Digestion parameters: Sample temperature and digestion time
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The choice of digestion temperature and time is crucial for successful digestion. By working in a closed vessel, significantly higher temperatures can be generated than in open vessels. This significantly increases the reaction rate. Temperature and pressure are the decisive parameters that lead to an acceleration of the reaction rate and thus to digestion in closed vessels. As a rule of thumb, one can assume that an increase in temperature by 10°C results in a doubling of the reaction rate.
Pressure generation during digestion
The parameters pressure and temperature correlate directly with each other. The total pressure in a digestion vessel is composed of the vapor pressure of the acid mixture at the respective temperature and the partial pressure of any gaseous substances formed (mostly carbon dioxide during the decomposition of organic samples). While the vapor pressure of the acids can be taken from tables, the partial pressure of the carbon dioxide can be estimated from the ideal gas equation and the sample weight. The estimation of the resulting total pressure is helpful in choosing the appropriate digestion vessel.
Digestion of 500 mg carbon in HNO3 at 200°C; vessel containing 60 ml.
pCO2 = Vapor pressure of the CO2 produced
mC = Weight in g
T = Digestion temperature in K
V = total volume of vessel - volume of acid mixture (mL)
V = 60 mL,
mc = 0.5 g carbon
T = 200°C -> p(CO2) = 26 bar
Total pressure: = p(CO2) + p(acid) = 26 + 10 bar = approx. 36 bar
For economic reasons, a short digestion time is preferable. However, it must be at least long enough to ensure complete sample digestion. Digestion temperatures that are greater than 250°C lead to very short digestion times, but the maximum temperature is still limited by following factors:
- Vapor pressure of the digestion acids
- Temperature resistance of the vessels
- Pressure resistance of the vessels
If the total digestion time is longer than 45min and temperatures higher than 260°C are necessary, the use of conventional pressure digestion systems is recommended. With these systems, digestion times of several hours or days can be realized (e.g. digestion of high-fired oxides or graphite & carbides).