How can we find out the lag time t experimentally for calculating diffusion coefficient by this formula D= L2/6t
The attached paper might be of interest to you ..
https://pubs.acs.org/doi/abs/10.1021/j150547a018?journalCode=jpchax
Hi.
The lag time is a graphic construction obtained from mass in a receiver vs. time. I've illustrated how to get the lag time in the attached graph of mass in the receiver (I didn't put any units) vs. the time in hours. The graph shows an initially slow release, and the slope gradually increases until the line becomes straight.
The lag time is obtained by taking the later points in the plot (when the plot is straight) and drawing a straight line through them, then extrapolating back to the horizontal axis intercept. The intercept will be the lag time.
The equation t = L^2/6D can then be used, but only if the data were obtained from the following conditions. The concentration in the donor is constant and the membrane separating the donor and receiver is initially void of the drug or chemical. (Any other conditions and the lag time equation does not hold.)
Also, as a rule of thumb, there is a little trial and error... the points that you take as being on the straight portion of the curve should be obtained at times that are at three lag times (roughly) or longer. The graph I provided shows that the points were taken from 12 hours out, which are all at least three times the lag time of about four hours.
There is another equation for what is know as the burst effect, which holds when the membrane separating the donor and receiver is fully loaded with the drug. In that case, the burst time is the horizontal intercept again, but is negative. (That is OK because it isn't a real time... it is just a number from the graph that has units of time.)
I hope this helps.
Thanks sir. But problem is when we supply the gas to the membrane, it comes out immediately may be within a second or so and it is almost impossible to measure this time practically particularly when you are comparing this time for different gases.
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