The autoclave not only provides temperature control above 100 degrees C, but also external pressure on the vessels to limit the chance of rupture. What other options there are I do not know

@ Linkon, you may try with pressure cooker.

There are other methods for digestion of samples for the measurement of phosphorus and nitrogen concentration, such as the use of microwave digestion or pressurized liquid extraction (PLE). These methods can be quicker and require less sample volume than acid persulfate digestion using an autoclave, but results may vary depending on the specific sample and method used. It is always recommended to consult the literature to compare the results of different digestion methods and choose the one that best suits your needs.

Thank you Dr. Paul Milham, Dr. J. C. Tarafdar, and Dr. Ahmad Al Khraisat for your suggestions.