The surface area of activated carbon can be determined using gas adsorption through a technique called Brunauer-Emmett-Teller BET method. The BET method is based on the adsorption of a gas, usually nitrogen, onto the surface of the activated carbon at a constant temperature, typically 77 K liquid nitrogen temperature . The method involves the following steps:1. Sample preparation: First, the activated carbon sample needs to be degassed to remove any adsorbed gases or contaminants from its surface. This is usually done by heating the sample under vacuum or in an inert gas atmosphere e.g., nitrogen or argon for several hours.2. Gas adsorption: The degassed activated carbon sample is then exposed to a known amount of nitrogen gas at a constant temperature 77 K . The nitrogen molecules adsorb onto the surface of the activated carbon, forming a monolayer or multilayer coverage depending on the pressure.3. Adsorption isotherm: The amount of nitrogen gas adsorbed onto the activated carbon is measured at different relative pressures P/P0 , where P is the equilibrium pressure of the gas and P0 is the saturation pressure. This results in an adsorption isotherm, which is a plot of the amount of gas adsorbed versus the relative pressure.4. BET analysis: The BET equation is used to analyze the adsorption isotherm data. The equation relates the amount of gas adsorbed, the relative pressure, and the surface area of the adsorbent. By fitting the experimental data to the BET equation, the specific surface area of the activated carbon can be calculated.The BET equation is given by:V = V_m * C * P/P0 / [ 1 - P/P0 * 1 + C - 1 * P/P0 ]where V is the volume of gas adsorbed at a given relative pressure P/P0 , V_m is the volume of gas adsorbed at monolayer coverage, and C is the BET constant related to the energy of adsorption.5. Surface area calculation: Once the BET parameters V_m and C are obtained from the analysis, the specific surface area S_BET of the activated carbon can be calculated using the following equation:S_BET = V_m * N_A * A_m / Mwhere N_A is Avogadro's number 6.022 x 10^23 mol^-1 , A_m is the cross-sectional area of the nitrogen molecule 0.162 nm^2 , and M is the mass of the activated carbon sample.The BET method provides an accurate and reliable way to determine the surface area of activated carbon and other porous materials. It is widely used in research and industry for characterizing the adsorption properties of various materials.