EN ISO 21068-2:2008

SLOVENSKI STANDARD
01-december-2008
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Chemical analysis of silicon-carbide-containing raw materials and refractory products -
Part 2: Determination of loss on ignition, total carbon, free carbon and silicon carbide,
total and free silica and total and free silicon (ISO 21068-2:2008)
Chemische Analyse von Siliciumcarbid enthaltenden Rohstoffen und feuerfesten
Erzeugnissen - Teil 2: Bestimmung des Glühverlustes und Gehaltes an
Gesamtkohlenstoff, freiem Kohlenstoff und Siliciumcarbid, des Gehaltes an gesamtem
und freiem Silicium(IV)-oxid sowie an gesamtem und freiem Silicium (ISO 21068-2:2008)
Analyse chimique des matières premières et des produits réfractaires contenant du
carbure de silicium - Partie 2: Détermination de la perte au feu, du carbone total, du
carbone libre et du carbure de silicium, de la silice totale et libre, et du silicium total et
libre (ISO 21068-2:2008)
Ta slovenski standard je istoveten z: EN ISO 21068-2:2008
ICS:
81.080 Ognjevzdržni materiali Refractories
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 21068-2
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2008
ICS 81.080
English Version
Chemical analysis of silicon-carbide-containing raw materials
and refractory products - Part 2: Determination of loss on
ignition, total carbon, free carbon and silicon carbide, total and
free silica and total and free silicon (ISO 21068-2:2008)
Analyse chimique des matières premières et des produits Chemische Analyse von Siliciumcarbid enthaltenden
réfractaires contenant du carbure de silicium - Partie 2: Rohstoffen und feuerfesten Erzeugnissen - Teil 2:
Détermination de la perte au feu, du carbone total, du Bestimmung des Glühverlustes und Gehaltes an
carbone libre et du carbure de silicium, de la silice totale et Gesamtkohlenstoff, freiem Kohlenstoff und Siliciumcarbid,
libre, et du silicium total et libre (ISO 21068-2:2008) des Gehaltes an gesamtem und freiem Silicium(IV)-oxid
sowie an gesamtem und freiem Silicium (ISO 21068-
2:2008)
This European Standard was approved by CEN on 11 July 2008.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the CEN Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as the
official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2008 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 21068-2:2008: E
worldwide for CEN national Members.

Contents Page
Foreword.3

Foreword
This document (EN ISO 21068-2:2008) has been prepared by Technical Committee ISO/TC 33 "Refractories"
in collaboration with Technical Committee CEN/TC 187 “Refractory products and materials” the secretariat of
which is held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an identical
text or by endorsement, at the latest by February 2009, and conflicting national standards shall be withdrawn
at the latest by February 2009.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.
Endorsement notice
The text of ISO 21068-2:2008 has been approved by CEN as a EN ISO 21068-2:2008 without any
modification.
INTERNATIONAL ISO
STANDARD 21068-2
First edition
2008-08-01
Chemical analysis of silicon-carbide-
containing raw materials and refractory
products —
Part 2:
Determination of loss on ignition, total
carbon, free carbon and silicon carbide,
total and free silica and total and free
silicon
Analyse chimique des matières premières et des produits réfractaires
contenant du carbure de silicium —
Partie 2: Détermination de la perte au feu, du carbone total, du carbone
libre et du carbure de silicium, de la silice totale et libre, et du silicium
total et libre
Reference number
ISO 21068-2:2008(E)
©
ISO 2008
ISO 21068-2:2008(E)
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ii © ISO 2008 – All rights reserved

ISO 21068-2:2008(E)
Contents Page
Foreword .v
Introduction.vi
1 Scope.1
2 Normative references.1
3 Terms and definitions .1
4 Determination of change in mass.2
4.1 General .2
4.2 Determination of the loss on drying at 250 °C (LOD ) gravimetric method.2
4.3 Determination of the loss on calcination in argon (LOI ) .3
Ar
4.4 Determination of the change in mass by heat pretreatment in air.5
4.5 Determination of the change in mass at 750 °C (LOI ).6
4.6 Determination of loss on ignition at 850 °C (LOI ).7
4.7 Determination of loss on ignition at 1 050 °C (LOI ) .7
1 050
5 Determination of the total carbon content.8
5.1 General .8
5.2 Combustion techniques.9
5.3 Detection techniques .12
5.4 Detection methods .17
6 Determination of free carbon content .24
6.1 General .24
6.2 Sample decomposition by combustion .24
6.3 Detection techniques .24
6.4 Direct detection methods .24
6.5 Indirect detection methods.29
6.6 Direct determination of free carbon by wet oxidation.33
7 Determination of silicon carbide content, SiC.33
7.1 General .33
7.2 Determination of silicon carbide, SiC, by indirect quantitative method.34
7.3 Determination of silicon carbide, SiC, by direct quantitative method .34
7.4 Determination of silicon carbide SiC by ignition method at 750 °C.35
7.5 Determination of silicon carbide, SiC, by chemical method.36
8 Determination of total silicon content via silica.38
8.1 Principle.38
8.2 Reagents.38
8.3 Apparatus.38
8.4 Procedure.39
8.5 Blank test.39
8.6 Calculation .40
9 Determination of free silicon content.40
9.1 Principle.40
9.2 Pretreatment with hydrochloric acid .40
9.3 Silicon determination by hydrogen evolution .40
9.4 Silicon determination by silver displacement .43
10 Determination of silica content, SiO .45
10.1 General .45
10.2 Determination of free and/or combined silica content, SiO .45
ISO 21068-2:2008(E)
10.3 Determination of free silica, SiO .46
10.4 Determination of surface silicon dioxide, SiO .46
11 Expression of results .46
12 Test report .46
Annex A (informative) Examples of CRMs for calibration of carbon analyser .47
Bibliography .48

iv © ISO 2008 – All rights reserved

ISO 21068-2:2008(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. Each 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 (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. 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.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 21068-2 was prepared by Technical Committee ISO/TC 33, Refractories.
ISO 21068 consists of the following parts, under the general title Chemical analysis of silicon-carbide-
containing raw materials and refractory products:
⎯ Part 1: General information and sample preparation
⎯ Part 2: Determination of loss on ignition, total carbon, free carbon and silicon carbide, total and free silica
and total and free silicon
⎯ Part 3: Determination of nitrogen, oxygen and metallic and oxidic constituents
ISO 21068-2:2008(E)
Introduction
[6]
ISO 21068, Parts 1 to 3, have been developed from the combination of a Japanese standard JIS 2011 and
work items originally developed within CEN. Because there is a wide variety of laboratory equipment in use,
the most commonly used methods are described.
[2] to [5]
This part of ISO 21068 is applicable to the analysis of all refractory products classified in ISO 10081
[1]
(shaped) and ISO 1927 (unshaped) and raw materials containing carbon and/or silicon carbide. Therefore,
this part of ISO 21068 covers the full range of analysis from pure silicon carbide to oxidic refractory
composition with a low content of silicon carbide and/or nitrides. Primarily, this part of ISO 21068 provides
methods to distinguish between different carbon-bound types like total carbon (C ) and free carbon (C )
total free
and derives from these two the silicon carbide content.
If free carbon is present, this part of ISO 21068 includes different types of temperature treatment, in order to
determine the mass changes gravimetrically. Frequently, the resulting residue is used for other determinations.
The determination of other groups of analytes described in this part of ISO 21068 are free metals, free silicon
(Si ), free aluminum (Al ), free magnesium (Mg ), free iron (Fe ) and the group of oxides from main to
free free free free
trace components.
This part of ISO 21068 also describes the chemical analysis of SiO , total Si, oxygen and nitrogen and other
oxidic-bound metals which typically occur in the materials.
This part of ISO 21068 represents a listing of analytical methods which is approximately structured according
to material composition. However, it is still the user who should prove the applicability of the method,
depending on the material and analytical requirements.

vi © ISO 2008 – All rights reserved

INTERNATIONAL STANDARD ISO 21068-2:2008(E)

Chemical analysis of silicon-carbide-containing raw materials
and refractory products —
Part 2:
Determination of loss on ignition, total carbon, free carbon and
silicon carbide, total and free silica and total and free silicon
1 Scope
This part of ISO 21068 specifies analytical techniques for the determination of change in mass by thermal
treatment at specified temperatures, and methods for the determination of the total carbon content, free
carbon, silicon carbide, silicon, total silica and free silica content of silicon-carbide-containing raw materials
and refractory products.
2 Normative references
The following referenced documents are indispensable for the application 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 5725-2, Accuracy (trueness and precision) of measurement methods and results — Part 2: Basic method
for the determination of repeatability and reproducibility of a standard measurement method
ISO 9286:1997, Abrasive grains and crude — Chemical analysis of silicon carbide
ISO 10060, Dense, shaped refractory products — Test methods for products containing carbon
ISO 21068-1:2008, Chemical analysis of silicon-carbide-containing raw materials and refractory products —
Part 1: General information and sample preparation
EN 12698-1:2007, Chemical analysis of nitride bonded silicon carbide refractories — Part 1: Chemical
methods
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 21068-1 apply.
ISO 21068-2:2008(E)
4 Determination of change in mass
4.1 General
The determination of change in mass is defined as a loss or increase in mass caused by heat treatment.
Several methods are distinguished based on the sample material, temperature and atmosphere. The
determination of change in mass is carried out by a gravimetric method. Heat treatments in air will lead to a
loss of volatile matter and carbon and an increase of mass due to oxidation, whereas a heat treatment in
argon will only lead to a loss of volatile matter.
Normally, the residue is used for other determinations. Therefore, the change in mass is considered for the
calculation of the analytical result. The analytical method which is applied to the residue depends on the
matrix and the parameters to be determined. It is up to the user to select the appropriate analytical method.
Table 1 gives an overview of methods of determination of change in mass by heat pretreatments and their
different applications.
Table 1 — Methods and application purpose of determination of change in mass
Short title of method Temperature Subclause Application
Attached water and chemically combined water are
Loss on drying (LOD) 250 °C 4.2
removed, e.g. in clay containing plastic formulations
Loss on calcination in argon All volatile compounds out of pitch- or resin-bonded
750 °C 4.3
(LOI ) formulations are removed
Ar
Volatile compounds are removed from resin-bonded
200 °C 4.4
formulations
Change in mass in air
Volatile compounds are removed from pitch-bonded
400 °C 4.4
formulations
Change in mass (LOI) 750 °C 4.5 Both procedures are suitable to remove carbon (e.g.
graphite) from refractory formulations. If fine-grained
SiC is present, care should be taken because SiC may
Loss on ignition (LOI) 850 °C 4.6
be oxidised as well.
Loss or gain of mass; attached water, chemically
Loss on ignition (LOI ) 1 050 °C 4.7 combined water, carbon, organic compounds (e.g.
1 050
pitch, resin), silicon carbide, and metals are removed

4.2 Determination of the loss on drying at 250 °C (LOD ) gravimetric method
4.2.1 Principle
The test sample is heated at 250 °C ± 10 °C and the loss of mass from attached water is determined.
4.2.2 Apparatus
4.2.2.1 Heat-resistant container, for example with dimensions 200 mm × 150 mm × 30 mm and made
from stainless steel.
4.2.3 Procedure
Heat the heat-resistant container at 250 °C ± 10 °C for 30 min. Cool in a desiccator, weigh and record its
empty mass , m , to the nearest 0,01 g.
Transfer the sample into the container and spread it out flat. Then weigh and record the mass, m , of the
container and sample to the nearest 0,01 g.
2 © ISO 2008 – All rights reserved

ISO 21068-2:2008(E)
Place the container without a lid in an air bath and heat it at 250 °C ± 10 °C for 16 h. Allow to cool in a
desiccator. Weigh and record the mass, m , of the container plus the dried sample to the nearest 0,01 g.
4.2.4 Calculation
Calculate the loss on drying at 250 °C, LOD , as a percentage by mass, using Equation (1).
mm−
LOD=×100 (1)
mm−
where
m is the mass of the empty container, in grams;
m is the mass of the container plus the sample before drying, in grams;
m is the mass of the container plus the sample after drying, in grams.
4.3 Determination of the loss on calcination in argon (LOI )
Ar
4.3.1 Principle
Pretreatment under argon at 750 °C to remove volatile matter. The loss of volatile matter is determined by a
gravimetric method.
The residue (R ) is normally used for determination of C , SiC and C (therefore these parameters will be
Ar total free
additionally indexed with ), and the change in mass has to be considered for the calculation of the result.
Ar
4.3.2 Apparatus
Ordinary laboratory apparatus and the following.
4.3.2.1 U-tube, with ground stoppers and filled with magnesium perchlorate.
4.3.2.2 Resistance furnace, heatable and adjustable at (750 ± 25) °C, in the centre of the heating zone.
4.3.2.3 Thermocouple with display, registering up to 1 200 °C.
4.3.2.4 Ceramic tube, with cones or other gastight connector, with an inner diameter W 16 mm, made
from porcelain, sillimanite, quartz or other suitable material.
4.3.2.5 Open combustion boats, of unglazed ceramic material, the length of which is adapted to the
oven's zone of constant temperature. The boats shall be broad enough to accommodate the amount of
sample required for the determination.
4.3.2.6 Gas flowmeter, with an upper scale reading of around 20 l/h.
The argon-conducting parts, such as tubes and connections, should be made of material proofed against
oxygen diffusion. Preferable materials are glass and copper. Silicone is unsuitable.
4.3.2.7 Test assembly
The test assembly is set up as shown in Figure 1.
ISO 21068-2:2008(E)
Key
1 valve for pressure control 5 argon cylinder 9 combustion boat
2 cold zone B 6 glass wool 10 gas flowmeter
3 cold zone A 7 magnesium perchlorate
4 ceramic tube 8 resistance furnace
Figure 1 — Apparatus set-up for determination of loss on calcination in argon

4.3.3 Reagents
4.3.3.1 Argon, 99,997 %.
4.3.4 Procedure
4.3.4.1 Check of test assembly, blank value determination
To check a newly set up test assembly or to carry out routine checks, a few samples of known volatile-matter
content shall be calcined as described in 4.3.4.2 before examining the analytical sample.
The difference between the result found in accordance with 4.3.4.2 and the known volatile-matter content shall
be taken into account as the blank value.
4.3.4.2 Determination
Carry out at least two determinations.
Before use, flush the apparatus for at least 15 min with argon.
Weigh the empty combustion boat that has previously been heated at (750 ± 25) °C and record the mass m .
Weigh 2 g of the sample to the nearest 0,001 g into the combustion boat and record the mass m .
Place the combustion boat and sample in cold zone A of the apparatus at u 200 °C. Pass argon through it at
a rate that ensures five changes of gas in the tube within 15 min.
Place the sample in the centre of the heating zone and calcine for 20 min at (750 ± 25) °C, without interruption
of the argon stream.
Move the combustion boat into cold zone B and cool in the argon stream at u 200 °C.
NOTE A period of 20 min is usually required to cool the sample.
Allow the boat to cool to room temperature in a desiccator, weigh to the nearest 0,001 g and record the final
mass, m .
4 © ISO 2008 – All rights reserved

ISO 21068-2:2008(E)
Repeat the calcination in the argon stream at (750 ± 25) °C until constant mass is obtained, i.e. when two
measurements taken at an interval of 30 min do not differ by more than 5 mg.
If the residue is required for the determination of other components, homogenize it and keep it in a closed
weighing bottle in a desiccator.
4.3.5 Calculation
Calculate the loss on calcination in argon at 750 °C, LOI , as a percentage by mass, using Equation (2).
Ar
mm−
LOI=×100 (2)
Ar
mm−
where
m is the mass of the empty combustion boat, in grams;
m is the mass of the combustion boat plus sample before ignition, in grams;
m is the mass of the combustion boat plus sample after ignition, in grams.
NOTE The decrease in mass is denoted by a minus sign.
4.4 Determination of the change in mass by heat pretreatment in air
For resin- and pitch-bonded materials, a sample pretreatment shall be performed in accordance with
ISO 10060, or the following procedure shall be followed.
Usually before crushing and grinding, subject a sample of approximately 1 kg to heat treatment as follows:
a) resin-bonded materials: 200 °C for 18 h in air;
b) pitch-bonded materials: 400 °C for 18 h in air.
Calculate the change in mass at 200 °C/400 °C, D, as a percentage by mass, using Equation (3).
mm−
D=×100 (3)
mm−
where
m is the mass of the empty container, in grams;
m is the mass of the container plus sample before heat pretreatment, in grams;
m is the mass of the container plus sample after ignition, in grams.
Report the result to the nearest 0,1 %.
When the nature of free carbon is not known, first carry out thermogravimetric analysis to determine the
temperature of the heat treatment.
ISO 21068-2:2008(E)
4.5 Determination of the change in mass at 750 °C (LOI )
4.5.1 Principle
Determination of the change in mass as a function of time by ignition at 750 °C in an electric muffle furnace.
The loss or increase of mass is determined by a gravimetric method.
4.5.2 Apparatus
4.5.2.1 Muffle furnace, with a temperature control from 100 °C to 1 000 °C.
4.5.2.2 Fused silica dish, porcelain, or platinum, approximately 60 mm long and 35 mm wide.
4.5.2.3 Balance, capable of weighing to 0,1 mg.
4.5.3 Procedure
Weigh the empty dish that has previously been heated at (750 ± 25) °C and record the mass m . Weigh 2,5 to
3,5 g of the sample, dried at 110 °C, to the nearest 0,001 g into the combustion boat and record the mass m .
Place the dish and the sample in the muffle furnace at 500 °C for 20 min.
Increase the furnace temperature to 750 °C and ignite the sample for a further 1 h 30 min when the furnace
has reached the test temperature.
Take the dish out of the furnace and allow it to cool down to room temperature in a desiccator.
Weigh the dish and sample. Record the mass m .
Replace the dish and sample in the furnace for a further 30 min and check whether there is a further loss in
mass. If so, repeat the whole procedure.
NOTE If a mass increase is observed after the second ignition, do not carry out further ignition because it can
indicate possible oxidation of some elements.
4.5.4 Calculation
Calculate the loss on ignition at 750 °C, LOI , as a percentage by mass, using Equation (4).
mm−
LOI=×100 (4)
mm−
where
m is the mass of the empty dish, in grams;
m is the mass of the dish plus sample before ignition, in grams;
m is the mass of the dish plus sample after ignition, in grams.
NOTE The result obtained in this way cannot be considered as the free carbon content.
6 © ISO 2008 – All rights reserved

ISO 21068-2:2008(E)
4.6 Determination of loss on ignition at 850 °C (LOI )
4.6.1 Apparatus
4.6.1.1 Platinum dish, platinum or porcelain (e.g. Type B 20 ml).
4.6.2 Procedure
Heat the platinum dish at 850 °C ± 25 °C for 15 min, cool it in a desiccator, and then weigh the platinum dish.
Record the mass of the empty dish m .
Weigh 5,0 g of a sample, to the nearest 0,1 g, into the platinum dish and spread it widely and thinly. Record
the mass of the dish plus sample m .
Put the platinum dish without a cover into an electric furnace and raise the temperature from room
temperature to 850 °C ± 25 °C, and keep heating for 3 h. Put the lid on the platinum dish, cool in a desiccator,
weigh it and record the mass m .
In the case of fusion of the sample containing silicon carbide, silicon nitride and metal silicon, several hours
should be spent on a rise of temperature to the 700 °C to 850 °C region. There is a risk of eroding a platinum
dish with a rapid rise of temperature to above 850 °C.
4.6.3 Calculation
Calculate the loss on ignition at 850 °C ± 25 °C, R, expressed as a percentage by mass using Equation (5).
mm−
R=×100 (5)
mm−
where
m is the mass of the empty dish, in grams;
m is the mass of the dish plus sample before ignition, in grams;
m is the mass of the dish plus sample after ignition, in grams.
Transfer the residue, after measurement, to an agate mortar, grind and mix lightly to be homogeneously,
transfer to a flat weighing bottle (50 mm × 30 mm), and keep in a desiccator. Use it for determination of each
component.
4.7 Determination of loss on ignition at 1 050 °C (LOI )
1 050
4.7.1 Principle
A sample is heated at 1 050 °C ± 25 °C and loss or gain of mass from loss of attached water, water of
crystallization, carbon, organic compounds, silicon carbide, and metals is determined by a gravimetric method.
4.7.2 Apparatus
4.7.2.1 Crucible, platinum or porcelain (e.g. Type B 20 ml).
4.7.3 Procedure
Heat the crucible to 1 050 °C ± 25 °C for a specified time, cool in a desiccator and weigh the empty platinum
or porcelain crucible and record the mass m .
ISO 21068-2:2008(E)
A platinum crucible should be heated for about 15 min and a porcelain crucible should be heated for about
60 min.
Weigh 1,0 g of the dry sample into the platinum or porcelain crucible to the nearest 0,1 mg, spread thinly and
weigh the mass of the crucible and sample and record the mass m .
Place the crucible without a lid in an electric muffle furnace and slowly raise the temperature to
1 050 °C ± 25 °C. Maintain this temperature for about 60 min and allow to cool with a lid. Weigh the crucible
plus the ignited sample and record the mass m .
4.7.4 Calculation
Calculate the loss on ignition, LOI , as a percentage by mass, using Equation (6).
1 050
mm−
LOI =×100 (6)
mm−
where:
m is the mass of the empty crucible, in grams;
m is the mass of the crucible plus sample before ignition, in grams;
m is the mass of the crucible plus sample after ignition, in grams.
If a gain on ignition is observed, a minus sign should be added in front of the numerical value.
5 Determination of the total carbon content
NOTE Suitable certified reference materials (CRMs) for the calibration of a carbon analyser are given in Annex A.
5.1 General
The total carbon content, w , can be determined using the following methods:
Ctotal
⎯ combustion with oxygen, using either
⎯ a resistance furnace (RF), with lead borate fusion or tin powder as accelerator/decomposition agent,
or
⎯ an induction furnace (IF), with metal fusion as accelerator;
⎯ detection methods using
⎯ coulometry,
⎯ gravimetry,
⎯ conductometry,
⎯ infrared absorption,
⎯ thermal conductivity.
Usual combinations of available equipment are shown in Table 2.
8 © ISO 2008 – All rights reserved

ISO 21068-2:2008(E)
Table 2 — Usual combinations of equipment for carbon determination
Equipment Coulometry Conductivity Gravimetry IR absorption Thermal conductivity
Resistance furnace
+ + + +
Induction furnace
+ + +
The procedures for the determination of total carbon are therefore structured as combustion techniques,
detection techniques and detection methods constituting the laboratory procedure.
5.2 Combustion techniques
5.2.1 General
Two different combustion techniques with different decomposing agents/accelerators can be used.
5.2.2 Resistance furnace in oxygen and lead borate as decomposing agent
5.2.2.1 Principle
The sample is heated together with lead borate in a stream of oxygen in a resistance tube furnace to convert
the carbon to carbon dioxide by combustion. The sample mass and the details of the combustion depend on
the method of determination used. The combustion gases are conducted through a tube containing
percarbamide to absorb the oxidation products of the sulfur contained in the sample. The carbon dioxide is
absorbed in an alkaline medium and determined either coulometrically, gravimetrically, conductometrically or
by infrared absorption.
5.2.2.2 Reagents
Use only reagents of analytical grade.
5.2.2.2.1 Oxygen, 99,99 % or 99,5 %.
NOTE Oxygen 99,99 % is used if the instrument does not have an oxygen-refining capability. Oxygen 99,5 % is used
if the instrument has oxygen-refining ability.
5.2.2.2.2 Lead borate, 2 PbO·B O , prepared by melting 45 g of analytical grade lead oxide, PbO,
2 3
together with 7 g of analytical grade boron trioxide, B O , for 10 min at (950 °C ± 25) °C, cooling the melt by
2 3
pouring it onto a clean aluminium plate and then pulverizing it.
5.2.2.3 Apparatus
Ordinary laboratory apparatus and the following.
5.2.2.3.1 Resistance furnace with ceramic tube, capable of being used up to 1 200 °C. The furnace shall
be capable of being maintained at a temperature of (1 050 ± 25) °C in the centre of the heating zone. The
furnace shall be fitted with a thermocouple connected to a device permitting measurement of the furnace
temperature.
Allowance should be made for the fact that, frequently, the temperature indicated on the built-in temperature
control display of the furnace deviates from the actual temperature in the ceramic tube. It should be adjusted
using an external thermocouple device, measuring the temperature of the heating zone inside the tube.
5.2.2.3.2 Open combustion boats of unglazed ceramic material, the length of which is adapted to the
heating zone of the furnace, and which are broad enough to accommodate the amount of sample required for
the determination. Before use, the boats shall be heated in a laboratory furnace at 1 000 °C for 1 h and stored
in a desiccator after cooling.
ISO 21068-2:2008(E)
5.2.2.4 Setting up of test assembly
Set up the test assembly in accordance with the manufacturer's instructions.
5.2.2.5 Procedure for RF combustion with lead borate
Weigh the required sample mass into the combustion boat and cover it with 1,5 g of lead borate, 2 PbB O .
2 3
Preheat the furnace to a temperature of (1 050 ± 25) °C and place the boat in the centre of the heating zone.
Adjust the oxygen flow rate so as to prevent the risk of air being sucked in from the outside.
For the total carbon in the case of coulometric and conductometric methods, the volume of the combustion
gas shall generally be reduced to one-tenth.
NOTE Combustion is usually complete after 5 min.
The carbon dioxide gas formed is supplied by carrier gas to the detection unit.
Carry out the determination of the carbon dioxide formed, as described in 5.3.
5.2.3 Resistance furnace in oxygen and tin powder as decomposing agent
5.2.3.1 Principle
The sample is burned with accelerator in an oxygen atmosphere inside a resistance heating furnace and the
yielded carbon dioxide (and carbon monoxide) is transferred in the detection unit selected by the user.
5.2.3.2 Reagents
Use only reagents of analytical grade.
5.2.3.2.1 Oxygen, 99,99 % or 99,5 %.
NOTE Oxygen 99,99 % is used if the instrument does not have oxygen-refining capability. Oxygen 99,5 % is used if
the instrument has oxygen-refining ability.
5.2.3.2.2 Accelerator, tin powder, with a grain size < 100 µm and with a low blank value.
5.2.3.3 Apparatus
5.2.3.3.1 Combustion boat, porcelain, outer diameter (OD) = 12 mm, inner diameter (ID) = 9 mm, 60 mm
long, annealed over 1 050 °C.
5.2.3.3.2 Combustion tube, porcelain, e.g. OD = 25 mm, ID = 20 mm, 600 mm long.
5.2.3.3.3 Furnace, of a carbon determination apparatus. It is composed of oxygen refining, sample burning,
combustion-gas refining and carbon-content determining parts.
5.2.3.3.4 Oxygen-refining assembly, composed of an oxidizing tube with electric furnace [copper(II) oxide,
CuO, or platinum-silica-wool], a carbon-dioxide-absorbing tube (soda lime) and a dehydration tube
[magnesium perchlorate, Mg(ClO ) , (for dryness)].
4 2
NOTE The oxygen-refining assembly is optional.
5.2.3.3.5 Sample-burning assembly, composed of a tubular electric furnace and porcelain combustion
tube. The tubular electric furnace shall be capable of maintaining (1 350 ± 25) °C at the centre of the
combustion tube.
10 © ISO 2008 – All rights reserved

ISO 21068-2:2008(E)
5.2.3.3.6 Combustion-gas-refining assembly, composed of a dust chamber filled with glass wool, a
desulfurization tube [manganese(IV) oxide, MnO ], with electric furnace, copper(II) oxide, CuO, oxidizing tube
and a dehydration tube [magnesium perchlorate, Mg(ClO ) ].
4 2
NOTE The desulfurization tube and oxidizing tube are optional.
Allowance should be made for the fact that, frequently, the temperature indicated on the built-in temperature
control display of the furnace deviates from the actual temperature in the ceramic tube. It should be adjusted
using an external thermocouple device, measuring the temperature of the heating zone inside the tube.
5.2.3.4 Setting up of test assembly
Set up the test assembly in accordance with the manufacturer's instructions.
5.2.3.5 Procedure of RF combustion with tin powder
Turn on the power source of the apparatus, raise the combustion tube temperature to 1 350 ± 25 °C, and wait
for the carbon determination apparatus to stabilize. Start oxygen flow at the specified pressure and amount
and check air-tightness.
NOTE The detailed procedures, for example the test of air tightness, are carried out in accordance with the
instruction manual attached to the apparatus.
Measure and spread the sample uniformly on a combustion boat. Cover it with 2 g of accelerator, mix 2 g of
accelerator into the sample and spread uniformly, or put the sample between 1 g each of accelerator, like a
sandwich, and spread.
Open the valve at the entrance of the combustion tube and put the combustion boat with the sample and
accelerator in the centre of the combustion tube. Immediately, close the valve tightly and start oxygen flow.
The carbon dioxide gas formed is supplied by carrier gas to the detection unit.
Carry out the determination of the carbon dioxide formed, as described in 5.3.
5.2.4 Induction furnace (IF) in oxygen and metallic powder as decomposing agent
5.2.4.1 Principle
The sample is heated together with a base metal additive in a stream of oxygen using a high-frequency
induction furnace. The carbon dioxide released is transferred by carrier gas to the detection unit.
5.2.4.2 Reagents
5.2.4.2.1 Granulated iron accelerator, e.g. as supplied by the supplier of the furnace.
5.2.4.2.2 Granulated tungsten accelerator, e.g. as supplied by the supplier of the furnace.
5.2.4.2.3 Granulated copper accelerator, e.g. as supplied by the supplier of the furnace.
5.2.4.2.4 Oxygen, 99,99 % or 99,5 %.
NOTE Oxygen 99,99 % is used if the instrument does not have an oxygen-refining capability. Oxygen 99,5 % is used
if the instrument has oxygen-refining ability.
ISO 21068-2:2008(E)
5.2.4.3 Apparatus
5.2.4.3.1 Combustion crucibles, of ceramic material with covers and holders. In principle, use the high-
frequency combustion crucible and the high-frequency combustion crucible holder as recommended by the
apparatus provider.
Use only accelerators with low blank test values.
The empty com
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