Thermometer Corrections CHM 254 and 265 Labs

Throughout the course, you will need to measure the temperature of a liquid or a vapor in various experiments. Although the thermometers should provide accurate measurements of temperature, there are four sources of potential error.

1. The thermometer is not properly calibrated.

2. The thermometer needs a stem correction.

3. The thermometer is improperly located.

4. In distillations and boiling point determinations, the effect of ambient pressure on the vapor pressure and consequently the observed boiling point can be significant.

In the past, we used mercury thermometers where the error was so small (except for case 3 in distillations!) that we could be sloppy and ignore it. The measured temperatures were reasonably close for our purposes. Mercury is toxic and an environmental hazard. Over time thermometers broke and even with the best efforts, the mercury could not be completely recovered. Fortunately, new thermometers are available that use organic oils with a small amount of a red or green dye. The oils are much safer but at a price. Mercury was chosen because for various reasons, it provides the most accurate and reproducible temperature readings. Nothing else behaves quite as well for standard thermometers.

Calibration should not be a problem for any thermometer. It wasn't in the past for mercury and shouldn't be for the oils. Some of the new thermometers were checked. Although they are accurate at and above room temperature, they might have a problem around 0° C. When the time comes to measure a temperature below 10° C, you should check your thermometer to see if it is properly calibrated. Your thermometers are the type known as stem or partial immersion thermometers. You will notice that nearly 2 cm below the -10° C line, there is a thin brown ring. Partial immersion thermometers are designed to give accurate readings if they are dipped (immersed) into a liquid up to that point. To check your thermometer at lower temperatures, suspend it in a stirring mixture of ice and water (an ice bath) up to the immersion ring. After five minutes, check the temperature. If it reads 0° C, it is properly calibrated, and you don't need to worry. If it reads 2° C for example, then you should subtract 2° C from any temperature reading below 10° C. If your thermometer reads for another liquid 8° C, the temperature is actually 6° C. You will also need to check at 100° C. Heat a beaker of water to boiling and suspend the thermometer to its immersion for 5 minutes. If it does not read 100° C, then you will have to adjust all future temperatures above 90° C accordingly.

Stem corrections are needed most often when measuring vapor temperature in a distillation. Usually it's not needed for mercury partial immersion thermometers. If you look at the diagram of the proper location of a thermometer in a distillation head, you can see the problem. The portion of the thermometer suspended in the hot vapor is at a different temperature from the remainder of the thermometer at the cooler room temperature. The mercury will not expand in the cool stem to the same extent as the portion in hot vapor. Perhaps more importantly for immersion thermometers, they were calibrated at 40° C and not room temperature and this must be taken into account.

The equation for a partial immersion thermometer to correct for this would be

Stem correction = a(DT)(T₁ - T₂)

a = coefficient of expansion for the liquid. For Hg, it is 0.000154. For most organic liquids, it is 0.001.

DT = the temperature difference between the temperature reading of the thermometer and where the stem emerges from the thermometer adaptor.

For example, the temperature may read that the vapor is 136° C. The first line on scale that you may read on thermometer sticking out of the adaptor may be 33° C. DT would then be 136 - 33 = 103° C.

T₁ = the temperature at which the thermometers were calibrated. The manufacturer told us that it was 40° C

T₂ = room temperature. It is best if the room temperature is measured near the cool portion of the stem with another thermometer.

Notice that for mercury, a is an order of magnitude smaller than for organic liquids. When we plug this value into the equations, the difference in temperature is small enough for many applications that we can be sloppy and ignore it. For the organic oils, corrections of several degrees are not unusual and perhaps we can't be sloppy without affecting our experiment. It will probably be necessary to do this calculation whenever you do a distillation. An example is shown below.

An organic solvent, cyclohexanone, was distilled. The vapor temperature read 152° C. Room temperature was 24° C, and the stem emerged from the adaptor at 29° C. The correction would be

X = 0.001(152 - 29)(40 - 24) = 2° C.

The actual vapor temperature would be 152 + 2 = 154° C.

Since errors were small for the mercury thermometers, then placement of the thermometer was not as crucial. For the oil thermometers, it will be important particularly for distillation.

Finally, the actual boiling point will depend on the pressure of the system (barometric pressure unless it is a vacuum distillation). The Clausius-Clayperon equation can be used to estimate the boiling point that should be observed. Unless told otherwise, we won't need to worry about it.