I've got a t-plot report that micropore volume and micropore area reported negative value. Slope of line is 3.3865 and intercept is -1.9077 and external surface area calculated is bigger than BET surface area. Isotherm plot of sample is type II.
How do I calculate external surface area by slope of line? As well as Vpore by intercept?
I attached this report.
This shows the importance of reporting uncertainties with all numerical quantities! If your data show that the micropore volume is (-0.003 +/- 0.007) cm3 g-1, for example, it is perfectly acceptable to report it like that.
(Note, though, that if the volume is (-0.003 +/- 0.05) cm3 g-1, you should report it as (0.00 +/- 0.05) cm3 g-1.)
Conversely, it would NOT make sense to report a volume of (-0.003 +/- 0.001) cm3 g-1 - that implies you have some unconsidered error in your experimental procedure.
You will need to propagate the uncertainties through from the initial measurement to the final reported quantity. I'm afraid I don't know enough about the t-plot technique to tell you how to do that. Good luck!
I'm not familiar with the software report attached. I'm not sure what the "thickness range" means exactly.
When applying the t-plot method it is vitally important to use data points in the appropriate P/Po range. For most materials a good place to start is from 0.2 to 0.5. The key here is to use points in the P/Po range that corresponds only to multilayer formation, i.e. between (statistical) monolayer and the onset of capillary condensation.
The other variable is which t-plot model is being applied; one might be a better fit to your experimental data than another. However in general, a good guide is that the model and data points chosen should not produce an external surface area greater than the BET value (say not more than 2% greater), otherwise something is amiss. Similarly, a small negative intercept (-ve micropore volume!) can be interpreted as zero.
One final note, the t-plot method is very sensitive to free space/void volume and saturation pressure (Po) errors - much more so than the BET equation.
If you are willing to share the raw data (vol adsorbed cc/g and P/Po), I'll be pleased to check the calculation.
I think it is experimental error
you can check this from the y plotting the Relative Pressure (P/Po) against 1/[Q(Po/P - 1)] and it should give a straight line with R2 (correlation coefficient), 1
One must choose correctly the thickness range in which the data is being fit, amongst other considerations.
I also found the negative value (-4.8) of v-t method using QuadraWintm software analyzer with R2=0.999 (P/Po= 0.2-0.5). The BET surface area value = the external surface. My I interpret as zero value, meanning my sample do not contain the micropore
Adel Fisli Yes, a negative intercept on a t-plot is interpreted as zero micropore volume. You could try a different t-plot model (default is deBoer aka Harkins Jura, Halsey is another choice, or if carbon black sample use that model).
Adel Fisli No micropores expected in your sample. Everything seems to be in order. the t-plot equations are empirical in nature and they can't be guaranteed to be a perfect model for multiplayer formation on all materials... sometimes this leads to a negative intercept.
Hi all,
I just came across to this forum as I am having the similar issue with my materials. After doing some literature investigation, I found some useful explanations that addressing the negative t-plot value:
' ....The micropore volume of FAU-Y is difficult to assess by the t-plot method due to the compressibility of nitrogen in micropores,[15] which, contrary to nitrogen in mesopores, leads to a non-constant nitrogen film thickness for a given pressure. The thickness of the film increases with decreasing micropore diameter[15]. For materials containing micropores, the intercept of the first slope with the Y-axis usually is used to determine the micropore volume. But as shown by computational studies for pure microporous or micro-/mesoporous materials, t-plots underestimate micropore volumes [15] and overestimate surface areas [13]. The underestimation of micropore volume by t-plot method has been demonstrated experimentally for materials containing more than 10% micropore volume [12]...'
Kindly have further reading here: Article Specific Surface Area Determination for Microporous/Mesoporo...
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