ISO 9277:2022
(Main)Determination of the specific surface area of solids by gas adsorption — BET method
Determination of the specific surface area of solids by gas adsorption — BET method
This document specifies the determination of the overall specific external and internal surface area of either disperse (e.g. nano-powders) or porous, solids by measuring the amount of physically adsorbed gas according to the method of Brunauer, Emmett and Teller method,[1] based on the 2015 International Union for Pure and Applied Chemistry (IUPAC) recommendations[3]. NOTE For solids exhibiting a chemically heterogeneous surface, for example, metal-carrying catalyst, the BET method gives the overall surface area, whereas the metallic portion of the surface area can be measured by chemisorption methods.
Détermination de l'aire massique (surface spécifique) des solides par adsorption de gaz — Méthode BET
General Information
Relations
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 9277
Third edition
2022-11
Determination of the specific surface
area of solids by gas adsorption —
BET method
Détermination de l'aire massique (surface spécifique) des solides par
adsorption de gaz — Méthode BET
Reference number
ISO 9277:2022(E)
© ISO 2022
---------------------- Page: 1 ----------------------
ISO 9277:2022(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
© ISO 2022 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 9277:2022(E)
Contents Page
Foreword .iv
Introduction .v
1 S c op e . 1
2 Nor m at i ve r ef er enc e s . 1
3 T erms and definitions . 1
4 S y mb ol s . 3
5 P r i nc iple . 3
6 P rocedure .4
6.1 S ample preparation . 4
6.2 E xperimental conditions . 7
6.3 M easuring methods for the assessment of the amount of adsorbed gas . 8
6.3.1 G eneral . 8
6.3.2 Static manometric (volumetric) method . 8
6.3.3 Flow manometric (volumetric) method . . 8
6.3.4 Gravimetric method . 9
6.3.5 Carrier gas method . . 10
6.3.6 Dynamic vapour sorption method . 11
7 E valuation of adsorption data .11
7.1 G eneral . 11
7.2 M ultipoint determination .12
7.3 Si ngle-point determination . 13
8 Te s t r ep or t .14
9 U se of reference materials .14
Annex A (informative) Cross-sectional areas of some frequently used adsorptives .15
Annex B (informative) BET area of microporous materials .16
Bibliography .21
iii
© ISO 2022 – All rights reserved
---------------------- Page: 3 ----------------------
ISO 9277:2022(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 24, Particle characterization including
sieving, Subcommittee SC 4, Particle characterization.
This third edition cancels and replaces the second edition (ISO 9277:2010), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— the IUPAC classification of adsorption isotherms has been updated according to Reference [3];
— the description of dynamic vapour sorption (DVS) method in 6.3.6 has been added;
— Annex A has been revised;
— Annex B has been removed;
— the former Annex C (now Annex B) has been revised.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
© ISO 2022 – All rights reserved
---------------------- Page: 4 ----------------------
ISO 9277:2022(E)
Introduction
Gas adsorption allows for assessing surface area of nonporous materials and porous materials with
accessible porosity (which depends on the chosen adsorptive) via the Brunauer, Emmett and Teller
[1],[2]
(BET) theory . The BET theory is applicable only to adsorption isotherms of type II (disperse,
nonporous or macroporous solids) and type IV (mesoporous solids) (see Figure 1, Type II and Type IVa
isotherms). However, in the case of Type IVb isotherms, caution is required since pore condensation can
[3]
occur at quite low p/p (see IUPAC recommendations ). The BET method cannot reliably be applied to
0
solids which absorb the measuring gas. A strategy for BET area determination of microporous materials
(type I isotherms) is described in Annex B.
Key
X relative pressure
Y amount adsorbed
[2]
SOURCE IUPAC Recommendations, 1994. Reproduced with the permission of the authors.
Figure 1 — IUPAC (2015) classification of adsorption isotherms
v
© ISO 2022 – All rights reserved
---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 9277:2022(E)
Determination of the specific surface area of solids by gas
adsorption — BET method
1 S cope
This document specifies the determination of the overall specific external and internal surface area of
either disperse (e.g. nano-powders) or porous, solids by measuring the amount of physically adsorbed
[1]
gas according to the method of Brunauer, Emmett and Teller method, based on the 2015 International
[3]
Union for Pure and Applied Chemistry (IUPAC) recommendations .
NOTE For solids exhibiting a chemically heterogeneous surface, for example, metal-carrying catalyst, the
BET method gives the overall surface area, whereas the metallic portion of the surface area can be measured by
chemisorption methods.
2 Normat ive 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 8213, Chemical products for industrial use — Sampling techniques — Solid chemical products in the
form of particles varying from powders to coarse lumps
ISO 14488, Particulate materials — Sampling and sample splitting for the determination of particulate
properties
3 T erms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
adsorption
enrichment of the adsorptive gas at the external and accessible internal surfaces of a solid material
3.2
physisorption
weak bonding of the adsorbate (3.2), reversible by small changes in pressure or temperature
3.3
adsorbate
adsorbed gas
3.4
adsorptive
gas or vapour to be adsorbed
3.5
adsorbent
solid material on which adsorption (3.1) occurs
1
© ISO 2022 – All rights reserved
---------------------- Page: 6 ----------------------
ISO 9277:2022(E)
3.6
isotherm
relationship between the amount of gas adsorbed and the equilibrium pressure of the gas, at constant
temperature
3.7
volume adsorbed
volumetric equivalent of adsorbed amount (3.8) expressed as gas at standard conditions of temperature
and pressure (STP)
3.8
adsorbed amount
number of moles of gas adsorbed at a given pressure and temperature
3.9
monolayer capacity
volumetric equivalent of monolayer amount expressed as gas at standard conditions of temperature
and pressure
3.10
surface area
area of the external surface of a solid plus the internal surface of its accessible macro-, meso- and
micropores (3.16)
3.11
specific surface area
absolute surface area (3.10) of the sample divided by sample mass
3.12
molecular cross-sectional area
molecular area of the adsorbate (3.2), i.e. the area occupied by an adsorbate molecule in the complete
monolayer
3.13
nanopore
pore with width of about 100 nm or less
3.14
macropore
pore with width greater than approximately 50 nm
3.15
mesopore
pore with width between approximately 2 nm and 50 nm
3.16
micropore
pore with width of approximately 2 nm or less
3.17
relative pressure
ratio of the equilibrium adsorption (3.1) pressure, p, to the saturation vapour pressure (3.18), p , at
0
analysis temperature
3.18
saturation vapour pressure
vapour pressure of the bulk liquefied adsorptive gas at the temperature of adsorption (3.1)
2
© ISO 2022 – All rights reserved
---------------------- Page: 7 ----------------------
ISO 9277:2022(E)
3.19
free space
head space
dead space
dead volume
volume of the sample holder not occupied by the sample
4 Symbols
Table 1 presents the symbols used in this document, together with their common units derived from
the SI. For comparison purposes, the lUPAC symbols are also given. All specific dimensions are related
to sample mass in grams.
Table 1 — Symbols
Symbol Quantity Unit
2
a molecular cross-sectional area nm
m
2 −1
specific surface area m g
a
s
a
C BET parameter 1
23 −1
L Avogadro constant (L = 6,022 × 10 ) mol
m mass of the solid sample g
a
specific mass adsorbed 1
m
a
−1
n specific amount adsorbed mol·g
a
−1
specific monolayer capacity of adsorbate mol g
n
m
−1
n specific monolayer capacity derived from multipoint measurement mol g
m,mp
−1
specific monolayer capacity derived from single-point measurement mol g
n
m,sp
p pressure of the adsorptive in equilibrium with the adsorbate Pa
p saturation vapour pressure of the adsorptive Pa
0
a
p/p relative pressure of the adsorptive 1
0
−1 −1
R molar gas constant (= 8,314) J mol K
r radius of uniform nonporous spheres nm
s
t time min
T temperature K
3 −1
V specific volume adsorbed cm g
a
3 −1
V specific micropore volume cm g
p,micro
−3
ρ (mass) density g cm
2 −1
u combined standard uncertainty for the certified specific surface area of a BET refer- m g
c
ence material
a
k coverage factor for the combined standard uncertainty 1
2 −1
U expanded uncertainty (U = k u ) for the certified specific surface area of a BET refer- m g
c
ence material
a
According to ISO 80000-1, the unit for any quantity of dimension one (at present commonly termed “dimensionless”) is
the unit one, symbol 1.
5 Principle
The method specified involves the determination of the amount of adsorbate or adsorptive gas required
to cover the external and the accessible internal pore surfaces of a solid (see Figure 2) with a complete
monolayer of adsorbate. This monolayer capacity can be calculated from the adsorption isotherm using
Formula (1) (see 7.1). Any gas may be used, provided it is physically adsorbed by weak bonds at the
3
© ISO 2022 – All rights reserved
---------------------- Page: 8 ----------------------
ISO 9277:2022(E)
surface of the solid (van der Waals forces) and can be desorbed by a decrease in pressure at the same
temperature.
Figure 2 — Schematic cross-section of a particle with surface detected
by the adsorption method shown by dotted line
Nitrogen at its boiling point (about 77 K) was for many decades the adsorptive generally used for
the determination of the specific surface area, mainly because liquid nitrogen was readily available
and relatively strong attractive adsorptive-adsorbent interactions for many systems. However, due
to nitrogen’s quadrupole moment the orientation of a nitrogen molecule is affected by the surface
chemistry of the adsorbent. This leads to uncertainties in the surface area determination by nitrogen
in the order of approximately 20 % for some surfaces. However, argon at 87 K is a great alternative
because argon does, contrary to the diatomic nitrogen molecule, not exhibit a quadrupole moment.
Hence, argon adsorption is less sensitive to the surface chemistry leading to a much more reliable
surface area determination for many adsorbent surfaces. If the sensitivity of the instrument when
2 −1
using argon or nitrogen is insufficient for low specific surface areas of about 1 m g or lower, the
application of krypton adsorption at liquid nitrogen temperature for the specific surface area analysis
is recommended. As a consequence of the low p of about 0,35 kPa for krypton at 77 K, the 'free
0
space' correction (see 3.19) for unadsorbed gas is significantly reduced (to 1/300th) compared to the
conditions of nitrogen adsorption at the same temperature and it becomes possible to manometrically
measure low uptakes of adsorptive with acceptable accuracy. Although at 77 K krypton is about
38,5 K below its triple point temperature, there is some evidence from microcalorimetry and neutron
diffraction studies that in the BET region, the adsorbate may well be in a liquid-like state and therefore
the value of the supercooled liquid is recommended as the effective p for the construction of the BET
0
plot.
The results of measurements with different adsorptives may deviate from each other because of
different molecular areas, different accessibilities to pores (classified into micro-, meso-, macro- and
nanopores) and different measuring temperatures. Moreover, it is well known from the concepts of
fractal analysis that experimental results for the quantities of length and area in the case of irregular
complex structures – such as those which are found in most porous and/or highly dispersed objects –
are not absolute but depend on the measurement scale i.e. the “yardstick” used. This means that less
area is available for larger adsorbate molecules.
The adsorptive gas is admitted to the sample container, which is held at a constant temperature.
The amounts adsorbed are measured in equilibrium with the adsorptive gas pressure p and plotted
against relative pressure, p/p , to give an adsorption isotherm. Adsorption isotherms may be obtained
0
by manometric (volumetric), by gravimetric or by the carrier gas method using continuous or
discontinuous operation (see 6.3).
6 Pr ocedure
6.1 S ample preparatio n
Sampling shall be carried out in accordance with ISO 8213 and ISO 14488. Prior to the determination
of an adsorption isotherm, remove physically adsorbed material from the sample surface by degassing,
while avoiding irreversible changes to the surface. Ascertain the maximum temperature at which
4
© ISO 2022 – All rights reserved
---------------------- Page: 9 ----------------------
ISO 9277:2022(E)
the sample is not affected by thermogravimetric analysis (see Figure 3), by spectroscopic methods,
or by trial experiments using different degassing conditions of time and temperature. When vacuum
conditions are used, degassing to a residual pressure of approximately 1 Pa or better is usually
sufficient. Degassing of the sample can also be performed at elevated temperature by flushing with an
inert gas (e.g. helium, nitrogen, argon). Degassing is complete when a steady value of the residual gas
pressure, p, of its composition or of the sample mass is reached.
Key
X degassing time 1 sample
Y sample mass 2 vacuum generating system
T temperature too low: long degassing time 3 balance
1
T optimum temperature 4 oven
2
T temperature too high: gas evolution due to
3
decomposition of the sample
Figure 3 — Thermogravimetric control of degassing
Using the vacuum technique, isolate the heated sample container from the pump and trap (at time t in
i
Figure 4). If the pressure is nearly constant over a period of 15 min to 30 min, degassing is complete.
This procedure also establishes the absence of leaks. The specific surface area should be related to the
mass of the degassed sample.
5
© ISO 2022 – All rights reserved
---------------------- Page: 10 ----------------------
ISO 9277:2022(E)
Key
X time p (t) leak
3
Y pressure 1 sample
t time of sample isolation 2 vacuum generating system
1
p (t) degassing complete, apparatus tight 3 manometer
1
p (t) incomplete degassing 4 oven
2
Figure 4 — Pressure control of degassing
After degassing, the sample container is cooled to the measuring temperature. It should be noted that,
at low gas pressures, the temperature of the sample needs some time to equilibrate due to the reduced
thermal conductivity within the sample cell.
6
© ISO 2022 – All rights reserved
---------------------- Page: 11 ----------------------
ISO 9277:2022(E)
Key
X time p fixed pressure limit
L
Y1 temperature 1 pressure curve
Y2 pressure 2 temperature curve
Figure 5 — Pressure controlled heating
For sensitive samples, a pressure-controlled heating (see Figure 5) is recommended. This procedure
consists in varying the heating rate in relationship to the gas pressure evolved from a porous material
during the degassing under vacuum conditions. When a fixed pressure limit p (usually around 7 Pa
L
to 10 Pa) is overtaken due to the desorbed material from the sample surface, the temperature increase
is stopped and the temperature is kept constant until the pressure falls below the limit, at that point
the system continues the temperature ramp. This procedure is particularly suitable to avoid structural
changes in microporous materials when fast heating rates can damage fragile structures due to a
vigorous vapour release. In addition, the method is very safe in preventing sample elutriation when
water or other vapour are released from the pores in very fine powder materials.
6.2 Experimental conditions
The precision of the measurement depends on the control of the following conditions.
a) The temperature or the p value of the adsorptive should be monitored during the analysis.
0
b) The purity of the adsorptive and any helium used to calibrate volumes or as a carrier gas should be
at least 99,999 %. If necessary, the gases should be dried and cleaned, for example, oxygen removed
from nitrogen.
c) The saturation vapour pressure p of the adsorptive at the measuring temperature can either be
0
determined directly using a nitrogen vapour pressure thermometer, or it can be monitored and
determined by measurement of the thermostat bath temperature.
d) The validity of the result depends on careful sampling and sample preparation.
In the discontinuous static procedure, at least three points within the relative pressure range for which
the BET equation is valid (typically 0,05 to 0,3) should be measured in equilibrium. For continuous
measurements, the deviation from equilibrium must be controlled either by occasional interruption of
the gas flow or by control measurements using the discontinuous method.
7
© ISO 2022 – All rights reserved
---------------------- Page: 12 ----------------------
ISO 9277:2022(E)
6.3 Measuring method s for the assessment of the amount of adsorbed gas
6.3.1 General
The various types of apparatus used for the determination of physisorption isotherms may be divided
into manometric and gravimetric methods, whereby static or dynamic techniques may be used in either
case. The manometric method is generally considered the most suitable technique for undertaking
physisorption measurements with nitrogen, argon and argon at cryogenic temperatures (i.e. 77 K and
87 K, the boiling point of nitrogen and argon respectively).
Gravimetric adsorption techniques are especially convenient for measurements with vapours (e.g.
water vapour or some organic adsorptives) at temperatures not too far removed from ambient. At low
temperatures (in particular at cryogenic temperatures), however, it can become difficult to control
convection effects and to measure the exact temperature of the adsorbent.
6.3.2 Static manometric (volumetric) method
In the static manometric method, a known amount of gas is admitted to a sample bulb thermostated
at the adsorption temperature (see Figure 6). Adsorption of the gas onto the sample occurs, and
the pressure in the confined volume continues to fall until the adsorbate and the adsorptive are in
equilibrium. The amount of adsorbate at the equilibrium pressure is the difference between the amount
of gas admitted and the amount of adsorptive remaining in the gas phase. Measurement of the pressure
is required together with temperatures and volumes of the system. The volumes are most easily
determined by gas expansion of an inert gas such as helium. The free space volume must be determined
before or after the measurement of the adsorption isotherm. The calibration of the volumes of the
system is done manometrically using helium at the measuring temperature. It should be noted that some
materials can adsorb and/or absorb helium. In this case, corrections can be made after measuring the
adsorption isotherm. If the measurement of the free space volume can be separated from the adsorption
measurement, the use of helium can be avoided. The void volume of the empty sample cell is measured
at ambient temperature using nitrogen. Subsequently, a blank experiment (with the empty sample cell)
is performed under the same experimental conditions (temperature and relative pressure range) as
the sorption measurements. The required correction for the sample volume is made by entering the
sample density or by pycnometric measurement with nitrogen at ambient temperature at the start of
the adsorption analysis (in case nitrogen adsorption effects can be neglected). The determination of the
free space volume may be avoided using difference measurements, that is, by means of a reference and
sample tube connected by a differential transducer. During sample measurement and determination
of the dead volume, it is recommended that the liquid level in the cooling bath be maintained, unless
otherwise compensated, at least 15 mm above the sample and constant to within 1 mm.
6.3.3 Flow manometric (volumetric) method
The flow manometric method is similar to the static manometric method, the difference is such that
gas is continuously fed to the sample at a relatively low flow rate rather than the series of batch doses.
By this method, the change in cell pressure is continuously measured at a controlled flow rate. The
adsorbed amount is determined by comparing the rate of pressure rise with the adsorptive gas to
that with a non-adsorbing gas such as helium used for calibration. An alternative approach, which
directly determines the quantity of gas adsorbed, is to measure the pressure difference between the
reference and sample tubes at the same volume of adsorptive gas fed. One volume is connected via a
flow control valve to the sample tube while the other volume is similarly connected to a balance tube
without sample. Because gas is continuously introduced, the analysis can, under some circumstances,
be completed in a shorter period than using a static manometric method. However, special care has to
be taken to ensure that the flow rate is low enough so that the adsorptive and adsorbate are close to
equilibrium at all times. Determination of the dead volume and control of the liquid level in the cooling
bath are recommended using the same requirements as described in 6.3.1.
8
© ISO 2022 – All rights reserved
---------------------- Page: 13 ----------------------
ISO 9277:2022(E)
6.3.4 Gravimetric method
In the continuous gravimetric method, the mass of gas adsorbed is measured as a function of the
gas pressure in the sample container using a microbalance (see Figure 7). Before measurement of
the adsorption isotherm, the buoyancy of the balance in the adsorptive should be measured at room
temperature and at measurement temperature. The buoyancy of the sample should be measured
at room temperature in helium. By using an equal-arm instrument, the buoyancy of the balance is
eliminated and that of the sample compensated for using a compact counterweight of the same density
as the sample. Because the sample is not in contact with the thermostat, it is necessary to ensure that it
is always at the measuring temperature. The sample temperature should be monitored.
−2
The zero point of the isotherm should be measured at a vacuum better than 10 Pa because disturbances
due to thermal gas flow exhibit a maximum at about 1 Pa.
In the discontinuous gravimetric procedure, the adsorptive is admitted stepwise and the pressure is
kept constant until the sample mass reaches a constant value.
Key
1 sample 5 calibrated volume (gas burette)
2 dewar vessel with cooling bath 6 saturation pressure tube
3 vacuum-generating system 7 adsorptive
4 manometer 8 gas for dead space measurement (e.g. He)
Figure 6 — Manometric (volumetric) method
9
© ISO 2022 – All rights reserved
---------------------- Page: 14 ----------------------
ISO 9277:2022(E)
Key
1 sample 4 manometer
2 cooling bath 5 balance
3 vacuum-generating system 6 adsorptive
Figure 7 — Gravimetric method
6.3.5
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.