#12 Under Pressure


After digesting eclogite in my bombs recently, I’ve really started to question one assumption. One of the reasons that I’ve been using the bombs in the first place is that I know that the high temperature and confined pressure reduces the boiling point of the acids used for sample digestion, thus meaning that effective digestion can take place at higher temperatures without the acids gassing off before dissolution is complete. Having said that, however, I have no definitive idea about the pressures generated in the bombs. This is because there is no simple easy way to know, as the pressure attained is a function of not only the ‘air space’ left in the bomb (that is the capacity of the bomb minus the volume of acid and sample added), but also of the composition of the liquid being heated.

After hunting in the manual for the bombs, I found some patchy data on the pressures assumed to be generated in the bombs by a small selection of liquids when present as a 10 ml volume in a 23 ml space. They are:

Comparing the pressures generated in 23 ml bombs by 10 ml of different liquids.

I thought I might plot up a simple graph comparing the pressure (P) and temperature (T) for these liquids in the bombs (above). The exercise also proved to be a good exercise to practice my skills in R programming. As you can see, even between the four different liquids, there’s quite a large variation in the pressure to be expected. Missing from this plot, however, are the two acids (HF and HNO3) which I use in my bomb dissolution. Who knows how they behave in these bombs? Well nobody, apparently. For the life of me I can’t find any data on those acids so as yet I’m unsure as to the actual pressures generated.

I’m almost certain that HNO3 alone generates pressures >> 2000 psi, as when cleaning the bombs, often a significant proportion of the acid is lost, i.e. the pressure is so high the top of the bomb (which is spring loaded) opens and some gas is released until it reseals. This does not happen when a sample is present in a mix of both HF and HNO3, suggesting that this mix generates a pressure somewhat lower. How much lower, however is the golden question which I am as yet unable to answer. As far I can see at the moment, there can be no sure way of knowing without conducting direct pressure measurements in the bombs, but watch this space as I might be able to do some modelling if I got my hands on some data!

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