Experiment 6 Simple and Fractional Distillation

What is and what do the following measure? vapor pressure The vapor pressure of a pure substance is the pressure exerted by the substance against the external pressure which is usually atmospheric pressure. Vapor pressure is a measure of the tendency of a condensed substance to escape the condensed phase. boiling point: When the vapor pressure of a liquid substance reaches the external pressure, the substance is observed to boil. normal boiling point: When the external pressure is atmospheric pressure, the temperature at which a pure substance boils is called the normal boiling point.

 H vap is the energy necessary to vaporize a mole of a pure substance A liq B liq A vap B vap  H vap B  H vap A

 f mixing liq A and liq B and mixing gas A and gas B results in no heat of mixing in both the liquid and gas phase, the system is considered to be ideal Then: P A obs =  A P o A ;P B obs =  B P o B where P A obs ;P B obs is the observed vapor pressure in the mixture;  A;  B is the mole fraction of A, B,...  A = n A / (n A +n B )  B = n B / (n A +n B ) and P o A ; P o B are the vapor pressures of pure A and B

A diagram of a simple distillation apparatus liq vapor liq This process is referred to as one theoretical plate

At what temperature will a 1:1 molar mixture of cyclohexane- methylcyclohexane boil? P T = P A obs + P B obs P T obs =  A P o A +  B P o B  A =  B = Atm = 101 kPa

T/KC/ kPaM/ kPaT/KC/ kPaM /kPa Vapor Pressures of Pure Cyclohexane (C); Methylcyclohexane (M)

T/KC/ kPaM/ kPaT/KC/ kPaM /kPa C = Cyclohexane; M = Methylcyclohexane

At what temperature will a 1:1 molar mixture of cyclohexane- methylcyclohexane boil? P T = P A obs + P B obs P T obs =  A P o A +  B P o B  A =  B = Atm = 101 kPa T/KC /kPa M /kPa P T obs  kPa ≈ 1 atm

What is the composition of the vapor at this temperature? T/KC /kPa M /kPa P C V C = n C RT ; P M V M = n M RT n C /n M = P C /P M = 128.5/71.1 = 1.8/1

An apparatus for fractional distillation 5 theoretical plates can be identified

A look at a schematic diagram of a gas chromatograph:

Instrumental components Carrier gas The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities. Sample injection port For optimum column efficiency, sample sizes should be small and should be introduced onto the column as a "plug" of vapor. The most common injection method is where a microsyringe is used to inject sample through a rubber septum into a heated port at the head of the column. SPME is an alternative method for introducing your sample. For packed columns, sample size ranges from tenths of a microliter up to 20 microliters. In preparative GC, sample sizes as much as a mL can be used in certain cercumstances. Capillary columns, on the other hand, need much less sample, typically less than mL. For capillary GC, split/splitless injection is used.

Columns There are two general types of column, packed and capillary. Packed columns contain a finely divided, inert, solid support material (commonly diatomaceous earth) coated with liquid stationary phase. Most packed columns are m in length and have an internal diameter of 2 - 6mm. Liquid coatings vary depending on polarity. Most are non-volatile waxes or organic liquids. Fused silica open tubular columns have much thinner walls than the glass capillary columns, and are given strength by the polyimide coating. These columns are flexible and can be wound into coils. They have the advantages of physical strength, flexibility and low reactivity. Temperature range of use varies from room temperature to about 300 ° C, depending on the coating. Cyclodextrin is bonded in chiral columns. Capillary column

It is not unusual for a capillary column to have more an efficiency characterized by more than 100,000 theoretical plates.

Detectors There are many detectors which can be used in gas chromatography. Different detectors will give different types of selectivity. A non-selective detector responds to all compounds except the carrier gas, a selective detector responds to a range of compounds with a common physical or chemical property and a specific detector responds to a single chemical compound. The FID is mostly commonly used. Flame ionization (FID) Mass flow Hydrogen and airMost organic cpds pg Thermal conductivity (TCD) Concen tration ReferenceUniversal1 ng Electron capture (ECD) Concen tration Make-upHalides, nitrates, nitriles, peroxides, anhydrides, organometallics50 fg Nitrogen- phosphorus Mass flow Hydrogen and airNitrogen, phosphorus10 pg Flame photometric (FPD) Mass flow Hydrogen and air possibly oxygen Sulphur, phosphorus, tin, boron, arsenic, germanium, selenium, chromium 10 0 pg Photo-ionization (PID) Concen tration Make-upAliphatics, aromatics, ketones, esters, aldehydes, amines, heterocyclics, organosulphurs, some organometallics 2 pg Hall electrolytic conductivity Mass flow Hydrogen, oxygen Halide, nitrogen, nitrosamine, sulphur