Here are two common methods for calibrating a Beckmann thermometer in the temperature range typically used in chemical experiments:
**First, the Constant Temperature Bath Method**
1. Begin by identifying the temperature range you will be measuring. For example, when determining the freezing point depression of an aqueous solution, the expected temperature range might be between 1°C and -5°C. For boiling point elevation, the range could be from 99°C to 105°C. When measuring heat of combustion, the mercury column is usually best set between 2°C and 3°C.
2. Based on the intended temperature range, estimate the temperature at which the mercury column should reach the end of the capillary. In a standard Beckmann thermometer, the mercury rises about 2°C from the highest point on the scale to the tip of the capillary. Adjust the amount of mercury in the bulb accordingly. For instance, if you want the maximum reading to be 1°C, the temperature at the capillary's end should be around 3°C.
3. Place the Beckmann thermometer in a constant temperature bath set to a temperature slightly above the target (e.g., 5°C or higher). This helps ensure proper expansion and control of the mercury column.
4. Remove the thermometer and hold it vertically with your right hand. Gently tap the right arm with your left hand, as shown in Figure 3-3. This action breaks the mercury column at the capillary elbow. Be quick during this step, as the mercury volume changes rapidly once the thermometer is removed from the bath.
5. Place the adjusted thermometer into a bath set to the estimated operating temperature and observe the reading. For example, in Experiment 2, you can test the freezing point using an ice-water bath at 0°C. If the reading falls between 3°C and 5°C, the calibration is appropriate. If it’s off, repeat the process to fine-tune the setting.
**Second, the Ruler Reading Method**
This method is suitable for experienced users who prefer not to use a constant temperature bath. Instead, they directly read the scale on the upper part of the Beckmann thermometer.
1. Estimate the maximum temperature you expect to measure.
2. Invert the thermometer so that the mercury flows from the capillary into the reservoir. Slowly tilt the thermometer until the mercury connects with the main reservoir.
3. If the estimated temperature is higher than room temperature, use warm water or let gravity assist in moving the mercury into the reservoir. When the mercury level reaches the desired temperature on the scale, gently tap the thermometer as shown in Figure 3-3 to break the column. If the temperature is below room temperature, immerse the thermometer in a cooler bath and lower the mercury until the reading matches your estimate. Then, break the column at the elbow.
4. Check whether the adjustment was correct by observing the reading. Make further adjustments if necessary.
**Precautions**
First, the Beckmann thermometer is made of fragile glass and is longer than a regular mercury thermometer. Handle it carefully—never place it randomly. Always store it in a case or on a stand.
Second, avoid exposing it to extreme temperatures or dropping it. Even small impacts can damage the delicate structure.
Third, be cautious not to connect the mercury in the capillary to the reservoir during the adjustment process. This can lead to incorrect readings or permanent damage.
By following these steps and taking care in handling the instrument, you can ensure accurate and reliable temperature measurements in your experiments.
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