Oh, no, you're not getting off that easily. You asked the questions so now you're getting a lecture
I use bits and pieces of this stuff in a practical application at work every day; it's fun for me to go back and consider it from the textbook side every once in a while and try to explain it. Makes me a better employee and a better writer.
Thank so much for this answer, math-troll and all. This is a great learning step, a lot to consider.
If we have 1g of freebase and add 15.6 drops - what happens? What is the pH of the water? How much water am I using? How much is freebase? How much is salt? Does that depend on the solubility of the freebase in the xylene and the solubility of the salt in water? How much xylene am I using?
Holy shit. I have to read some things...
Today's lesson is a bunch of me blathering about the pH equation: pH = -log10[H+]
is the concentration of acid in solution expressed in Moles/L. This is easy to do with HCl because it breaks apart completely into H+
(Hydronium ions aka protons) and Cl-
(chloride ions) in water; it's more difficult to do with other acids, like acetic, that don't completely break apart.
pH is a dimensionless quantity, and as such there is no unit. The equation simply describes the amount of free hydrogen (H+
ions) in solution from a range of 0 (lots of free hydrogen) to 14 (Very little free hydrogen) Because this is a logarithmic function, each whole number indicates a 10x jump in the amount of H+
ions. So a pH 6 solution has 10x more than pure water (pH 7), pH 5 solution has 100x more than pure water, ph 4 solution has 1000x more, etc
Looking back at our notes from last night we can see that 1mL of our 20% solution contains 0.006mol of HCl. Let's convert that to moles/L quick so we can calculate the pH of our 20% solution.
0.006mol x 1,000mL/L = 6mol/L
 = -0.79
Yes, pH can be negative, even though the scale goes from 0-14, but you only ever really see it when calculating the pH of concentrated acids.
If you have 1L of xylene that contains 1g of mescaline and you add 15.6 drops of 20% HCl you'll be able to see the mescaline HCl crystals crashing out with each drop you add, but in the end you'll have 1L of xylene with 0.78mL of water at the bottom. The salting reaction happens only at the interface between the xylene and the water, and with such a tiny amount of water you could shake that 1L container for a thousand years and that 0.78mL of water would only touch a fraction of the xylene to react with a fraction of the mescaline. The pH of the water would remain relatively unchanged at around -0.79 (I think... I hope pharmer can chime in a bit to clarify the Theory vs Practice on this for me)
If I remember correctly from your extraction thread you run 100mL of strong HCl solution through your xylene and check the pH, adding more acid dropwise until the pH is 7 or lower. This is excellent Practical Chemistry, as a pH of 7 or less indicates that all of the mescaline in the xylene has reacted with the H+
ions in the aqueous solution. The brown impurities we see in "over-acidified extractions" are probably other alkaloids that have a much lower pKa than mescaline that come over in the overly-strong HCl solution.
If we have estimated our xylene to contain 1g (0.0047mol) of mescaline freebase then we need to introduce it to 0.0047mol of HCl in a solution that has a pH that isn't so low that all the icky stuff comes over with it. Let's just see what happens if we put our 15.6 drops (0.0047mol) of 20% HCl in 1L of pure water.
[0.0047] = 2.33
So we have created a pH 2.33 solution that if we run it through the xylene in 5 or so pulls with vigorous mixing each time should theoretically convert 1g of mescaline freebase (211.26 g/mol) into 1.16g mescaline HCl(247.72g/mol)
I'm kind of getting into the weeds here, so I'm going to stop and encourage further questions and maybe some input from more experienced extractors...
Theory and Practice are VERY
different animals, and these numbers are not quite lining up with established extraction protocols as far as I recall; it's been a hot minute since my buddy Freddy was doing this kind of stuff regularly and textbooks only go so far...
Regarding the multiple pulls because of xylene and water's mutual dislike, I have a question:
If my salting-out water reaches pH 5 and on evap'ing yields for example 100mg of salt, approximately how much do you think might remain to be pulled from the xylene in the next round? (assuming I shake the mixture for a minute after adding drops of acid).
Multiple pulls are done because the reaction only happens at the places where the xylene and the water touch. Each time they touch they decrease the probability of the reaction happening again so we shake them up to increase the number of places they touch.
Forgive me, but I don't quite know your salting process well enough to speak to the numbers. I know it's a "titration salting" but can you run me through it again please?
Perhaps with example numbers from previous extractions? (I know that's a big ask, and if you don't have actual data you can just make up numbers)
I applaud you're desire to get to the finicky fine details of proper chemistry.
Some of us don't have that kind of discipline for math (god bless those who do) and just add a drop of concentrate and watch what happens. You'll be able to see the reaction in the liquid and then the crystals drop out. Some of us do this until nothing precipitates out of solution. Usually what happens is you get brown crystals once you've evap'd away the over-acidified water.
It's far from perfect but gets the job done.
Knowing the math, and why it's important, is master class level stuff. Knowing what the reactions are and why they happen at each step of the process is where the meat and potatoes of home chem projects live.
That's the beauty of Chemistry! You can go at it in several different ways. None of the famous Counterculture chemists (Nicholas Sand, Owlsley Stanley, William Leonard Pickard etc) had chemistry degrees; they were simply exceptionally intelligent individuals with the means and drive to produce these exciting molecules...
"Far from perfect but gets the job done" is just as good as "I got perfect results."
Either way the job is done!
Can you tell us more about this adding concentrated acid dropwise into xylene method please? I've never read anything about it & I'm mighty intrigued by watching anything precipitate out of anything else. That's some meat and potatoes I want to know more about...
Edited by Phineas_Carmichael, 08 May 2021 - 01:28 AM.