Earlier this spring I got into quite a heated debate on physicsforums.com about the nature of wave function collapse. In the course of that debate I leaned pretty heavily on the assumption that the basic chemistry of the photographic process involved a thermodynamically spontaneous transition from the undeveloped film to the exposed film. To be fair I never said I knew this to be true for a fact; I just said it seemed like a pretty good assumption, partly based on the fact that I never heard of anyone being able to "regenerate" used photographic film by, say for example, gently heating it to convert the exposed crystals back to their unexposed state.
I was ridiculed pretty soundly for trying to inject thermodynamics into the argument. Not knowing the chemistry, I kept pressing my opponents (who claimed some expertise in that field) to write out the reactions so we could evaluate it. Eventually it came out: at least in a simplified form, it appeared that we had to account for the reduction of silver bromide to elemental silver. Ignoring what happens to the bromine, we are nevertheless faced with a reaction enthalpy of 99 kJoules/mole in the positive direction: in other words, the reaction, far from being thermodynamically spontaneous, requires a significant input of energy. If we convert this to atomic terms, it comes to a near infrared "photon" for each atom of silver. It appeared that I was completely wrong.
And yet within a few days I had posted a counterargument which unexpectedly, and quite effectively rescued my argument from the ashes. It was a clever argument, backed up with sound mathematics; and one which I was uniquely predisposed to be able to make. Because thirty years previously, I had done a thermodynamic analysis of a chemical system in a completely different context which turned out to have the same essential features as the present case. I'm going to tell you about that now.
When I graduated from engineering in 1984, my first job was with Atomic Energy of Canada at their nuclear research station in Pinawa, Manitoba. You know that there are several different reactor designs out there, with the coolant system being one of the design variables. Some reactors are cooled with ordinary "light" water, and others with heavy water. The Pinawa reator was unique in being oil-cooled, which meant it ran at higher temperature with relatively lower coolant pressure.
Nevertheless one or the reactor's protective systems involved checking for cracks in the pressure tubes containing the coolant. This was done via a tube-within-a-tube geometry whereby the pressure tubes were surrounded by a containment tube full of CO2 gas, which was constantly purged and sampled at a monitoring station. Potential cracks in the pressure tubes could theoretically be detected by monitoring for trace hydrocarbon contamination in the CO2 purge gas.
The system seemed to work reasonably well for about twenty years, although it had a few quirks that no one worried to much about. Nevertheless, for one reason or another a decision had been made to upgrade and modernize the instrumentation, and as a new junior engineer I was given this relatively straightforward assignment.
That's where the story gets interesting. I'll continue with my next blogpost.