# Determining Charge of Protonated Alkaloids

During the extraction of many alkaloids, you lower the pH in order to protonate the compound, making it polar and water soluble. With Mescaline, you must precipitate it as a salt.

What I don't understand, is how can you figure out the mole ratio between the Mescaline and the conjugate base of the acid (for example acetate). My understanding is you'd have to figure out the charge of the protonated mescaline. How? Thanks

Edit:

The reason is not so I can ionize all of the mescaline. I want to know this so I can calculate the molar mass of the mescaline salt. That way I can take the weight I have of product, and calculate (using the molar mass) the weight equivalence of freebase mescaline, for dosage purposes. Also, it's just interesting knowledge that I'd like to know.

• I don't think anyone is counting. You don't use exact equivalents of the acid. You just add hydrochloric acid (probably 1 or 3 M) until the pH is low enough. – Zhe Dec 7 '16 at 22:21
• Mescaline has one amine group so what would be there to count? – Mithoron Dec 7 '16 at 22:47
• Yes, but I'm not counting for that reason. I'm counting so I can estimate the molar mass of the salt. – Thor Correia Dec 8 '16 at 1:55

Are you sure you're trying to calculate the molar mass of the mescaline salt? Isn't that known, depending on the acid you use?

It seems to me the key you need is the mole ratio. For a weak acid-base conjugate pair, you can use the Henderson-Hasselbalch equation:

$pH = pK_a + log(\frac{[B]}{[A]})$

where [B]/[A] is the ratio of concentrations of conjugate base to conjugate acid. If calculated for a single solution containing both forms, the total solution volume cancels and this is the mole ratio. In a more convenient form,

$\frac{B}{A} = 10^{pH - pK_a} = \frac{K_a}{[H^+]}$

(The first equality is more practical, the second is useful for theoretical manipulations.)

Note that when $pH = pK_a$, $B = A$ (the analyte will be 50% protonated and 50% deprotonated).

For mescaline (more precisely, its conjugate acid), $pK_a =$ 9.56, typical of an amine base/alkaloid. So at a pH of 9.56, approximately half of your mescaline should be protonated. You could extract this, measure the weight of the salt, use the known molar mass to convert to moles of the protonated mescaline (I recommend using a monoprotic acid such as acetic if possible to simplify the situation). By the Henderson-Hasselbalch, the moles of protonated mescaline you extracted at its $pK_a$ will equal the moles of freebase mescaline remaining in your organic solvent.

Rule of thumb is to go 2 pH units below the pkb of the compound you're going to protonate, in order to get ~100 % protonation (and vice versa; 2 units above pka for complete deprotonation).

So for mescaline: pkb: 4.44, you would be pretty safe to add HCL to pH ~2.

• Thanks! I know that much, but that's not why I'm trying to find the charge. I'm trying to calculate the molar mass of the salt. Also it's just useful information I'd like to know. – Thor Correia Dec 8 '16 at 1:56
• Ok, and you are using which acid for salt formation? – Daniel H H Dec 8 '16 at 9:43
• Debating between acetic acid and citric. – Thor Correia Dec 9 '16 at 14:23