OSIST prEN 12698:2005
(Main)Chemical analysis of nitride bonded silicon carbide refractories
Chemical analysis of nitride bonded silicon carbide refractories
This part of EN 12698 describes the methods for the analysis of all refractory products containing nitride bonded silicon carbide, irrespective of the Silicon carbide level. It includes details of sample preparation, general principles of chemical analysis and detailed methods for the determination of carbon, silicon carbide, free aluminium, free silicon, total nitrogen and oxygen.
Silicon nitride, silicon oxynitride, silicon, free silica and silicon aluminium oxynitride (sialon) can be determined by XRD.
Chemische Analyse von feuerfesten Erzeugnissen aus nitridgebundenem Silicumcarbid
Dieses Dokument legt Analysenverfahren für sämtliche feuerfesten Erzeugnisse auf der Basis von
nitridgebundenem Siliciumcarbid, unabhängig vom Siliciumcarbidanteil, fest. Es enthält Einzelheiten für die
Probenvorbereitung, allgemeine Grundlagen der chemischen Analyse und ausführliche Verfahren für die
Bestimmung von Kohlenstoff, Siliciumcarbid, freiem Aluminium, freiem Silicium, Gesamtstickstoff und
Sauerstoff.
Siliciumnitrid, Siliciumoxynitrid, Silicium, freies Siliciumdioxid und Silicium-Aluminium-Oxy-Nitrid (Sialon)
können mittels XRD bestimmt werden.
Analyse chimique des produits réfractaires contenant du carbure de silicium lié au nitrure
La présente partie de l'EN 12698 décrit les méthodes d'analyse applicables a tous les produits réfractaires contenant du carbure de silicium lié au nitrure, quelle que soit la teneur en carbure de silicium. Elle contient les détails relatifs a la préparation des échantillons, les principes généraux de l'analyse chimique ainsi que les méthodes détaillées de détermination du carbone, du carbure de silicium, de l'aluminium libre, du silicium libre, de l'azote total et de l'oxygene.
Le nitrure de silicium, l'oxynitrure de silicium, le silicium, la silice libre et l'oxynitrure de silicium et d'aluminium (sialon) peuvent etre déterminés par DRX (Diffraction aux rayons X).
Chemical analysis of nitride bonded silicon carbide refractories
General Information
Relations
Standards Content (Sample)
SLOVENSKI OSIST prEN 12698:2005
PREDSTANDARD
maj 2005
Chemical analysis of nitride bonded silicon carbide refractories
ICS 71.040.40; 81.060.10 Referenčna številka
OSIST prEN 12698:2005(en)
© Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno
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EUROPEAN STANDARD
DRAFT
prEN 12698
NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2005
ICS
English version
Chemical analysis of nitride bonded silicon carbide refractories
Chemische Analyse von feuerfesten Erzeugnissen aus
nitridgebundenem Silicumcarbid
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee CEN/TC 187.
If this draft becomes a European Standard, 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.
This draft European Standard was established by CEN 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 Management Centre has the same
status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without notice and
shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 12698:2005: E
worldwide for CEN national Members.
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prEN 12698:2005 (E)
Contents Page
Foreword .3
1 Scope .4
2 Normative references.4
3 Choice of method of determination.4
4 Determination of free aluminium .5
5 Determination of total nitrogen.6
6 Determination of silicon nitride, silicon oxynitride and other phases.16
7 Determination of free silicon .19
8 Determination of free silica .19
9 Determination of carbon .21
10 Direct determination of free carbon content .24
11 Determination of silicon carbide.24
12 Determination of free alumina.25
13 Determination of β’-Sialon.26
Annex A (informative) Determination of free carbon — Hot chromic sulfuric iodic acid
method (9,2,1): Explanation for the evaluation of the different possible detection
methods.29
A.1 Coulometric detection system (Figure A,1) .29
A.2 Infrared absorption detection system .30
A.3 Conductometric detection system.31
Annex B (normative) x-ray diffraction data for the determination of β’-Sialon content.32
B.1 General .32
B.2 Theoretical peak positions and Miller indices (h, k, l) of β’-sialon for z=3. .32
B.3 Example of calculation of Z-Number for β’-sialon .33
Bibliography.37
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Foreword
This document (prEN 12698:2005) has been prepared by Technical Committee CEN/TC 187
“Refractories”, the secretariat of which is held by BSI.
This document is currently submitted to the CEN Enquiry.
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prEN 12698:2005 (E)
1 Scope
This part of EN 12698 describes the methods for the analysis of all refractory products containing nitride
bonded silicon carbide, irrespective of the Silicon carbide level. It includes details of sample preparation,
general principles of chemical analysis and detailed methods for the determination of carbon, silicon
carbide, free aluminium, free silicon, total nitrogen and oxygen.
Silicon nitride, silicon oxynitride, silicon, free silica and silicon aluminium oxynitride (sialon) can be
determined by XRD.
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.
EN ISO 21068 Chemical analysis of silicon carbide refractories
ISO 3310-1 Test sieves -- Technical requirements and testing -- Part 1: Test sieves of metal wire cloth
ISO 5022 Sampling of shaped refractories
ISO 8656 Sampling of unshaped refractories
ISO 5725 Precision of test methods — Determination of repeatability and reproducibility for a standard
test method by inter-laboratory tests
3 Choice of method of determination
A list of methods and the relevant European Standards is given in Table 1.
Table 1 — Methods and relevant European Standards
Item tested European Standard
Carbon and silicon carbide EN ISO 21068
Free silicon EN ISO 21068
Oxygen EN ISO 21068
Free aluminium EN 12698
Total nitrogen EN 12698
Silicon nitride/oxynitride, free silica EN 12698
XRD EN ISO 21068
and EN 12698
Sialon EN 12698
Free carbon EN ISO 21068
or EN 12698
Total carbon EN ISO 21068
Silicon carbide EN ISO 21068
and EN 12698
Free alumina EN 12698
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Carbon and silicon carbide can be determined by evolution of carbon dioxide on combustion in a stream
of oxygen at selected temperatures. The carbon dioxide evolved can be conveniently measured
coulometrically, gravimetrically or by infra-red detection by absorption onto soda lime.
Free aluminium can be determined by evolution of hydrogen on treatment with hydrochloric acid and
measurement of the gas volume in a nitrometer. Free silicon can be determined on the same sample by
treatment with sodium hydroxide after washing by decantation.
Total nitrogen can be determined by a variety of methods.
Distinction between silicon nitride and aluminium nitride can be made by the reaction with sodium
hydroxide solution, while aluminium nitride is quantitatively decomposed in this way yielding free
ammonia, silicon nitride is unaffected.
A convenient commercial apparatus for the determination of total oxygen consists of an induction furnace
in which the sample is heated with pure carbon. Carbon dioxide and carbon monoxide are measured by
infra red absorption and the integrated signals are combined to give the total oxygen content.
Specification of many of the constituents can often be made by use of XRD techniques, e.g. free silicon,
silicon nitride, silicon oxynitride, quartz and cristobalite peaks.
4 Determination of free aluminium
4.1 Principle
The volume of hydrogen generated by the action of dilute hydrochloric acid on any free aluminium in a
sample is measured.
It is not a very specific method.
If the sample is known to contain carbonate, then the volume of hydrogen evolved is corrected for the
known carbonate present.
NOTE The free aluminium content can also be determined by evolution of hydrogen using sodium hydroxide. In
this case, the volume of hydrogen evolved shall be corrected for the known silicon content. Free iron will also evolve
hydrogen; correction shall be made for the known for the known iron content.
4.2 Reagents
Use distilled water or water which has been fully demineralized by ion exchange (de-ionized water) and
reagents of analytical grade.
4.2.1 Dilute hydrochloric acid, 1+1 by volume
4.3 Apparatus
4.3.1 Nitrometer, as used for the determination of free silicon.
4.3.2 Balance capable of reading to the nearest 0.1 mg
4.4 Procedure
Weigh 0,5000 g ± 0,0100 g of sample into a clean, dry nitrometer tube. Place a dry ignition tube also
inside the tube, and holding the nitrometer tube upright, introduce about 5 ml of dilute hydrochloric acid
into the ignition tube, using a long dropping pipette, and without letting any touch the sample.
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Carefully fit the bung of the apparatus making sure of a good seal. Equalise the pressure as in the free
silicon determination and leave the 3-way tap in a position that connects the sample and graduated tube.
Tip the acid out of the ignition tube onto the sample. Shake gently and allow to stand about 15 minutes.
Read off the volume after equalising the pressure. Shake gently and read again after another 5 min to
10 min. Record the final volume reading when consecutive readings are the same. Also note the
temperature and barometric pressure.
NOTE For frequent use of this method, it is recommended that a conical flask with airtight sample insert device,
e.g. a side-on positioned ground-in connection and ground-in stopper with weighing bottle (special version) is used.
The weighed sample is placed into the stopper-connected weighing bottle. The hydrochloric acid added to the flask.
After equalising of pressure, the sample powder is added to the acid by turning the stopper.
4.5 Calculation and expression of results
Correct the volume reading to the gas volume at Standard Temperature, V in ml, using the equation:
(STP)
(P − ρ) 273
V = V ⋅ ⋅ (1)
(STP) 1
760 (273 +T )
where
V is the measured volume, in ml;
1
P is the atmospheric pressure at time of measurement, in mPa;
ρ is the partial pressure of water vapour at the measured temperature, in mPa;
o
T is the measured temperature, in C.
Calculate the percentage of free aluminium, A, using the equation:
100
(2)
A = V ⋅0,000804⋅
(STP)
m
where
m is the mass of the sample, in g.
Report the result to the nearest 0,1 %.
5 Determination of total nitrogen
5.1 Determination of total nitrogen by carrier gas fusion (CGF)
5.1.1 Principle
A sample, prepared as described in clause 5.1.4, is decomposed in a graphite crucible in a stream of
carrier gas (helium) by heating to a temperature above 2 400 ºC in a resistance furnace (electrode
furnace).
The gases released are mainly nitrogen, carbon monoxide and hydrogen. The carbon monoxide and
hydrogen are oxidized, respectively, to carbon dioxide and water and then removed by absorption.
Alternatively, oxidation may be confined to the carbon monoxide and gases other than nitrogen are
removed, for example, using a molecular sieve. The change in thermal conductivity due to the nitrogen
component is then measured.
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The details of the determination procedure may vary with the type of apparatus used and it is therefore
only possible to give general instructions which can be used with any type of apparatus, whose nitrogen
content has been determined by an absolute method (see 5.2 and 5.3). Using the gas calibration, the
validity of the results shall be confirmed by analysing a reference material having a similar extraction
behaviour.
5.1.2 Reagents
Use distilled water or water which has been fully demineralized by ion exchange (de-ionized water) and
reagents of analytical grade.
5.1.2.1 Helium, having a minimum purity of 99,99 %.
5.1.2.2 Nitrogen, having a minimum of 99,99 %.
5.1.2.3 Catalysts, such as copper oxide.
5.1.2.4 Sorption agents for removing water vapour and carbon dioxide, e.g. magnesium perchlorate,
sodium hydroxide on a support, or a molecular sieve.
5.1.3 Apparatus
5.1.3.1 Commercially available apparatus consisting of a resistance furnace and a measuring unit for the
determination of nitrogen in a stream of carrier gas using a thermal conductivity cell. An example of a
suitable apparatus is given in Figure 1.
Key
1. Resistance furnace
2. Apparatus for oxidizing carbon monoxide and hydrogen
3. Carbon dioxide and water vapour absorption tubes
4. Thermal conductivity cell
Figure 1 — Gas flow diagram for the determination of total nitrogen by carrier gas fusion
5.1.3.2 Analytical balance capable of measuring to the nearest 0,01 mg.
5.1.3.3 Graphite crucibles having approximately the same electrical resistance. The crucibles shall
contain only extremely small amounts of nitrogen which can be removed by degassing.
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5.1.4 Sampling
Sample shaped and unshaped products using the procedures given in ISO 5022 and ISO 8656
respectively.
In sampling large fragments, take care to collect samples from different points of individual pieces.
Homogenize the sample by reducing the maximum particle size to 150 µm and take the test sample from
this material.
5.1.5 Sample preparation
o o o o
Dry the test sample to constant mass at a temperature of 105 C ± 5 C to 135 C ± 5 C. It is advisable
to encase the sample in tin, nickel or platinum and compress it so that as little air as possible is included.
The use of the bath metals as casing materials (capsules or foil) will ensure the formation of a
homogeneous melt for the extraction.
NOTE The addition of bath metals, e.g. nickel or tin, may also be necessary to complete the extraction.
5.1.6 Procedure
The mass of the sample depends on the type of apparatus used, but shall not be less than 4 mg.
The lower limit depends on the homogeneity of the sample material and the upper limit on the calibration
range of the apparatus; the initial sample mass will depend on the anticipated nitrogen content.
Follow the manufacturer’s operating instructions for the apparatus.
o
Place the crucible in the furnace and degas it at a temperature of about 100 C above the analysis
temperature. Weigh the sample and record its mass to the nearest 0,01 mg. Add the sample to the
crucible and heat.
NOTE Reliable analytical results will only be obtained if adequate information relating to sample preparation,
procedure, calibration, recalibration and checking, and apparatus maintenance is available from in-house
experiments and experience.
5.1.7 Calibration and recalibration
5.1.7.1 Gas calibration
Gas calibration shall be carried out by adding a known amount of nitrogen gas. Calculate the amount of
gas added, m in mg, using the equation:
ρV ⋅ ρN
T 2
m = (3)
ρ()1 + γT
n
where
ρ is the corrected barometric pressure, in mPa;
o
T is the temperature, in C;
o
V is the gas volume added, in ml at T C and ρ mPa;
T
ρN is the density of nitrogen gas under standard conditions, i.e. 1,2504 mg/ml;
2
γ is the cubic coefficient of thermal expansion of nitrogen (0,003671 /K);
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ρ is the standard pressure, 1013,25 mPa.
n
By this procedure the linearity of the evaluation curve is fixed. This can also be done with a computer
connected to the measuring equipment. The calibration, however, will not provide any information about
the efficiency of the extraction process. This can only be determined by analysing suitable reference
samples. The latter approach is the only one possible in the case of systems not designed for gas
calibration.
5.1.7.2 Calibration using solids
For calibration with solids, the reference material shall be analysed using widely varying sample masses,
if possible, covering the entire calibration range of the apparatus. However, the increase in the relative
analytical error has to be considered, if smaller sample masses are used.
NOTE If a linearity has been found beforehand by calibration using gas addition or a reference sample, any
variation is the analytical result in connection with the initial sample mass can be unambiguously ascribed to an
inefficient extraction process.
5.1.8 Checking and maintaining the apparatus
Before a new apparatus is used, the manufacturer’s data on the measurement range, the initial sample
mass, the reproducibility and the stability shall be checked using suitable samples with known nitrogen
contents. The manufacturer’s instructions for regular checks and maintenance shall be carried out. The
replacement of the oxidation and sorption reagents is particularly important and incorrect results are to be
expected if it is not carried out in due time.
5.1.9 Precision of the method
Under the conditions specified, the values of the repeatability limit r and the reproducibility limit R as
conducted in ISO 5725 are:
r = 1 %
R = 2 %
NOTE An improved reproducibility and accuracy may be expected if certified reference materials are used for
calibration.
5.2 Determination of total nitrogen content by fusion decomposition
5.2.1 General
The method referred to in clause 3 of this standard is used to determine nitrogen in silicon nitride, Si N ,
3 4
by fusion decomposition. Analogous methods can be used to determine nitrogen in materials containing
not less than 5 % of nitrogen as silicon and aluminium nitrides.
5.2.2 Principle
o
The sample is fused with lithium hydroxide at a temperature of not more than 700 C to convert the
nitrogen into ammonia. A gentle stream of inert gas is used to transfer the ammonia to a receiving vessel
containing boric acid solution and the amount of nitrogen is determined by titration with an acid of known
concentration.
5.2.3 Reagents
Use distilled water or water which has been fully demineralized by ion exchange (de-ionized water) and
reagents of analytical grade.
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5.2.3.1 Powdered lithium hydroxide, LiOH.
5.2.3.2 Sulfuric acid, ρ = 1,84 g/m1.
5.2.3.3 Standard 0,1 mol/l hydrochloric or sulfuric acid of known standardization for titration. The
coefficient of variation of the standardization shall not exceed 0,001.
5.2.3.4 Boric acid solution, prepared by dissolving 40 g of boric acid, H BO , in 1 l of hot water.
3 3
5.2.3.5 Inert gas, argon or nitrogen, with a purity of 99,99 % as inert gas.
5.2.3.6 Sodium carbonate, Na CO , 99,95% to 100,05 %.
2 3
5.2.3.7 Dried calcium chloride, CaCl .
2
5.2.4 Apparatus
5.2.4.1 Analytical balance, capable of reading to the nearest 0,01 mg.
5.2.4.2 Apparatus for releasing, carrying over and absorbing ammonia (see figure 2), comprising:
a) flow meter;
b) gas washing bottles;
c) vitreous silica reaction tube with ground joints, stoppers and gas inlet;
d) unglazed porcelain boats;
e) tubular furnace, e.g. heated by infrared radiation, capable of being heated to, and maintained at
(700 ± 10) ºC;
f) vitreous silica wool;
g) gas inlet tube with ground joint and capillary tip;
h) absorption vessel.
In this apparatus, the inert gas from a pressurised gas cylinder passes through a gas washing bottle filled
with sulphuric acid having a density, ρ = 1,84 g/ml, preceded and followed by an empty washing bottle for
safety reasons.
NOTE 1 No gas purification is necessary if the ammonia content of the inert gas does not exceed 0,005 % by
volume.
The inert gas is then passed through a flow meter and into the vitreous silica reaction tube at the side gas
inlet. The ground joint through which the sample is inserted is also located at this point. The other end of
the reaction tube is connected by a ground joint to a gas inlet tube whose tip has been drawn out to form
a capillary and extends almost to the bottom of a narrow absorption vessel.
NOTE 2 The reaction tube may also be a gas-tight ceramic tube with the sample inlet and borosilicate glass
ground joint shown in figure 1, which are fused on or attached by means of silicone hoses.
The reaction tube shall be heated with a tubular furnace which can be maintained at (700 ± 10) °C. The
still hot part of the tube outside the tubular furnace and adjacent to the absorption vessel is packed with
loose vitreous silica wool which is capable of condensing any lithium hydroxide which evaporates.
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Key
1 inert gas 6 tubular furnace
2 sulfuric acid 7 vitreous silica wool
3 washing bottles 8 gas inlet tube with capillary tip
4 vitreous silica tube with connections 9 boric acid
5 porcelain boat 10 absorption vessel
Figure 2 — Nitrogen determination apparatus for fusion decomposition
5.2.4.3 Potentiometric titrator, having a metering volume of 50 ml and a maximum relative tolerance of
0,1 %.
5.2.5 Sampling
Sample as described in 5.1.4.
5.2.6 Procedure
5.2.6.1 Decomposition by fusion
Coat the entire inside of the porcelain boats with 500 mg of lithium hydroxide at 600 °C and store the
boats in a desiccator. Weigh 100 mg of the sample to the nearest 0,01 mg, into a coated porcelain boat
and mix it thoroughly with 1,5 g of lithium hydroxide. Flush the apparatus with inert gas, pour 40 ml of
boric acid solution into the absorption vessel and immerse the gas inlet tube in it. Set the inert gas flow to
70 to 100 normal ml/min, open the ground joint closure and push the porcelain boat into the centre of the
reaction tube to the point where the thermocouple is located. After closing the tube again, heat the
tubular furnace slowly to 700 °C in steps so as to prevent the melt spattering. In the case of tubular
furnaces which heat up rapidly, the heating phase shall not be less than 15 minutes. After about 30 min
at (700 ± 10) °C, the nitride nitrogen will have been quantitatively converted into ammonia. Ensure that
the furnace temperature does not under any circumstances exceed 730 °C, because the lithium
hydroxide will start to evaporate above that temperature.
NOTE The time for complete reaction shall be established before the method is applied.
5.2.6.2 Standardization of titration acid
Weigh 200 mg of dried sodium carbonate to the nearest 0,01 mg in a sealable weighing bottle.
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NOTE Dry the sodium carbonate at 270 °C to 300 °C for about one hour, stirring occasionally, and store it in a
desiccator over calcium chloride.
Dissolve the sodium carbonate in 50 ml of distilled water and add the titration acid to be standardized
using a potentiometric titrator until the equivalent point in the pH range 4,6 ± 0,2 is reached. Take the
mean value of not less than 3 titrations. The coefficient of variation shall not exceed 0,001.
5.2.6.3 Titrating the absorption solution
When the reaction is complete, remove the gas inlet tube from the absorption vessel and rinse its inside
and outside with a few millilitres of water. Titrate the amount of absorbed ammonia to the equivalence
point, which is generally a pH value of 4,6 ± 0,2 with the standardized titration acid using the
potentiometric titrator.
5.2.6.4 Blank value
To determine the blank value, carry out the determination described in the clauses 5.2.6.1 and 5.2.6.3,
without a test sample.
5.2.7 Calculation and expression of results
5.2.7.1 Calculation of acid titration factor
Calculate the titration correction factor, t, of the acid using equation (4):
m
Na
t = (4)
5,2994V
1
where
m is the sample mass of sodium carbonate, in mg;
Na
V is the volume used of the 0,1 mol/l acid to be standardized, in ml;
1
5,2994 is the titrimetric factor, in mg/ml.
5.2.7.2 Calculation of nitrogen content
Calculate the nitrogen content, M , as a percentage by mass using the following:
N
()V − V × t ×1,4007 ×100
2 3
M = (5)
N
m
where
V is the volume of titration acid used for the sample, in ml;
2
V is the volume of titration acid used for the blank value, in ml;
3
t is the titration correction factor of the acid;
m is the sample mass, in mg;
1,4007 is the titrimetric factor, in mg/ml.
Report the result to the nearest 0,1 %.
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5.2.8 Precision
Under the conditions specified, the values of the repeatability limit r and the reproducibility limit R as
conducted in ISO 5725 are:
r = 0,5 %
R = 0,9 %
5.3 Determination of total nitrogen content by Kjeldahl distillation
5.3.1 Principle
The sample is dissolved in hydrofluoric acid under pressure and the nitrogen is distilled over as ammonia
into a receiving vessel containing boric acid solution, using an ammonia distillation apparatus, and
determined by potentiometric titration.
5.3.2 Reagents
Use distilled water, or water which has been fully demineralized by ion exchange (de-ionized water) and
reagents of analytical grade of known analytical purity.
Unless otherwise specified, solutions are aqueous solutions.
5.3.2.1 Hydrofluoric acid, HF, ρ = 1,13 g/ml;
5.3.2.2 Boric acid, H BO
3 3;
5.3.2.3 Boric acid solution, to be prepared by dissolving 40 g of boric acid in 1 l of hot water;
5.3.2.4 Sodium carbonate, Na CO , 99,95 to 100,05 %;
2 3
5.3.2.5 Hydrochloric acid, or sulfuric acid solution, c(HCl) or c(H SO ) = 0,1 or 0,05 mol/l. The coefficient
2 4
of variation shall not exceed 0,001;
5.3.2.6 Sodium hydroxide solution, to be prepared by dissolving 400 g of sodium hydroxide in 1 l of water.
5.3.3 Apparatus
Ordinary laboratory apparatus and the following:
5.3.3.1 Analytical balance capable of reading tot the nearest 0,01 mg;
5.3.3.2 Laboratory oven or microwave oven suitable for temperatures up to 230 °C;
5.3.3.3 Apparatus for the determination of ammonia, e.g. by the Parnas-Wagner method, with a steam
generator;
5.3.3.4 Digestion apparatus to be used under pressure, with a poly-tetrafluoroethylene insert having a
capacity of 100 ml and suitable for temperatures up to 200 °C;
5.3.3.5 Potentiometic titrator comprising a pH measuring cell and a metered volume of (50 ± 0,025) ml.
5.3.4 Sampling
Sample as described in 5.1.4.
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5.3.5 Procedure
5.3.5.1 Decomposition of sample under pressure
Weigh about 500 mg of the test sample, pre-treated as specified in 5.1.4, to the nearest 0,01 mg, transfer
this quantitatively to the polytetrafluoroethylene insert of the pressurized digestion apparatus and add 5
ml of water and 10 ml of hydrofluoric acid. Seal the apparatus as directed by the manufacturer and heat
(see 5.3.3.4). Ensure that the solution and sample are kept at (200 ± 5) °C for 4 to 6 hours. After cooling
and opening the apparatus, bind the excess hydrofluoric acid by adding 4,7 g of boric acid. Ignore any
dark particles occasionally left behind. Transfer the solution quantitatively to a 250 ml volumetric flask
and make up to the mark with water.
5.3.5.2 Standardization of titration acid
Weigh 200 mg of sodium carbonate to the nearest 0,01 mg into a sealable weighing bottle.
NOTE Dry the sodium carbonate at 270 to 300 °C for about one hour, stirring occasionally, and store it over
calcium chloride in a desiccator.
Dissolve the sodium carbonate in 50 ml of water and add the titration acid using a potentiometric titrator
apparatus until the equivalence point in the pH range of 4,1 to 4,3 is reached. Take the mean of not less
than 3 titrations. The coefficient of variation shall not exceed 0,001.
5.3.5.3 Distillation and potentiometric titration
Use a one-mark bulb pipette to pipette a 50 ml aliquot portion of the solution prepared as described in
clause 5.3.5.1 into an ammonia determination apparatus which has been steamed out. Pour 20 ml of
boric acid solution into the receiving vessel of the still and immerse the con
...
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