CHM343 Lab Report 5

CHM343 Lab Report 5 - Choon Liang Tan 03.03.08 CHM343...

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Choon Liang Tan 03.03.08 CHM343 Separation Methods Lab Report 5 Determination of Kovats Retention Indices Name: Choon Liang Tan Lab Partner: Yolanda Diaz Bronco ID: 007156760 Date: 02.25.08 Class: CHM343 Lecturer: Dr. Hossein Ahmadzadeh 1
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Abstract: Both DB-23 and MXT-5 columns are non-polar columns and are great instruments for analysis of hydrocarbon chains and organic compounds. Yet, with two sides of analysis, the unknown sample was determined to only have two compounds, not three as was expected. The two compounds were butanol and ethylbenzene. Any proof of having pentanone was not found in both columns. Introduction: Many studies have been conducted with any of these two columns provided, DB- 23 and MXT-5. Both columns are non-polar columns, meaning that the lesser the polarity of the analytes the greater the retention in the stationary phase will be. This method is to test for an indication of the hydrocarbon chain length of the three compounds within an unknown solution. In separation methods, the main aspects that affects retention time are the volatility and degree of interaction with the stationary phase that an analyte has in gas chromatography. The strength of the interactions are determined by the polarities of the analyte and the stationary phase. To see the clearer picture, like associates with like, thus, for a non-polar column, non-polar analytes are retained longer and would reach the detector only after all the more polar analytes. This is the first half of determining whether the unknown solution is polar or not. Kovats introduced a retention index, I, that is referred to as the retention of a homologous series of normal aliphatic hydrocarbons. For all n-paraffins on a stationary phase, I is 100nc at all temperatures, whereby nc is the number of carbons present in the hydrocarbon standards. The way this index was formed was by referring to he logarithm of the adjusted retention time and the number of carbons in members of homologous series. Logtr’ = Knc + b (1) The K value is the proportionality constant and the b refers to the intercept. The correlation between the time analytes are bounded in the stationary phase and how it increases per additional carbon atom to more specifically methylene group can be all shown by a specific constant value. This value calculates how the changes are in the analytes volatility and strength of interaction simply between methylene groups and the stationary phases. Delta logtr’ = Constant value (2) CH 2 Logtr’ (nc+x) = logtr’nc + x.delta logtr’ (3) CH 2 2
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I = 100nc + 100.(logtr’x – logtr’nc) (4) Tm = (ad – bc) (5) (a+d –b –c) Experimental: Apparatus: DB-23 Column (HP6890 Series GC System Plus +) Non Polar Column MXT-5 Column (Buck Scientific Model 910 Gas Chromatograph) None Polar Column GC Syringe (10μL) Chemicals: Methanol 100% (Fisher Scientific®) Standards: n-decane, n-nonane, n-octane, n-heptane, n-hexane, and n-pentane (CPP) (Each standard is approximately 100ppm) Procedure: Working on the MXT-5 column (Buck Scientific) was the first machine to be used. There were two machines in total to be used, the other being the DB-23 column.
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CHM343 Lab Report 5 - Choon Liang Tan 03.03.08 CHM343...

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