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With the Thiele tube ( Fig. 12.9 ), you use hot oil to transfer heat evenly to your sam-ple in a melting-point capillary, just like the metal block of the Mel-Temp apparatus does. You heat the oil in the side arm and it expands. The hot oil goes up the side arm, warming your sample and thermometer as it touches them. Now the oil is cooler, and it falls to the bottom of the tube, where it is heated again by a burner. This cycle goes on automatically as you do the melting-point experiment in the Thiele tube.

Thermometer periscope knob

Mercury column, line in mirror, and marks on metal

case should line up

mp tube and sample

Magnifying lens Thermometer

bulb

5060

FIGURE 12.8 Reading the temperature.

USING THE THIELE TUBE 99

100 CHAPTER 12 THE MELTING-POINT EXPERIMENT

Don’t get any water in the tube, or when you heat the tube the water can boil and throw hot oil out at you. Let’s start from the beginning.

Cleaning the Tube

This is a bit tricky, so don’t do it unless your instructor says so. Also, check with your instructor before you put fresh oil in the tube.

Thermometer

Notched cork holds thermometer without pressure buildup

Thiele tube clamped here

Rubber ring above hot oil!

Heats sample in capillary tube 2

1

Heat here Hot oil rises

Oil cools, falls to bottom, and recirculates 3

FIGURE 12.9 Taking melting points with the Thiele tube.

1. Pour the old oil out into an appropriate container, and let the tube drain.

2. Use a hydrocarbon solvent (hexane, ligroin, petroleum ether—and no fl ames! ) to dissolve the oil that’s left.

3. Get out the old soap and water and elbow grease, clean the tube, and rinse it out really well.

4. Dry the tube thoroughly in a drying oven (usually !100°C). Carefully take it out of the oven, and let it cool.

5. Let your instructor examine the tube. If you get the OK, then add some fresh oil. Watch it. First, no water. Second, don’t overfi ll the tube. Normally, the oil expands as you heat the tube. If you’ve overfi lled the tube, oil will crawl out and get you.

Getting the Sample Ready

Here you use a loaded melting-point capillary tube (see “Loading the Melting-Point Tube” earlier in the chapter) and attach it directly to the thermometer. Unfortunately, the thermometer has bulges; there are some problems, and you may snap the tube while attaching it to the thermometer.

1. Get a thin rubber ring or cut one from a piece of rubber tubing.

2. Put the bottom of the loaded mp tube just above the place where the thermom-eter constricts ( Fig. 12.10 ), and carefully roll the rubber ring onto the mp tube.

Hold mp tube on straight part

Move ring up and over tube

No pressure here!

(Tube may snap!)

Ring near top of tube

Final resting place!

Middle of sample at middle of bulb Move ring up

Slide mp tube down

FIGURE 12.10 Attaching the mp tube to the thermometer without a disaster.

USING THE THIELE TUBE 101

102 CHAPTER 12 THE MELTING-POINT EXPERIMENT

3. Reposition the tube so that the sample is near the center of the bulb and the rubber ring is near the open end. Make sure the tube is vertical.

4. Roll the rubber ring up to near the top of the mp tube, out of harm’s way.

Dunking the Melting-Point Tube

There are more ways of keeping the thermometer suspended in the oil than I care to list. You can cut or fi le a notch on the side of the cork, drill a hole, and insert the ther-mometer (be careful!). Finally, cap the Thiele tube and dunk the mp tube ( Fig. 12.9 ).

The notch is there so that pressure will not build up as the tube is heated. Keep the notch open, or the setup may explode.

But this requires drilling or boring corks, something you try to avoid. (Why have ground-glass jointware in the undergraduate lab?) You can gently hold a ther-mometer and a cork in a clamp ( Fig. 12.11 ). Not too much pressure, though!

Undrilled cork helps hold thermometer

(careful)

Thiele tubes completely open

and safe

Quasi-legal use of thermometer adapter

to hold thermometer

FIGURE 12.11 Safely suspended thermometer with Thiele tube.

Finally, you might put the thermometer in the thermometer adapter and sus-pend that, clamped gently by the rubber part of the adapter, not by the ground-glass end. Clamping ground glass will score the joint.

Heating the Sample

The appropriately clamped thermometer is set up in the Thiele tube as in Fig. 12.9 . Look at this fi gure now , and remember to heat the tube carefully—always carefully—

at the elbow. Then,

1. If you don’t know the melting point of the sample, heat the oil fairly quickly, but no more than 10°C per minute, to get a rough melting point. And it will be rough indeed, since the temperature of the thermometer usually lags that of the sample.

2. After this sample has melted, lift the thermometer and attached sample tube carefully (it may be HOT) by the thermometer up at the clamp, until they are just out of the oil. This way the thermometer and sample can cool, and the hot oil can drain off. Wait for the thermometer to cool to about room temperature before you remove it entirely from the tube. Wipe off some of the oil, reload a melting-point tube (never remelt melted samples), and try again. And heat at 2°C per minute this time.

EXERCISES

1. “If you can see it, you can see it melt.” This should put the correct amount of a solid in a melting-point tube. If someone has put about one-quarter of an inch of solid in the tube, should they go for an eye exam?

2. Defi ne “melting point.”

3. How fast should a melting-point capillary sample be heated?

4. Why not remelt samples? Don’t they just cool and solidify in exactly the re-verse manner that they heated and melted?

5. Defi ne “freezing point.”

EXERCISES 103

RECRYSTALLIZATION

■ Too much solvent and your crystals don’t come back .

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chapter

105 The essence of a recrystallization is purifi cation . Messy, dirty compounds are cleaned up, purifi ed, and can then hold their heads up in public again. The sequence of events you use will depend a lot on how messy your crude product is and on just how soluble it is in various solvents.

In any case, you’ll have to remember a few things.

1. Find a solvent that will dissolve the solid while hot.

2. The same solvent should not dissolve it while cold.

3. The cold solvent must keep impurities dissolved in it forever or longer.

This is the major problem. And it requires some experimentation. That’s right!

Once again, art over science. Usually, you’ll know what you should have prepared, so the task is easier. It requires a trip to your notebook and, possibly, a handbook (see Chapter 2 , “Keeping a Notebook,” and Chapter 3 , “Interpreting a Handbook”). You have the data on the solubility of the compound in your notebook. What’s that, you say? You don’t have the data in your notebook? Congratulations, you get the highest F in the course.

Information in the notebook (which came from a handbook) for your com-pound might say, for alcohol (meaning ethyl alcohol), s.h. Since this means s oluble in h ot alcohol, it implies that it is i nsoluble (i.) in cold alcohol. Then alcohol is prob-ably a good solvent for recrystallization of that compound. Also, check on the color or crystalline form . This is important since:

1. A color in a supposedly white product is an impurity.

2. A color in a colored product is not an impurity.

3. The wrong color in a product is an impurity.

You can usually assume that impurities are present in small amounts. Then you don’t have to guess what possible impurities might be present or what they might be soluble or insoluble in. If your sample is really dirty, the assumption can be fatal. This doesn’t usually happen in an undergraduate lab, but you should be aware of it.