Remove the flame and quickly press the stopper down to form a vacuum-tight seal. Before doing so, be sure to protect your hands and arms by wearing long sleeves and a pair of hot-water gloves used by professional dishwashers. Also, hold a towel against the flask.
If you immerse the hot, tightly sealed cell in a cool bath, the water inside the flask will boil again. This delightful effect occurs as the water vapor within the cell condenses, lowering the internal pressure, which then decreases the boiling temperature. When the cell cools completely, you should test the quality of the vacuum by giving the cell a gentle vertical shake. (Be careful, because a vigorous jolt could shatter the glass.) You should hear a sharp ¿snap¿ caused by the so-called water-hammer effect: the water, uncushioned by air, will slam full-force into the glass. If you don't hear the sound, regenerate the vacuum.
To reach the triple point, first chill the cell overnight in a refrigerator. Next, you'll need to form a thick ice mantle around the inner test tube. Professionals usually pour a frigid mixture of dry ice and alcohol into the inner well, but Schmermund gets fantastic results with liquid nitrogen, which is much colder. You'll find both refrigerants at your local welder's supply store. Before you add the coolant, dry the inner surface of the test tube thoroughly because the glass could crack if ice forms inside the well. Keep in mind that refraction will make the ice mantle appear to grow faster than it actually does. When the mantle looks like it is nearly touching the flask, dump out the remaining refrigerant.
Using liquid nitrogen entails a complication. The ice mantle will form fastest at the bottom where it is in contact with the nitrogen for the longest time. To make the mantle more even, Schmermund periodically lets all the nitrogen boil away and then drops in progressively longer wooden dowels. Additional nitrogen boils energetically around the dowel, and the expanding gases tend to keep the coolant above the dowel's top.
Separate the mantle from the test tube by filling the well with distilled water and 10 percent isopropyl alcohol to melt the mantle's inner surface. Don't be alarmed, though, when the ice cracks violently. If when you rotate the flask the mantle stays put, the ice is no longer stuck to the glass. When that happens pour off enough of the water-alcohol mixture so that its level is two centimeters below the top of the ice.
Last, place the cell inside an insulated drink container filled with crushed ice and water. Because the ice mantle is buoyant, it will press up against the bottom of the inner test tube, making this spot slightly colder than the triple point. A cutting from a pencil eraser makes an ideal spacer. Rest the thermometer on top of the eraser cutting inside the well. In about an hour, the thermometer will settle on the triple point.
To determine temperature, the best thermometers work by measuring the resistance across a thin platinum wire. Because the change in resistance caused by a given temperature difference is well known for platinum, the triple point is all you'll need to calibrate the instrument. Sadly, such thermometers are very expensive. But Schmermund has an answer for that too, as you'll see in my next column.