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Theil Melting Point Tube
Convection-Loop Geometry: Precision-engineered lateral arm facilitates continuous natural convection, maintaining thermal equilibrium in oil baths without mechanical stirring.
High-Strain Borosilicate 3.3: Low-expansion glass matrix 3.3 x 10-6 K-1 ensures structural integrity during rapid thermal cycling up to 300 °C.
$1.86
Quick Answer: A Thiele tube, listed here as the Theil Melting Point Tube, is a glass tube with a looped side arm that holds a heating oil, used to find the melting point of a solid packed in a capillary tube beside a thermometer. Heating the side arm sets up convection that warms the oil evenly.
Convection Does the Stirring
The design, named after the German chemist Johannes Thiele, solves the problem of heating a thermometer and a tiny sample evenly without a stirrer. The tube is filled with a high-boiling oil to just above the upper junction of the side arm. When the lower bend of the arm is warmed with a small flame, the hot oil rises through the arm and flows back into the main tube, setting up a circulation that keeps the temperature around the thermometer bulb uniform.
The sample is packed a few millimetres deep into a capillary tube sealed at one end, which is fixed to the thermometer with a thin rubber ring so the sample sits beside the bulb. The thermometer is held in a cork with a notch cut in its side, so the apparatus is never sealed. Melting is watched through the glass, and the range from the first drop of liquid to a clear melt is recorded.
The listing title spells the name Theil; the apparatus is the Thiele tube. Oil, thermometer and capillaries are used with it, and the tube size is not stated in the listing.
Specifications
| Item | Melting point tube with looped side arm (Thiele pattern) |
| Heating medium | High-boiling oil, commonly liquid paraffin or silicone oil |
| Heating point | Lower bend of the side arm, with a small flame |
| Sample holder | Capillary tube attached beside the thermometer bulb |
| Size | Confirm at enquiry |
Care & Handling
- Keep the rubber ring that holds the capillary above the oil surface; hot oil softens rubber and the capillary can fall in.
- Fill the oil only slightly above the upper junction of the side arm, because oil expands as it heats and can spill over.
- Never heat the oil close to its smoke point, and keep water out of hot oil, which makes it spit violently.
- Let the oil cool fully before emptying or moving the tube, and cover it in storage to keep out dust and moisture.
Applications
- Determining the melting point of an organic solid as a test of identity
- Checking purity: a sharp melting range suggests a pure sample, a broad, low range an impure one
- Mixed melting point tests to confirm whether two samples are the same compound
- Boiling point determination of small liquid samples with an inverted capillary
Why Choose LabEquip
College organic laboratories and pharmacy departments use Thiele tubes for melting point work when an electrical apparatus is not available for every student. LabEquip lists it in the chemistry lab equipment range, and organic solids for these practicals keep well in the clear glass wide mouth reagent bottle; send quantities via the contact page.
Frequently Asked Questions
Why is the side arm heated instead of the bottom of the tube?
Heating the side arm makes the warm oil rise through the loop and return to the main tube, so it circulates continuously. This convection keeps the oil around the thermometer at an even temperature without a stirrer.
Which oil should be used for melting point work?
Liquid paraffin is common for melting points up to roughly 200 °C, and silicone oil is used for higher temperatures because it is more stable. Choose an oil whose safe working range is above the expected melting point.
How fast should I heat near the melting point?
Heat quickly to about 10 to 15 °C below the expected melting point, then slow to about 1 to 2 °C per minute. Slow heating gives the thermometer and sample time to reach the same temperature.
What does a broad melting range tell me?
Impurities lower the melting point and spread it over several degrees. A pure organic compound usually melts sharply, within about one or two degrees, so a broad range suggests the sample needs recrystallising.
What is a mixed melting point test?
The unknown is mixed with a known sample of the suspected compound and the melting point measured. If they are the same compound the mixture melts at the same temperature; if they differ, it melts lower and over a wider range.
Why is a notched cork used to hold the thermometer?
The notch leaves an opening to the air, so the heated oil and air can expand without building pressure. A fully sealed tube could push the cork out or crack the glass when heated.
Last Updated: September 2026
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