Purple Dyes from Lichen
Background
A number of posts have shown purple dye baths and dye results from “AFM” methods with various lichens. One in particular is a widely available, abundant lichen, Evernia prunastri.
Underlying Chemistry
Literature about orcein dyes states they are traditionally produced from 5 or 6 lichen genera, including Evernia. Orcein is a traditional purple dye, it is likely the chemical structure responsible for successful blue/purple dyes.
One article (Upreti et al 2010) seems to authoritatively describe precursors and reactions to get orcein compounds from lichens:
Assuming these reactions are credible, it would offer guidance for how to achieve successful orcein production. Note that in the final reaction both ammonium hydroxide and oxygen are necessary.
I’m going to call this equation 3:
Eq 3: 2C7H8O2 + 2NH4OH + 3O2---> 2C7H7NO3 (orecein) + 6H2O
Condensation Reaction(s)
The third and final condensation reaction above shows one mole of Nitrogen and 3 moles of O2, i.e. 6 moles of oxygen are needed, suggesting to me that insuring sufficient O2 is important.
Solubility of oxygen in water
“Water solubility of oxygen at 25oC and pressure = 1 bar is at 40 mg/L water. In air with a normal composition the oxygen partial pressure is 0.2 atm. This results in dissolution of 40 . 0.2 = 8 mg O2/L in water that comes in contact with air.
Oxygen solubility is strongly temperature dependent and decreases at higher temperatures. Oxygen solubility is negatively correlated with the amount of dissolved solids. Consequently, oxygen solubility in freshwater exceeds that in seawater by 1-3 mg/L, depending on temperature."
Read more: https://www.lenntech.com/periodic/water/oxygen/oxygen-and-water.htm#ixzz7IX350h91
If there was 8mg O2 per liter water (1,000 g), and we had, e.g., 2% NH3 in solution, that would be 200 grams of NH3, or 400 g as NH4OH per 1,000 g solution. That is 400,000 mg of NH4OH, or 50,000 times as much as O2, but you need 3X O2 compared to NH4OH. Seems to me we are woefully short of oxygen! And note that it actually helps to be colder rather than warmer! The effect is not overwhelming, though – only goes up to 14 mg/L at 0°C (75% more O2). Raising temperature – say from 40°F (4°C) to 90° F (32°C) diminishes it from about 12 mg/L down to 7 mg/L, so nearly 50% less. Realistically, though, our AFM options are something more like room temperature (say 20°C, 68°F) or warm somewhat - (say 30°C or 86F). The warmed treatment only reduces the O2 from 9 down to 7 mg/L, or 22% less. On the other hand, the rule of thumb for chemical reactions is that 10°C increase in temperature doubles the reaction rate. This means that warmer would be far preferable for the reaction rate, even though slightly less O2 present.
A quick search seems to show that pH has a very large impact. Dissolved Oxygen (DO) almost quadruples going from pH 8 to about 9.2. Hard to say what it does going clear to pH 11 or higher.
Ammonia and ammonium hydroxide
Ammonia (NH3, a gas) dissolves readily in water. The solubility allows a w/w solution of up to about 36% at typical room temperatures (20°C or 68°F). (My plot of data from Merck Index).
Most available “Ammonia water’ solutions are well below this concentration, typically reported as 1-5% w/w, with occasional sources at “Janitorial Strength” at 10%. Laboratory reagent grades can be purchased up to 25% or so.
But ammonia in solution doesn’t readily go to “Ammonium Hydroxide”, NH4OH, as it is often mis-named. According to Wikipedia entry for “Ammonium solution”:
“Although the name ammonium hydroxide suggests an alkali with composition [NH4+][OH−], it is actually impossible to isolate samples of NH4OH. The ions NH4+ and OH− do not account for a significant fraction of the total amount of ammonia except in extremely dilute solutions.
In aqueous solution, ammonia deprotonates a small fraction of the water to give ammonium an hydroxide according to the following equilibrium:
NH3 + H2O ⇌ NH4+ + OH−.
In a 1 M ammonia solution, about 0.42% of the ammonia is converted to ammonium” (my emphasis)
However, the pH of the solution has a huge impact on this equilibrium. This effect is shown elegantly here:
Note that purchased “Ammonia Water” is typically very high pH – 11.5 to 12.5. At room temperature virtually none of the dissolved NH4 would be in ammonium form. Yet if the pH were in the 9-10 range, between 20 to 75% of the NH4 goes to NH4OH or NH4+ ammonium ion. Assuming Upreti’s 2010 Eq.3 is correct, our final step requires NH3OH, not NH3, yet normal conditions of AFM would seem to greatly restrict this.
One hypothesis would be that the historical use of aged urine as the ammonia source “worked” because the ammonia content was low (one source I could find showed only 0.2% NH3 in urine aged for 4 hours), and this (plus other compounds in urine) kept the pH low (relatively – Wikipedia says it ranges from4 to 8) and thus much of the NH3 generated during aging shifted to NH4OH+, fostering good results via Eq.3. This is purely speculation.
My experiments including adjusting the pH of ammonia solutions down to as low as pH6.
Impact of Oxygen
Equation 3 describes that stoichiometrically, 3 moles of O2 are required for each mole NH4OH. Unlike ammonia gas, O2 is not very soluble in water, as shown below.
Tentative Conclusions
I currently think that concentration of starting ammonia solution is not that crucial – most of the time there will be a vast excess of NH3 to begin. Having a reduced pH (say 10 to 11) to foster equilibrium to NH4OH is critical, as is getting O2 into the reaction. I suspect that Eq. 3 reaction is faster at higher temperatures, although this reduces the dissolved O2 available, but by far less than the expected reaction rate increase, so I think warmer is "better" (faster).
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