Melting Point Spring Fall 09
- 2. Background Information Melting point: temperature at which solid is in equilibrium with its liquid Melting point range: span of temperature s from the point at which the crystals first begin to liquefy to the point at which the entire sample is liquid Pure compounds usually melt over a narrow temperature range of 1-2 °C Presence of soluble impurity almost always decreases and broadens the melting point range
- 3. Melting Point of Mixture (X and Y) Eutectic temperature : lowest possible melting point for a mixture of 2 compounds Eutectic composition: composition at which both solid X and solid Y are in equilibrium with the liquid mixture (at eutectic temperature) Eutectic point
- 4. Taking a Melting Point: Packing Pack the capillary tube by pressing the open end gently into a sample of the compound to be analyzed. (Capillary tubes break easily!) The solid should fill the tube to a depth of ~ 2 mm. Tap the bottom of the capillary on a hard surface so that the crystals pack down into the bottom of the tube.
- 5. Melting Point Apparatus Pack the crystals at the bottom of the tube and place the tube in the slot behind the eye-piece on the Mel-Temp. Turn on the Mel-Temp and turn the knob to adjust heating rate (not too fast or too slow). Watch through eyepiece and record temperature at which the crystals first begin to liquefy and record temperature at which the entire sample is liquid (mp range)
- 6. Experimental Procedure Preview Part I Practice melting range: Measure the mp of only one known compound, benzoic acid Part III Identifying an unknown compound by mixture melting point range: Measure the mp of an unknown compound and identify it by testing with known compounds
- 7. Experimental Procedure: Part I Use benzoic acid (mp = 122-123 °C). Use proper size of sample (not more than 2 mm in capillary) just enough to see!!! Pack firmly by dropping through a long glass tube Do not move the Mel-Temp to your bench or remove the probe Increase Temperature by 10°C/min until 10-15°C below the known mp range, then reduce rate to 1-2°C/min Allow mp apparatus to cool down naturally after experiment ( no wet paper towels or ice – may use air )
- 8. Part II – Eutectic Temperature (skip this) Unknown mp = 133-135 °C Benzoin (mp = 135-137°C) or trans -Cinnamic Acid (mp = 133-134°C) mp = 135-137°C Unknown is Benzoin mp << 133°C Unknown is not Benzoin mp << 133°C Unknown is not trans -Cinnamic Acid mp = 133- 134°C Unknown is trans -Cinnamic Acid Mix Unknown and Benzoin Mix Unknown and trans -Cinnamic Acid
- 9. Part III: Identifying Your Unknown Unknown is given to you by TA (see list - page 20) Record the code. Take 2 melting points of your unknown (2 different samples): - 1 st one taken quickly to approximate range (~10°C/min) - 2 nd one taken slower (1-2°C/min) when 10°C below mp range approximated from 1 st run Get a spatula tip of the compound that could match the mp of your unknown (from labeled samples on TA bench) and add it to a spatula tip of your unknown on a watch glass (1:1 mixture). Mix thoroughly by grinding with a scoopula or spatula. Take mp of the mixture: - If mp is the same as mp of unknown, unknown is identified. - if mp of mixture is different from mp of unknown, repeat process with another compound.
- 10. Waste Handling Don’t discard capillaries in the trash. Put them in the collection beaker labeled “3221 Melting Capillaries”. Any unused unknown is to be returned to the TA in the original sample vial (properly capped).