GB/T 2922-1982 Determination of specific surface area of chromatographic carriers for chemical reagents
Some standard content:
National Standard of the People's Republic of China
Chemical reagents
Determination of specific surface area of solid supports used in chromatography
Chemical reagents
Determination of specific surface area of solid supports used in chromatography This standard is applicable to the determination of specific surface area of chromatographic supports. The determination range of this method is 0.5～1000m2/g. Basic principle
UDC 543.544
.25 — 42
GB 2922 —82
This method is based on the BET multilayer adsorption principle and uses continuous flow gas chromatography to determine the specific surface area. The BET isotherm equation is as follows,
PN,/P.
V.(1- Pn,/P.)
In the formula: PN, equilibrium pressure of adsorbate, mmHgPo
- saturated vapor pressure of adsorbate, mmHg; V adsorption amount of adsorbent, ml,
gas adsorption amount of monolayer, ml,
C——constant of BET equation.
（1）
BET equation is applicable to the range of 0.05～0.35 relative pressure (PN,/P.). Select 3 or 4 points in this range, and the adsorption amount (V.) obtained by experiment and its corresponding adsorption amount will be plotted according to BET equation with V.(1-Pn,/P.)
, and a straight line will be obtained, whose slope is the saturated adsorption amount of α sublayer (Vm). Vm
can be used to calculate the specific surface area per gram of sample. 2 Instrument
The intercept is b-
For PN,/P,
The single molecule
can be obtained from the slope and intercept
Then the specific surface area is determined according to the area occupied by each adsorbed molecule on the adsorption surface. The flow chart of the instrument device is as follows: Issued by the State Administration of Standards on March 3, 1982
Implemented on February 1, 1983
GB 2922-
Ai, A2—pressure regulating valve, B,, B2—pressure gauge, C1, C2—adjustable gas resistance, D,, D2—three-way valve, E1, E2—mixer, F—cold trap, Gi, G2—Dewar flask, H—insulation layer; I1, I2—spiral tube, J-thermal conductivity cell, K—sample tube; L—soap film flowmeter, M—six-way valve, N—known volume measuring tube. The instruments suitable for this method should meet the following indicators: 2.1 Thermal conductivity detector
2.1.1 Sensitivity
Under the condition that the instrument works stably, with oxygen as carrier gas and benzene as sample, the average sensitivity of the instrument measured three times is greater than 1000mV·ml/mg.
2.1.2 Baseline stability
Use hydrogen as carrier gas, bridge current at 200mA, attenuation at level 1, baseline drift after 30min is no more than 0.1mV, noise no more than 0.05mV.
2.2 Pressure regulator
The flow rate fluctuation of carrier gas or adsorbate gas during experimental operation is no more than 1%. 2.3. Mixer
The volume of the front mixer of this instrument shall not be less than 20ml, and the rear mixer can use a stainless steel straight tube with a length of 250mm and an inner diameter of 10mm. 3 Determination
Weigh an appropriate amount of dried sample, place it in the sample tube, connect it to the gas path, and put a Dewar flask filled with liquid nitrogen on the cold trap. The bridge flow of the thermal conductivity detector is controlled at 140mA. Use a soap film flowmeter to measure the flow rate of carrier gas hydrogen (or hydrogen), then add a certain amount of adsorbate (nitrogen), and measure the total flow rate of the mixed gas. After the baseline is stable, put the Dewar flask filled with liquid nitrogen on the sample tube. At this time, an adsorption peak will appear on the recorder. After the adsorption reaches equilibrium (the recording pen returns to the baseline), remove the liquid nitrogen Dewar flask and use room temperature water to accelerate desorption. The ·-desorption peak will appear on the recorder, and the desorption peak area is used as the basis for calculation. Note: When measuring each point, the Dewar flask covered with the sample tube should maintain the same liquid nitrogen level. The purity requirement of the carrier gas (nitrogen or hydrogen) and the adsorbate (nitrogen) is 99.9%.
4 Calculation
The peak area is measured by multiplying the peak height by the half-peak width. The reading microscope with an accuracy greater than 0.02mm or other tools with equal accuracy are used for reading. The calculation of the adsorption maximum can be done by the external standard method and the instrument constant method. 4.1 External standard method
GB 2922—82
The external standard method is to add pure nitrogen of known volume into the measuring tube again after the desorption peak of the sample is measured by the above method. The peak that appears on the recorder at this time is the standard peak.
The formula for calculating the adsorption amount by the external standard method is: Vs
Where: Va—adsorption amount in standard state, ml; Vs
—known volume of nitrogen, ml,
As-—peak area of known volume of nitrogen, mm2, Aa—desorption peak area, mm2,
Pa-—atmospheric pressure under experimental conditions, mmHgRoom temperature under experimental conditions, “K.
4.2 Instrument constant method
When the circuit parameters of the instrument remain unchanged, the peak area and the gas volume corresponding to the peak area, as well as The function between the carrier gas flow rate is a constant.
The calculation formula of the instrument constant K (min/mm2) is: K
Where: α-
As·Re
The ratio of the partial pressure of nitrogen in the mixed gas to the atmospheric pressure V
—The known volume of the measuring tube used, ml,
A-—The corresponding peak area with the known volume V of the measuring tube used; mm2, Rc—Carrier gas flow rate, ml/minz
Atmospheric pressure under experimental conditions, mmHg;
Room temperature under experimental conditions, "K.
Note: The conditions for testing the instrument constant K value must be consistent with the experimental conditions. The formula for calculating the adsorption amount using the instrument constant method is: V.-K.Rt.Aa
Wu Zhong: K
-Instrument constant, min/mm2,
-Total flow rate of the mixed gas, ml/min;
A. —desorption peak area, mm2,
-adsorption amount in standard state, ml.
（3）
（4）
In the relative pressure range applicable to the BET equation, select 3 or 4 points and measure the corresponding adsorption amount V in standard state. According to the measurement principle, the gas adsorption amount Vm of a single layer can be calculated. The specific surface area calculation formula with inert gas nitrogen as adsorbent is: Vm
Sa= 4.36-
W—sample weight, g,
-gas adsorption capacity of monolayer, ml.
5 Precision and accuracy
The precision of this method is within ±5%, and its accuracy is within ±5% when compared with the BET volumetric method. (5)
GB2922—82
Appendix A
Relationship between sample weight and specific surface area
(reference)
It is generally believed that the sample weight should be such that the adsorption of nitrogen is about 5ml, so samples with large specific surface area should be weighed less, while samples with small specific surface area should be sampled more. However, the specific surface area of samples varies from small to large by 10,000 times, so for samples with a specific surface area of less than 10m2/g, in addition to weighing more samples, the adsorption capacity can only be reduced accordingly. The estimated relationship between sample weight and sample specific surface area is shown in the figure below for reference. 2000
GB 2922-82
Appendix B
Relationship between sample tube and weighing
(reference)
When filling the sample, the solid sample is placed at the bottom of the U-shaped glass tube, and a gap should be left between the top of the sample and the wall of the sample tube to avoid resistance. In order to reduce the error of non-representative and non-uniform sampling, the sample weight should be increased as much as possible. In order to reduce the weighing error, a balance with a precision of one ten-thousandth must be used to weigh the sample.
Different volumes of sample tubes should be selected according to the specific surface area of the sample. Samples with small specific surface area should use large-volume sample tubes to increase the sample weight, but considering the influence of the reverse peak during desorption, the dead volume should be as small as possible. The sample tube should be as small as possible without being blocked by the sample. The commonly used large-volume sample tubes are listed in the figure below. 127
GB 2922—82bZxz.net
Appendix C
Requirements for cold trap tubes
(Supplement)
When measuring small specific surface areas, in addition to increasing the size of the mixer to ensure uniform mixing of the balance gas, the role of the cold trap cannot be ignored. The purity requirements of this method for carrier gas and adsorbate gas mainly rely on cold traps to achieve the purpose of purification, so the cold trap tube must be immersed in a Dewar flask filled with liquid nitrogen during the entire test process. When adding liquid nitrogen, the flow rate fluctuates due to temperature changes, which can easily blow away the sample, and the flow rate takes a long time to stabilize. Therefore, when adding liquid nitrogen, the cold trap tube should not be completely separated from the liquid nitrogen Dewar flask. Additional remarks:
This standard was proposed by the Ministry of Chemical Industry of the People's Republic of China, and the Beijing Chemical Reagent General Factory is responsible for the technical coordination. This standard was drafted by the Jilin Chemical Industry Company Research Institute and the Shanghai Chemical Reagent Research Institute. The main drafters of this standard are Zheng Jingwan and Sun Jinmu. 128
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