Distilling units of this kind are used for the most widely varying analyses in laboratories. The system, being closed during the distilling operation, is particularly important when substances which are gaseous or which evaporate quickly are to be bonded or combined by the distilling receiver. The invention will be described hereinafter with reference to a distilling unit for determining nitrogen in samples of milk, by direct distillation. However, apparatus according to the invention can be used for any distillation unit of the kind indicated.
Determining nitrogen by direct distillation is based on the fact that a sample, upon being boiled in alkaline solution, gives off ammonia. The major proportion of the ammonia originates from the rapid hydrolysis, which is concluded after a few minutes, of the albumin components, glutamine and asparagine. This fact makes it possible to determine experimentally the relationship between total nitrogen or protein respectively and the ammonia nitrogen which is liberated upon boiling in an alkaline solution. The whole analysis operation is reduced, by virtue of such direct distillation, to the following steps: introduction by means of pipette, dilution, alkaline ajustment, distillation, titration and calculation. It is important however that the distillation time is strictly observed and that the distillate discharge pipe is immersed into the receiver so that even gaseous ammonia components are combined in the distillate receiver.
When the distilling operation is stopped, there occurs a reduced pressure which, when the distillate discharge pipe is immersed, has the result that the distillate is sucked back into the distilling vessel. In order to avoid this, the receiver with the distillate must be removed precisely after the predetermined distilling time, before distillation is interrupted.
Equipment is already known for substantially automating the distillation operation, in order to achieve the most rational sample processing possible. With such equipment, after the preset distillation time has expired, the distilling receiver is automatically lowered before the distillation operation is interrupted. This mode of operation reliably prevents the distillate from being drawn back into the distilling vessel. However, this equipment suffers from the disadvantage that the lifting means and the electronic control arrangement which is required for that purpose is costly and complicated. The lifting mechanism is also liable to trouble and requires careful maintenance and care.