Open vs. closed Digestion
Table of Contents
The main advantage of the closed procedure compared to open acid digestion at reflux or with the classic 'hot plate' is the significantly higher working temperatures. While these are limited by the boiling points of the acid solutions in open systems, temperatures of 200-260°C can be reached in closed digestion vessels. Temperature and pressure are the decisive parameters that lead to an acceleration of the reaction rate and thus to digestion in closed vessels. The Arrhenius equation describes the correlation between reaction rate constant and temperature. As a rule of thumb, it can be assumed that an increase in temperature by 10°C results in a doubling of the reaction rate.
k Reaction rate constant
A Frequency factor
E_A Activation energy in J/mol
R universal gas constant
T absolute temperature in K
When working with a closed vessel, the enclosed steam leads to an increase in pressure. The dependence of pressure to temperature is described by the Clausius-Clapeyron equation. An increase in pressure is accompanied by an increase in temperature, which in turn leads to accelerated reaction kinetics. Due to these effects, an acceleration of the reaction and a reduction of the digestion time can be achieved in closed systems. Even though the reaction rate can be significantly increased by using closed vessels, there are still major differences with regard to the heating sources. Either classical electrical heating jackets, as used in pressure digestion vessels (e.g. Tölg bombs), or microwave heating can be used to heat the samples. In classical pressure digestion bombs, the heating of the sample-acid mixture takes place indirectly via the heated outer vessel. This way the heating process is more time-consuming. In the microwave however, heating occurs by direct coupling of the electromagnetic radiation into the sample solution. Due to this, the time required to heat the samples can be significantly reduced. In addition, the PTFE components of the microwave are subjected to significantly less thermal stress, since they are cooled from the outside.
In conclusion, closed systems offer the following advantages over open digestion systems:
- Higher working temperatures possible
- Higher pressure possible
- Accelerated reaction kinetics
- Reduction of the digestion time