Temperature measurement

A word must be said here about the accuracy of thermometers. A thermometer purchased from a scientific supply house should be accurate to 0.1 deg. C. but don't count on it. Thermometers purchased at a drugstore or a winemaker's supply store can be off by as much as 2 degrees. We recommend that you always check the accuracy of a thermometer by placing it in boiling water and recording the temperature. You may be lucky and find you have purchased one which reads 100 deg. C. but if it doesn't, simply make a note of the deviation and apply the appropriate correction whenever you use it to read a temperature. And don't forget that atmospheric pressure affects the boiling point of water. Digital thermometers are extremely useful in that they are much easier to read than the glass type, sit right in front of you on the bench and are accurate enough for our purposes, more accurate in many cases than the other sort.

Fortunately for us it is not necessary to rely on the exact temperature during a fractional distillation in order to indicate when the heads have finished coming over and it is safe to start collecting ethanol. For one thing the temperature is influenced markedly by atmospheric pressure (see Appendix V). Constancy of temperature is sufficient and is what we are looking for. Thus, if the temperature has risen to just over 78 deg. C. and has stayed there for 15 minutes or so you can be fairly sure that all the heads are gone.

Briefly then, proceed as follows: Operate under total reflux for a couple of hours to equilibrate the column, bleeding off the heads periodically into a spoon and sniffing them until there is very little smell and until the temperature remains constant at just over 78°C. Then start to collect the dis-tilate by opening the valve in the stillhead.

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