Question

When making the measurements for the Rf values, it is conceivable to measure the distance the spot travels from the top, middle or bottom of the spot. In practice, however, it is best to measure only from the middle of the spot. Why?

Answer 1

Calculate So values for the following reactions by using tabulated So values from Appendix C.

(a) N2O4(g) 2 NO2(g) So = J/K

(b) 2 PCl3(g) + O2(g) 2 POCl3(g) So = J/K

(c) 2 NOCl(g) Æ 2 NO(g) + Cl2(g) So = J/K

(d) SO2(g) + 1/2 O2(g) SO3(g) So = J/K

The middle of the spot is the place, where the average of the sample has reached climbing up the chromatographic plate by capillary action. Since a symmetrical spot is placed ideally at the base line of the TLC plate, the distance travelled must be measured from the absolute center of the spot at the final place that the spot has reached.