1. The data in Table I below were obtained by using a GLC column operated at con

Question

1. The data in Table I below were obtained by using a GLC column operated at constant T Retention Rime (min) 0.32 4.49 6.23 7.08 7.78 11.52 16.24 18.11 Adjusted Retention Time calculated) tR-tR-tm Log tR Compound Air n-pentane n-Hexane n-Heptane n-Octane Air pulled in the syringe when injected on a GLC column is usually taken as dead time because air is a mixture of small molecular weight gases not retained on GLC column. Calculate the Kovats retention indices for the sample Component A, B and C. From a plot ofI as a function of log (tr-tm) estimate the retention time of n-butane

Explanation / Answer

adjusted retention time tr' log Tr'

n-pentane 4.17 0.62

A 5.91 0.77

n-hexane 6.76 0.83

B 7.46 0.87

n-heptane 11.2 1.05

C 15.92 1.20

n-octane 17.79 1.25

Tr' was calculated by substracting retention time of air from the given samples.

the formula for Kovats retention indice (I) is

I= 100 * [n + {(log tx - log tn)/ (log tn+1-logtn)}]

n= number of carbon atomes in n-alkane (tn)

tx= retention time of sample

tn= ret. time of sample just before tx

tn+1= ret. time of sample just after tx

using this formula calculate I for A,B,C

IA= 100 * [ 5 +{( 0.77-0.62)/ (0.83-0.62)}] ( for A, tn is n-pentane and tn+1 is n-hexane)

IA= 571.43

similarly for B and C

IB =618.18 (tn is n-hexane, tn+1 is n-heptane)

IC= 775 (tn is n-heptane, tn+1 is n-octane)

I am sorry I cannot upload graph because chegg allows pictures upto 2mb and my phone clicks around 3mb!

But, with this formula you can calculate remaining I for the samples make a graph easily to find I for n-butane!

hope this was helpful

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