Introduction

In general, different pores (micro-, meso-, and macropores) can 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 is more precisely defined as the ratio of the volume of the accessible pores and voids to the total volume occupied by a given amount of the solid. In addition to the accessible pores, a solid can 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 can 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 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 properties;

⎯ polymers and ceramic.

It is well established that the performance of a porous solid (e.g. its strength, reactivity, permeability of adsorbent power) 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 appropriate diameter of 0,003 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 (2,0 nm to 100 nm), and is an extension of the surface area estimation technique;

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 2,0 nm, and is an extension of the surface area estimation technique.

Scope

This International Standard describes a method for the evaluation of the pore size distribution and the specific surface in pores of solids by mercury porosimetry according to the method of Ritter 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 psia) corresponding to a minimum equivalent pore diameter of approximately 0,003 μm. The maximum diameter will be 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 interparticle and intraparticle porosity. In general, it cannot distinguish between these porosities where they co-exist.

The method is suitable for the study of most non-wettable, by mercury, porous materials. Samples that amalgamate with mercury, such as certain metals, e.g. gold, aluminium, reduced copper, reduced nickel and silver, can be unsuitable for this technique or can require a preliminary passivation. Under the applied pressure, some materials are deformed, compacted or destroyed, whereby open pores can be collapsed and closed pores opened. In some cases, it is possible to apply sample compressibility corrections and useful comparative data can still be obtained. For these reasons, the mercury porosimetry technique is considered to be comparative.

Principles

A non-wettable liquid can enter a porous system only when forced by pressure. The pore size distribution of a porous solid can be determined by forcing mercury into an evacuated sample under increasing pressure and measuring the volume of mercury intruded as a function of pressure. The determination 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.

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, having a uniform bore capillary tube through which the sample can be evacuated and through which mercury can enter.

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 void volume of the sample. Since different materials exhibit a wide range of open porosities a number of sample holders with different diameter capillary tubes and sample volumes may be required. A special design of sample holder is often used with powdered samples to avoid loss of powder during evacuation.

6.2 Porosimeter, capable of carrying out the test as two sequential measurements, a low-pressure test up to at least 0,2 MPa (30 psia) and a high-pressure test up to the maximum operating pressure of the porosimeter [circa 400 MPa (60 000 psia)].

The porosimeter may have several ports for high- and low-pressure operations, 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 vacuum pump, equipped with mercury retainer, to a residual pressure of 7 Pa or less 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 mm3 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, of analytical quality, with a purity of at least ratio of 99,4 mass %.

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