28.2 Further practical techniques

steps of filtration under pressure

1) connect one end of the pressure tubing to the vacuum outlet, connect the other end to the Buchner flask. fit the Buchner funnel to the Buchner flask ensuring a tight fit

2) switch on the vacuum pump/ tap, check for good suction by placing your hand across the top of the funnel

3) place a piece of filter paper inside the Buchner funnel and wet this using solvent

4) slowly pour the reaction mixture from the beaker in the centre of the filter paper. rinse out the beaker with the solvent

5) rinse the crystals in the Buchner funnel with minimum volume of solvent, leave under suction so crystals start to dry

steps of recrystalisation

1) pour some of the solvent into a conical flask and warm in a water bath

2) tip the impure sample into a second conical flask

3) slowly add the minimum volume of solvent to the impure sample until the solid dissolves

4) allow the solution to cool, crystals should form

5) filter the crystals under reduced pressure

steps of melting point determination

1) ensure sample is dry and free flowing

2) hold one end of a capillary tube into flame and rotate until end of capillary tube is sealed. allow to cool

3) push open end of capillary tube into solid sample to fill to around 3mm depth

4) taking melting point electrically or using Thiele tube

melting point of impure sample compared to pure sample

impure sample will have a lower melting point

steps of using electrically heated melting point apparatus

1) place the capillary tube containing the sample into a sample hole and a 0/300°C thermometer in the thermometer hole

2) heat the sample using the rapid heat setting whilst observing the sample through the magnifying window

3) record the melting point once the solid is seen to melt

4) prepare a second sample and place in the melting point apparatus and heat the sample

5) as melting point is being approached, set to low and raise the temp slowly whilst observing the sample

steps of melting point analysis using the Thiele tube method

1) set up the Thiele tube

2) attach the capillary tube containing the sample to a thermometer using a rubber band

3) insert the thermometer through a hole in the cork. the end of the thermometer should dip into the oil

4) using a micro-burner, slowly heat the arm of the Thiele tube whilst observing the solid. when the solid starts to melt, remove the heat and record the temp

5) repeat melting point determination