Oxygen Role in AFM Lichen Dyes

 Oxygen in the AFM for Lichen Dyes

Background

·      Assume that the reactions shown by Upreti et al 2010 are correct. That is:

·      Reaction 3 (Rx 3) requires both Ammonium hydroxide and Oxygen, that is, O2 dissolved in the dye solution.

·      O2 is not very soluble in water – only about 9 mg/L at 20°C. That is about 0.009% (much lower than the NH4OH likely present)

·      Accordingly, increasing O2 in solution should help.

·      This suggests leaving significant “head space” – air inside the AFM vessel.

o   Upreti et al 2010, for example, specifies only filling the vessel 1/3 with liquid, perhaps for this reason.

·      Many anecdotal suggestions for AFM specify:

o    daily shaking of solution in order to assist O2 dissolution. It may help – certainly can’t hurt.

o   Removal of the lid occasionally to allow new air / O2 into the head space. Makes sense if the O2 is being depleted.

Oxygen Tracking

A “thought experiment” may suggest the relative importance of shaking, opening, or bubbling air through AFM solution.

·      Assume you have a 2L container. Add 1L distilled H2O at 20°C. Close container and agitate for “a while” until equilibrium solubility is achieved.

o   At that point there should be about 9 mg of O2 in the water.

o   In the headspace prior to dissolution there should have been 1L of air

§  Air is about 21% O2 and 78% N2 by volume. Since the molecular weights of each are similar, say there is 20% O2 by mass.

§  Air weight per liter according to web calcs, is about 1.225 g, or 1,225 mg. If 20% is O2 by weight, then it contained 245 mg of O2. 

·      Therefore….less than 4% of the O2 in the original 1L of air in the jar went into solution at equilibrium. That is, 96% is still in the headspace. Until that is consumed by the reaction, no more O2 would be depleted from the headspace air. 

My Conclusion

Opening a jar daily to allow “fresh air” with O2 is not really necessary to keep O2 in solution. On the other hand, NH3 in solution is quite volatile (hence the smell when the jar is open) and daily opening allows gaseous NH3 to escape, and eventually diminishes the NH3 in solution (although by my thinking we have a considerable excess of NH3, compared to oxygen, so probably not an issue.)

In my experiment I have two open jar treatments – one with bubbling air. I will try to detect a diminishment of NH3 (by lack of odor) and may supplement additional NH3 at some point.