There are a number of reasons to make measurements on blank samples (non-absorbing samples of low surface area, or empty sample cell). One of the main reasons is to validate the accuracy of experimental measurements at the temperatures and pressures that will be used for measurements on actual storage materials. A blank measurement under these conditions should give a quantity of sorbed gas that is very close to zero. The amount by which the measurement is non-zero is an indication of the accuracy of the measurement system and data analysis. The effective H2 uptake or release in these blank tests should be small relative to the uptake expected from the sample. If the expected sample uptake is very small at the measurement conditions (e.g. hydrogen spillover adsorption at room temperature and low pressures), then it is especially critical that the “blank” H2 uptake should be small. If the instrument volumes and pressure gauges are well calibrated, the void volume is well characterized, and the temperature gradients are properly accounted for, then H2 uptake in blank tests can be on the order of 100 μg.

blank sample testing at various temperatures in the range 0-50 atm

Another reason for doing blank sample measurements is to empirically characterize the response of the instrument to a set of measurement conditions. For example, it is possible to determine the equivalent room temperature gas volume of a sample cell that is being held at a temperature far from room temperature using a blank sample rather than the actual sample to be measured. While it is more typical to use inert gas (generally Helium) measurement on the real sample to determine an “apparent volume” there may be some concern that the sample itself will adsorb significant amounts of the inert gas causing large errors in the “apparent” void volume of the sample cell. This becomes an important concern for high-surface area materials being measured at low (cryogenic) temperatures where inert gas uptake may be large.

In such cases, the void volume of the sample cell, with the sample in it, is usually determined from helium expansion measurements at room temperature and low pressures (under 5 bar). Under these conditions the effect of helium adsorption is generally considered to be small enough to have little impact on the volume calibration. By using a blank sample measured with hydrogen under the same conditions as the sample will be measured a “Temperature Correction Factor” can be determined. These tests are run under identical conditions, but instead use inert, non-porous materials (e.g. stainless steel rods, silicon granules, glass beads) which have as close as possible the same gas displacement volume as the actual sample to be measured. The correction factor determined from these tests is the ratio of the apparent (at temperature) void volume of the cell containing the blank sample to the physical void volume of the cell containing the sample at room (or instrument) temperature. This ratio will be pressure and temperature dependent. Once this “Temperature Correction Factor” is determined using the blank sample, it can then be applied to correct the room (or instrument) temperature void volume value of the actual sample at equivalent experimental temperatures and pressures. This correction then compensates for the temperature induced density change in the gas in different isothermal and gradient sections of the instrument to aid in calculating the actual amount of hydrogen ad/absorbed by the sample at low temperatures.