Suppose you carry out a TLC separation of acctaminophen and phenacelin on silica

Question

Suppose you carry out a TLC separation of acctaminophen and phenacelin on silica gel using a nonpolar developing solvent. Which would have a higher R_f value and why What will be the possible effect on your results of the following experimental errors or variations You allowed your chromatogram to develop too long, and you couldn't find the solvent front. You marked the starting line with a ballpoint pen When the TLC plate was placed in the developing chamber, you found out that the original spots were below the solvent level. What would be the result of adding too much sample to the TLC plate In a TLC analysis of analgesics, what would be the result if a solvent of extremely low polarity is used to develop the plate

Explanation / Answer

1.

Phenacetin has – O-CH2-CH3 and Acetaminophen has –OH as distinctive group. The ether group has a lower afinity for the stationary phase (by comparison to - OH) and thus there is a higher Rf for phenacetin.

2.

a.Not a big problem, if you can see all spots developed (here only two). Use Rf as relative value.

b. you distroyed the continuity of the stationary phase. Fatal error

c. the original spots were diluted/dissolved in the solvent. Fatal error

3.

You will obtain a long smear and a poor separation.

4.

By comparison to a nonpolar sovent, the Rf will be a little higher.

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