-IJ3

c
L
I
c
L z
+aJ x
CL

2i
m
m
.-u
-:
z

:_______

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ISO 351:1996
https://standards.iteh.ai/catalog/standards/sist/25ba2256-c4bc-4c29-8604-
cb124182a6f9/iso-351-1996

z
3
t
c
.-0
+
z
+-
Eh
13
E
x
0
L
u
f
2
1 a

.-E
:

Figure 1 - Arrangement of furnace and absorption train

3
ISO 351:1996(El) 0 ISO

6.5 Oxygen cylinder, fitted with a needle valve to through a pressure regulator adjusted to maintain
control the rate of flow of Oxygen (5.7), and a slight suction at the inlet end of the combustion tube.
flowmeter to measure up to 300 ml/min. The Oxygen
should, as a precautionary measure, be passed A convenient assembly of the apparatus is illustrated
through a U-tube packed with sodium hydroxide on in figure 1.
an inert base (5.8).
7 Preparation of test Sample
6.6 Combustion boat, of iron-free, unglazed
Prepare the test Sample for general analysis in ac-
porcelain, of minimum dimensions 45 mm long,
cordante with ISO 1988, ISO 2309 or ISO 5069-2, as
12,5 mm wide and 10 mm deep, for hard coal and
appropriate. Ensure that the moisture content of the
coke samples, and approximately 75 mm long,
Sample is in equilibrium with the laboratory atmos-
15 mm wide and 10 mm deep for brown coal and
phere, exposing it, if necessary, in a thin layer for the
lignite.
minimum time required to achieve equilibrium.

6.7 Heat-resistant wire, approximately 2 mm thick Before commencing the determination, thoroughly
and 450 mm long, with a bent end to transfer the mix the test Sample for at least 1 min, preferably by
used boat from the combustion tube onto a rigid mechanical means.
ref ractory s heet.
If the results are to be calculated other than on an
“air-dried” basis (see clause IO), after weighing the
6.8 Silica or steel Pusher, made from rod or sealed test Portion (see clause 8), determine the moisture
tube, approximately 6 mm in diameter and 450 mm content using a further Portion of the test Sample by
long, with a disc end 12 mm in diameter, for pushing the method described in ISO 331, ISO 687, ISO 1015
the boat into the hot zone of the combustion tube. or ISO 5068, as appropriate. A further Portion should
iTeh STANDARD PREVIEW
The Pusher Passes loosely through a glass or metal
T-piece, one end of which fits into the rubber stopper
be set aside if the determination
Eschka method (ISO 587) is required.
of chlorine by the

(standards.iteh.ai)
which closes the inlet end of the combustion tube,
the other being sealed by a rubber sleeve (see 8 Procedure
note 1) through which the Pusher slides. The sleeve ISO 351:1996
prevents the escape of Oxygen, which enters at the
https://standards.iteh.ai/catalog/standards/sist/25ba2256-c4bc-4c29-8604-
Raise the temperature of the furnace (6.3) to
stem of the T-piece. The Pusher is usually marked cb124182a6f9/iso-351-1996
1 350 “C. Weigh, to the nearest 0,l mg, about 0,5 g
from the disc end, for convenience in ascertaining the of the test Sample (clause 7) and transfer to a com-
Position of the boat in the combustion tube during bustion boat (6.6), taking care to spread uniformly.
pushing. Cover evenly with about 0,5 g of the aluminium Oxide
(5.1). Measure 100 ml of the hydrogen peroxide sol-
NOTE 1 The rubber sleeve should be changed period-
ution (5.2) and either divide this amount between the
ically to avoid leakage.
two absorbers (6.9) or pour it all into the Single ab-
sorber.
6.9 Two absorbers, of capacity about 150 ml,
which may be large boiling tubes, wide-necked Adjust the suction-pump and pressure regulator (see
bottles or Dresche1 bottles, each containing a sintered 6.9) so that a rapid stream of air is drawn at constant
glass disc of 0,015 mm to 0,040 mm maximum pore pressure through the absorber(s). Insert the silica
size in the gas distribution tube. The diameter of the adapter into the combustion tube (6.4) and secure the
absorber shall be such that the disc is covered to a stopper. Adjust the Oxygen (5.7) flow to 300 ml/min.
depth of at least 25 mm by the absorbing Solution.
Connect the silica adapter, or the reaction tube fitted Insert the charged boat from the inlet end of the
combustion tube so that its centre is 240 mm from
with a tubulure, to the first absorber. Connect this in
the centre of the hottest zone and secure the stopper
series with the second absorber.
carrying the Pusher (6.8) and the Oxygen inlet. After
Alternatively, use a Single narrow absorber about 1 min, move the boat forward 40 mm. At the end of
35 mm in diameter and 150 mm deep, so that the each of the next five 1 min periods, push the boat
bubbler disc (0,015 mm to 0,040 mm maximum pore forward about 40 mm, pulling back the Pusher each
size) is covered to a depth of at least 90 mm. time to prevent distortion. After the last push, the
boat should be at the centre of the hottest zone (see
To avoid leakage at the rubber sleeve of the inlet end note 2). Allow the boat to remain in the hottest zone
due to the resistance of the sintered glass bubbler, for a further 4 min. Disconnect the absorber(s) and,
connect the second absorber to a suction-pump using the bent heat-resistant wire (6.7), withdraw the

4
0 ISO ISO 351:1996(E)

used com bustion boat onto a thick sheet of suitable 10 Expression of results
refractory materia
The total Sulfur content, ws, of the Sample as ana-
Wash the adapter, collecting the washings in the
lysed, expressed as a percentage by mass, is given
Single absorber or in the first of the two absorbers.
by the equation in either 10.1 or 10.2, as appropriate.
Transfer the contents of the absorber(s) into a
250 ml conical filtration beaker, wash out the
absorber(s) and collect the washings in the same fil- Report the result, as the mean of duplicate determi-
tration beaker. Add 2 or 3 drops of the mixed indicator nations, to the nearest 0,Ol % (m/m).
Solution (5.6) and titrate with the sodium tetraborate
Solution (5.4) to a neutral steel-grey colour. Note the The results of the determination described in this
titration volume. This procedure gives the total acidity International Standard are reported on the “air-dried”
due to oxides of Sulfur and chlorine (see the equations basis. Calculation of results to other bases is dealt
in 4.1 to 4.4). Determine the correction for chlorine with in ISO 1170.
as described in either 8.1 or 8.2.

For certain coals which liberate volatile matter at a

fast rate, the early stages of heating may give a
carry-over of unburnt carbon particles. For such coals,
and for certain Cokes of very low volatile matter (for 10.1 Calculation using the mercury(ll)
example foundry Cokes), the rate of pushing should oxycyanide method for chlorine correction
be reduced according to the following procedure.

Insert the charged boat so that its centre is about 3206Cq (v, - v*) - c*(v3 - KJ1
W C= m
240 m m from the centre of the hottest Zone. After
iTeh STANDARDwhere
1 min, move the boat forward 20 mm. At the end ofPREVIEW
each of the next eleven 1 min periods, move the boat
(standards.iteh.ai)
forward about 20 mm. Allow the boat to remain in the
hottest zone for a further 4 min. m is the mass, in grams, of the test
ISO 351:1996 Portion;
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8.1 Chlorine correction - Mercury( cb124182a6f9/iso-351-1996
li) Vl is the volume, in millilitres, of the
oxycyanide method sodium tetraborate Solution used in the
determination;
After titration with the sodium tetraborate Solution
(5.4), the chloride ion is present as sodium chloride. V2 is the volume, in millilitres, of the
Add 20 ml of the mercury(ll) oxycyanide Solution sodium tetraborate Solution used in the
(5.3) to convert the sodium chloride to sodium hy- blank test;
droxide (see note 2). Titrate the liberated alkali with
the sulfuric acid (5.5) (see the equations in 4.5 and V3 is the volume, in millilitres, of the
4.6). Note the titration volume. sulfuric acid Solution used in the deter-
mination;
NOTE 2 For 0,5 g test portions of samples containing up
to 1,2 % chlorine, 20 ml of mercury(ll) oxycyanide Solution is the volume, in millilitres, of the
v4
is sufficient excess. sulfuric acid Solution used in the blank
test;

8.2 Chlorine correction - Eschka method is the concentration, expressed in

Cl
moles per litre, of the sodium
Determine the chlorine content of the test Sample,
tetraborate Solution;
using a further test portion, according to the pro-
cedure described in ISO 587. is the concentration, expressed in
C2
moles per litre, of the sulfuric acid sol-
ution;
9 Blank test
0,032 06 is the mass, in grams of Sulfur, equiv-
Carry out a blank test using the same procedure as alent to 1 ml of 1 molar sodium
described in clause 8, but omitting the test Portion. tetraborate Solution.