ISO 15901-1:2016

INTERNATIONAL ISO
STANDARD 15901-1
Second edition
2016-04-01
Evaluation of pore size distribution
and porosity of solid materials
by mercury porosimetry and gas
adsorption —
Part 1:
Mercury porosimetry
Evaluation de la distribution de taille des pores et la porosité des
matériaux solides par porosimétrie à mercure et l’adsorption des gaz —
Partie 1: Porosimétrie à mercure
Reference number
©
ISO 2016
© ISO 2016, Published in Switzerland
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ii © ISO 2016 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 4
5 Principles . 5
6 Apparatus and material . 6
6.1 Sample holder . 6
6.2 Porosimeter . 7
6.3 Mercury . 7
7 Procedures for calibration and performance . 7
7.1 General . 7
7.2 Pressure signal calibration . 7
7.3 Volume signal calibration . 7
7.4 Vacuum transducer calibration . 7
7.5 Verification of porosimeter performance . 8
8 Procedures . 8
8.1 Sampling . 8
8.1.1 Obtaining a test sample . . 8
8.1.2 Quantity of sample . 8
8.2 Method . 9
8.2.1 Sample pre-treatment . 9
8.2.2 Filling of the sample holder and evacuation . 9
8.2.3 Filling the sample holder with mercury . 9
8.2.4 Measurement .10
8.2.5 Completion of test .10
8.2.6 Blank and sample compression correction . .10
9 Evaluation .11
9.1 Determination of the pore size distribution .11
9.2 Determination of the specific pore volume .11
9.3 Determination of the specific surface area .12
9.4 Determination of the bulk and skeleton densities .12
9.5 Determination of the porosity .13
10 Reporting .13
Annex A (informative) Mercury porosimetry analysis results .14
Annex B (informative) Recommendations for the safe handling of mercury .17
Bibliography .19
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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different types of ISO documents should be noted. This document was drafted in accordance with the
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The committee responsible for this document is ISO/TC 24, Particle characterization including sieving,
Subcommittee SC 4, Particle characterization.
This second edition cancels and replaces the first edition (ISO 15901-1:2005), which has been technically
revised. It also incorporates the Corrigendum ISO 15901-1:2005/Cor 1:2007.
ISO 15901 consists of the following parts, under the general title Evaluation of pore size distribution and
porosity of solid materials by mercury porosimetry and gas adsorption:
— Part 1: Mercury porosimetry
— Part 2: Analysis of mesopores and macropores by gas adsorption
— Part 3: Analysis of micropores by gas adsorption
iv © ISO 2016 – All rights reserved

Introduction
In general, different pores (micro-, meso-, and macropores) may be pictured as either apertures,
channels or cavities within a solid body or as space (i.e. interstices or voids) between solid particles
in a bed, compact or aggregate. Porosity is a term which is often used to indicate the porous nature of
solid material and in this International Standard is more precisely defined as the ratio of the total pore
volume of the accessible pores and voids to the volume of the particulate agglomerate. In addition to
the accessible pores, a solid may contain closed pores which are isolated from the external surface and
into which fluids are not able to penetrate. The characterization of closed pores is not covered in this
International Standard.
Porous materials may take the form of fine or coarse powders, compacts, extrudates, sheets or
monoliths. Their characterization usually involves the determination of the pore size distribution as
well as the total accessible pore volume or porosity. For some purposes it is also necessary to study the
pore shape and interconnectivity and to determine the internal and external specific surface area.
Porous materials have great technological importance, for example in the context of the following:
— controlled drug release;
— catalysis;
— gas separation;
— filtration including sterilization;
— materials technology;
— environmental protection and pollution control;
— natural reservoir rocks;
— building materials;
— polymers and ceramic.
It is well established that the performance of a porous solid (e.g. its strength, reactivity, permeability) is
dependent on its pore structure. Many different methods have been developed for the characterization
of pore structure. In view of the complexity of most porous solids, it is not surprising that the results
obtained are not always in agreement and that no single technique can be relied upon to provide a
complete picture of the pore structure. The choice of the most appropriate method depends on the
application of the porous solid, its chemical and physical nature and the range of pore size.
The most commonly used methods are as follows:
a) Mercury porosimetry, where the pores are filled with mercury under pressure. This method is
suitable for many materials with pores in the approximate diameter range of 0,004 µm to 400 µm.
b) Meso- and macropore analysis by gas adsorption, where the pores are characterized by adsorbing
a gas, such as nitrogen at liquid nitrogen temperature. The method is used for pores in the
approximate diameter range of 0,002 µm to 0,1 µm (2 nm to 100 nm).
c) Micropore analysis by gas adsorption, where the pores are characterized by adsorbing a gas,
such as nitrogen at liquid nitrogen temperature. The method is used for pores in the approximate
diameter range of 0,4 nm to 2 nm.
INTERNATIONAL STANDARD ISO 15901-1:2016(E)
Evaluation of pore size distribution and porosity of solid
materials by mercury porosimetry and gas adsorption —
Part 1:
Mercury porosimetry
WARNING — The use of this International Standard may involve hazardous materials,
operations and equipment. This International Standard does not purport to address all of the
safety problems associated with its use. It is the responsibility of the user of this International
Standard to establish appropriate safety and health practices and determine the applicability of
regulatory limitations prior to use.
1 Scope
This International Standard describes a method for the evaluation of the pore size distribution and
the specific surface area of pores in solids by mercury porosimetry according to the method of Ritter
[1][2]
and Drake . It is a comparative test, usually destructive due to mercury contamination, in which
the volume of mercury penetrating a pore or void is determined as a function of an applied hydrostatic
pressure, which can be related to a pore diameter.
Practical considerations presently limit the maximum applied absolute pressure to about 400 MPa
(60 000 psi) corresponding to a minimum equivalent pore diameter of approximately 4 nm. The
maximum diameter is limited for samples having a significant depth due to the difference in hydrostatic
head of mercury from the top to the bottom of the sample. For the most purposes, this limit can be
regarded as 400 µm. The measurements cover inter-particle and intra-particle porosity. In general,
without additional information from other methods it is difficult to distinguish between these porosities
where they co-exist. The method is suitable for the study of most porous materials non-wettable by
mercury. Samples that amalgamate with mercury, such as certain metals, e.g. gold, aluminium, copper,
nickel and silver, can be unsuitable with this technique or can require a preliminary passivation. Under
the applied pressure some materials are deformed, compacted or destroyed, whereby open pores may
be collapsed and closed pores opened. In some cases it may be possible to apply sample compressibility
corrections and useful comparative data may still be obtainable. For these reasons, the mercury
porosimetry technique is considered to be comparative.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 3165, Sampling of chemical products for industrial use — Safety in sampling
ISO 14488, Particulate materials — Sampling and sample splitting for the determination of particulate
properties
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
porosimeter
instrument for measuring pore volume and pore size distribution
3.2
porosimetry
methods for the estimation of pore volume, pore size distribution, and porosity
3.3
porous solid
solid with cavities or channels which are deeper than they are wide
3.4
powder
porous or nonporous solid composed of discrete particles with maximum dimension less than about
1 mm, powders with a particle size below about 1 µm are often referred to as fine powders
3.5
pore
cavity or channel which is deeper than it is wide, otherwise it is part of the material’s roughness
3.6
void
interstice
space between particles, i.e. interparticle pore
3.7
macropore
pore of internal width greater than 50 nm
3.8
mesopore
pore of internal width between 2 nm and 50 nm
3.9
micropore
pore of internal width less than 2 nm
3.10
closed pore
pore totally enclosed by its walls and hence not interconnecting with other pores and not accessible
to fluids
3.11
open pore
pore not totally enclosed by its walls and open to the surface either directly or by interconnecting with
other pores and therefore accessible to fluid
3.12
ink bottle pore
narrow necked open pore
3.13
pore size
internal pore width (for example, the diameter of a cylindrical pore or the distance between the
opposite walls of a slit) which is a representative value of various sizes of vacant space inside a porous
material
3.14
pore volume
volume of open pores unless otherwise stated
2 © ISO 2016 – All rights reserved

3.15
pore diameter
diameter of a pore in a model in which the pores typically are assumed to be cylindrical in shape and
which is calculated from data obtained by a specified procedure
3.16
median pore diameter
diameter that corresponds to the 50th percentile of pore volume, i.e. the diameter for which one half of
the pore volume is found to be in larger pores and one half is found to be in smaller pores
3.17
modal pore diameter
mode
pore diameter of the maximum in a differential pore size distribution curve
3.18
hydraulic pore diameter
average pore diameter, calculated as the ratio of pore volume multiplied by four to pore area.
3.19
bulk volume
volume of powder or solids, including all pores (open and closed) and interstitial spaces between
particles.
3.20
bulk density
ratio of sample mass to bulk volume
3.21
skeleton volume
volume of the sample including the volume of closed pores (if present) but excluding the volumes of
open pores as well as that of void spaces between particles within the bulk sample
[SOURCE: ISO 12154]
3.22
skeleton density
ratio of sample mass to skeleton volume
3.23
apparent volume
total volume of the solid constituents of the sample including closed pores and pores inaccessible or not
detectable by the stated method;
3.24
apparent density
ratio of sample mass to apparent volume
3.25
envelope volume
total volume of the particle, including closed and open pores, but excluding void space between the
individual particles
3.26
envelope density
ratio of sample mass to envelope volume
3.27
porosity
ratio of the volume of the accessible pores and voids to the bulk volume occupied by an amount of the solid
3.28
interparticle porosity
ratio of the volume of void space between the individual particles to the bulk volume of the particles
or powder
3.29
intraparticle porosity
ratio of the volume of open pores inside the individual particles of a particulate or divided solid sample
to the bulk volume occupied by the sample
3.30
surface area
extent of accessible surface area as determined by a given method under stated conditions
3.31
surface tension
work required to increase a surface area divided by that area.
3.32
contact angle
angle at which a liquid/vapour interface meets the surface of a solid material
4 Symbols and abbreviated terms
For the purposes of this document, the following symbols apply.
Symbol Term SI unit Derived and obso- Conversion factors
lete units
P pressure Pa MPa, psia, 1 MPa = 10 Pa
−2
Torr, mmHg 1 psi = 1 lb in = 6 895 Pa
1 Torr = 1 mmHg = 133,32 Pa
−9 −6
1nm = 10 m, 1µm = 10 m,
d pore diameter m nm, µm, Å
p
−10
1 Å = 10 m
t time s h 1 h = 3 600 s
2 −1 2 −1
S specific surface area m ·kg m ·g
intruded volume of
3 3 3 3 3 3 −6 3
V m cm , mm 10 mm = 1 cm = 10 m
Hg
mercury
initial intruded volume of
3 3 3
V m cm , mm
Hg,0
mercury
total intruded volume of
3 3 3
V m cm , mm
Hg,max
mercury
3 −1
mm ·g
3 . −1
V specific pore volume m kg
p
3 −1
cm ·g
−1 −1 −1 −3 −1
γ surface tension of mercury N·m dyne·cm 1 dyne·cm = 10 N·m
−3 −3 −3 3 −3
ρ density of mercury kg·m g·cm 1 g·cm = 10 kg·m
Hg
contact angle of mercury at
Θ rad ° 1° = (π/180) rad
the sample
m mass of the test sample kg g
S
m mass of empty sample holder kg g
SH
mass of sample holder with
m kg g
SH+S
sample
mass of sample holder with
m sample and filled with mer- kg g
SH+S+Hg
cury
4 © ISO 2016 – All rights reserved

Symbol Term SI unit Derived and obso- Conversion factors
lete units
3 3
V bulk volume m cm
B
3 3
V skeleton volume m cm
S
3 3
V volume of sample holder m cm
SH
−3 −3
ρ bulk density kg·m g·cm
B
−3 −3
ρ skeleton density kg·m g·cm
S
ε porosity —
5 Principles
Mercury porosimetry is a widely accepted method for pore size analysis of various materials such as
pharmaceutical tablets, building materials, catalysts and their supports, mainly because it allows pore
size/porosity analysis to be undertaken over a wide range of pore sizes from meso- to macropores with
[1]–[7]
pore widths about 0,004 µm to about 400 µm . In contrast to capillary condensation, where the
pore fluid wets the pore walls (i.e. the contact angle is smaller than 90 degrees), mercury porosimetry
describes a non-wetting situation (i.e. the contact angle is greater than 90 degrees) and therefore
pressure must be applied to force mercury into the pores. Thus, a progressive increase in hydrostatic
pressure is applied to enable the mercury to enter the pores in decreasing order of width. Accordingly,
there is an inverse relationship between the applied pressure, p, and the pore diameter, d , which in the
p
simplest case of cylindrical pores is given by the Washburn equation (see 9.1).
In the application of mercury porosimetry, the volume of mercury entering the pore structure is
measured as the applied pressure is gradually increased. The value V at the applied pressure, p, gives
Hg
the cumulative volume of all available pores of diameter equal to, or greater than, d . The determination
p
may proceed either with the pressure being raised in a step-wise manner and the volume of mercury
intruded measured after an interval of time when equilibrium has been achieved, or by raising the
pressure in a continuous (progressive) manner.
Figure 1 shows two intrusion/extrusion cycles of mercury into a porous powder as a function of
pressure. Region (a) corresponds to a re-arrangement of particles within the powder bed, followed
by intrusion of the interparticle voids (b). Filling of intraparticle pores occurs in the region (c) and
for some materials (reversible) compression is then possible at higher pressures (d). Hysteresis (h)
is observed and extrusion (e) occurs at different pressures than for the intrusion. On completion of a
first intrusion-extrusion cycle, usually some mercury is retained by the sample, thereby preventing the
loop from closing (f). Intrusion-extrusion cycles after the first cycle continue to show hysteresis (g) but
eventually the loop closes, showing that there is no further entrapment of mercury. On most samples,
entrapment is not observed anymore after just the second cycle, which also indicates that hysteresis
and entrapment are essentially of different origin.
Key
1 powder compression
2 interparticle filling
3 intraparticle filling
Y intruded mercury, V
Hg
X hydraulic pressure, lg p
Figure 1 — Characteristic features of mercury porosimetry curves
The hysteresis and entrapment phenomena is undoubtedly important in order to obtain a comprehensive
pore size analysis. Mercury entrapment appears to be caused by the rupture of mercury bridges in pore
constrictions during extrusion from ink-bottle pores. Different mechanisms have been proposed to
[6]–[12]
explain intrusion/extrusion hysteresis . The single pore mechanism implies that hysteresis can
be understood as an intrinsic property of the intrusion/extrusion process due to nucleation barriers
associated with the formation of a vapour-liquid interface during extrusion, or discussed in terms
of differences in advancing and receding contact angles. In contrast, the network models take into
account the ink-bottle and percolation effects in pore networks. It is now generally accepted that pore
blocking effects, which can occur on the intrusion branch, are similar to the percolation effects involved
in the desorption of gases from porous networks. Indeed, the shape of a mercury intrusion/extrusion
[9][10]
hysteresis loop often agrees well with that of the corresponding gas adsorption loop . Thus,
mercury intrusion and the capillary evaporation appear to be based on similar mechanisms. The
pore blocking/percolation effects are dominant in disordered pore networks, and a reliable pore size
distribution can only be calculated from the intrusion branch by applying complex network models,
based on percolation theory. The application of such models also allows one to obtain a limited amount
[11][12].
of structural information from the intrusion/extrusion hysteresis loop
6 Apparatus and material
WARNING — It is important that proper precautions for the protection of laboratory personnel
are taken when mercury is used. Attention is drawn to the relevant regulations and guidance
documents which appertain for the protection of personnel in each of the member countries.
6.1 Sample holder
The sample holder may consist of a vessel with a uniform bore capillary tube through which the sample
can be evacuated and the vessel filled with mercury. The capillary tube is attached to a wider bore
tube in which the test sample is located. If precise measurements are required, the internal volume of
the capillary tube should be between 20 % and 90 % of the expected pore and interparticle volume of
the sample. Since different materials exhibit a wide range of open porosities a number of sample vessel
holders with different tube diameters and vessel volumes is required. A special design of sample holder
is often used with powdered samples to avoid loss of powder during evacuation.
6 © ISO 2016 – All rights reserved

In order to evaluate the porosity and the bulk and skeleton densities, the volume of the sample holder,
including the capillary tube, must be known.
6.2 Porosimeter
An instrument capable of carrying out the test at two sequential measurements, a low pressure test
up to at least 0,2 MPa (30 psi) and a high-pressure test up to the maximum operating pressure of the
porosimeter [circa 400 MPa (60 000 psi)].
The porosimeter may have several ports for high and low pressure operation, or the low pressure test
may be carried out on a separate unit.
Prior to any porosimetry measurement it is necessary to evacuate the sample using a typical rotary
vacuum pump, equipped with a mercury retainer and then to fill the sample holder with mercury to a
given low pressure. A means of generating pressure is necessary to cause intrusion of mercury.
A means of detecting the change in the volume of mercury intruded to a resolution of 1 mm or less is
desirable. This is usually done by measuring the change in capacitance between the mercury column in
the capillary tube and a metal sleeve around the outside of the sample holder.
6.3 Mercury
Mercury in analytical quality should be used for the measurements (at least a mass ratio of
...