[This is a revision of post #12 in this
thread.
Post #12 is superceded by this post.]
-----------------------------
Determination of nicotine in e-liquid by titration
-----------------------------
First, a word from my ..., which prefers to be covered:
-----------------------------
ALL MATERIAL SAFETY DATA SHEETS APPLICABLE TO ANY MATERIAL REFERENCED HERE MUST BE READ AND UNDERSTOOD, PARTICULARLY AS THEY RELATE TO CHEMICAL EXPOSURE AND SPILL CLEANUP. APPROPRIATE PERSONAL PROTECTIVE EQUIPMENT MUST BE WORN INCLUDING (AT MINIMUM) ANSI Z87 RATED SAFETY GLASSES, CHEMICAL RESISTANT GLOVES APPROPRIATE TO THE MATERIALS BEING HANDLED, AND A PROTECTIVE LAB COAT. ALL WORK MUST BE PERFORMED IN A FUME HOOD UNDER THE SUPERVISION OF AN APPROPRIATELY TRAINED INDIVIDUAL. AN EYEWASH AND SAFETY SHOWER MUST BE READILY ACCESSIBLE AND ANYBODY ATTEMPTING ANY PROCEDURE REFERENCED HERE MUST BE TRAINED IN THEIR USE. NOTWITHSTANDING THESE SAFETY REQUIREMENTS, INJURY MAY STILL RESULT AND THE INDIVIDUAL BY PERFORMING ANY PROCEDURE SPECIFIED HERE ACKNOWLEDGES ALL RESPONSIBILITY AND LIABILITY AS HIS OWN.
-----------------------------
Acid/base titration is an analytical technique by which a standardized solution (in this case an acid) is added to a solution containing an unknown amount of a substance (in this case, nicotine, an organic base) to determine the amount/concentration of the unknown via neutralization. A color changing indicator is added to the solution containing the nicotine to tell us when the nicotine has been neutralized (all free nicotine converted to salt). It is at this point that the pH of the solution swings from alkaline (pH > 7) to acidic (pH < 7). Nicotine acts as a mono-hydroxy base (even though it's really a basic nitrogen at work), and titration with a strong acid gives a sharp endpoint. This is to say that as we add acid in the presence of excess free nicotine, the solution pH will drop slowly toward 7 (neutral) until the free nicotine is neutralized. At this point, adding another drop or two of the acid solution will cause a precipitous drop in the pH of the solution indicating what is referred to as the endpoint of the titration.
Example: we have 50 mL of water at pH 7. We add 1 drop (0.05 mL) of 0.1N HCl solution. What is the resulting pH? Doesn't seem like it should change by a lot, does it? wrong. The hydrogen ion concentration becomes:
0.1 * (0.05/50.05) = 0.0000999 N (round to 0.0001 N)
Since pH is defined as -log (base 10) of the hydrogen ion concentration, we find the negative log of 0.0001, which is 4. So that single drop of 0.1 N strong acid (full ionization) drops the pH from 7 to 4.
So as long as there is excess nicotine relative to the acid that's being added, the pH remains greater than 7, but once the nicotine is consumed, it takes a tiny amount of acid to drop the pH far below 7.
Enough theory.. let's make it work.
---------------------------------
Equipment:
1. 50 mL burette w/stopcock, 0.1 ml subdivisions.
(can be had for under $20)
2. A burette stand and burette clamp.
(or anything you can cobble together to make the burette stand upright in one spot).
3. 250 mL erlenmeyer flask.
(or suitably sized glass container)
4. 25 mL graduated cylinder, 0.5 ml subdivisions.
(under $10 on eBay or elsewhere)
5. Bromothymol blue, aqueous solution.
(API pH Test Kit, from Mars Fishcare) Walmart usually has it for about $5 in the aquarium supply section. If it has a color card in the box that shows a range of 6.0 - 7.6 (going from yellow, through green, to blue), this is the stuff.
6. 1000 mL volumetric flask
(or 1L plastic bottle marked with a Sharpie)
7. 0.1N HCl solution, 1 liter.
(you make this from 32% muriatic acid)
8. Distilled water, 1 gallon.
(from local supermarket)
Before use, all containers should be washed 3X with distilled water at 10% of the volume of the container and dried with a clean paper towel. E.G., A 250 mL erlenmeyer flask should be rinsed 3X with ~25 mL distilled water.
---------------------------------
Preparation of 0.1N HCl
You could simply look around on the internet and purchase a liter of 0.1N sulfuric acid. This shouldn't land you on any government watch-lists. Try to buy concentrated sulfuric acid, and someone is going to notice. If you don't want to go the 0.1N sulfuric acid route, you can make a pretty approximate 0.1N HCl solution by purchasing some 32% muriatic acid. You've just re-read the safety warning at the top of this post, you've read a bunch of MSDS sheets, and bought a bunch of personal protective equipment. You've got a fume hood, professional supervision, safety shower, and eyewash close at hand. Good.
Into a 25 mL graduated cylinder, add 9.63 mL of 32% muriatic acid. You'll not really be able to exactly add this amount to the graduated cylinder, but assuming you have the 0.5 mL subdivisions, just eyeball it to 1/5 of the way between the 9.5 and 10.0 mL mark. When reading liquid levels, always read to the BOTTOM of the meniscus at the top surface of the liquid.
Add approximately 500 mL distilled water to your 1000 mL volumetric flask (or 1 liter plastic bottle marked for 1L) (pre-washed 3X with distilled water, remember?) Carefully add the acid from the graduated cylinder to the 500 mL distilled water in the volumetric flask/plastic bottle. *CAREFULLY add 25 mL distilled water to the residual acid in the graduated cylinder to rinse any residual acid and add this rinse to the 1000 mL volumetric flask/plastic bottle. Swirl to mix well, and add additional distilled water to reach 1000 mL final volume. Mark this solution 0.1N HCl. *(NOTE: STRONG ACID SHOULD BE ADDED TO WATER, NEVER ADD WATER TO ANY APPRECIABLE VOLUME OF STRONG ACID). 0.1N HCl is generally not hazardous, but treat it caution and maintain safety as previously detailed. Store the remaining 32% muriatic acid in a secure location inaccessible to children. This stuff
is dangerous.
---------------------------------
Determining the amount of e-liquid to use:
When determining how much e-liquid to use, we'll start by assuming that the supplier is both competent and honest. We will use the mg/mL on the label to find the volume of e-liquid to add to the 250 mL erlenmeyer flask. This volume will correspond to the amount of nicotine at the vendor stated nicotine level that will be exactly neutralized by 25 mL of the 0.1N HCl solution. 25 mL of 0.1N HCl will neutralize exactly 0.0025 moles of a mono-hydroxy base (or of an organic base acting as a mono-hydroxy base).
The molecular weight of nicotine is 162.26, so 0.0025 moles of nicotine is:
162.26 grams * 0.0025 = 0.4056 g.
We'll just call it 406 mg.
So, if you have e-liquid that claims to be 36 mg/mL, you'll want to use:
406mg/(36 mg/mL) = 11.28 mL
If your e-liquid claims to be 48 mg/mL, you want to use:
406mg/(48 mg/mL) = 8.46 mL
We've now done the math.
---------------------------------
Procedure:
1. After 3X rinsing the 25 mL graduated cylinder to make damned sure we have all the concentrated acid out of it from the step where we made the 0.1N acid, add the volume of e-liquid to the graduated cylinder that was determined in the previous section. Again, assuming you have 0.5 mL subdivisions, estimate fractional mL's as best you can. Again, measure to the BOTTOM of the meniscus on the top of the liquid.
2. Add the e-liquid from the graduated cylinder to the 250 mL erlenmeyer flask. rinse the graduated cylinder 3X with equal volumes of distilled water and add each rinse to the erlenmeyer flask such that the total volume of liquid in the erlenmeyer flask is approximately 50 ml. (If you used 12 mL e-liquid, 3 rinses at ~ 12 or 13 mL each should do it). The rinses should be well stirred inside the graduated cylinder (use a clean soda straw) to make sure that none of the viscous e-liquid remains in the graduated cylinder.
3. Add 30 drops aqueous bromothymol blue solution to the e-liquid/distilled water solution in the 250 mL erlenmeyer flask. The color of the solution should be distinctly blue.
4. Rinse the burette 3X by filling it with 0.1N HCl solution, and drain it. Add 0.1N HCl solution to the burette and open the stopcock slowly and allow the meniscus of the acid solution to precisely reach the top mark on the burette (the top is often dual marked as 50 mL and 0 mL, I.E. 50 mL present, 0 mL dispensed).
5. Secure the burette in the burette holder and secure the burette holder to the ringstand (or whatever monstrosity you've cobbled together to hold the burette still). Place the tip of the burette 1 inch above the lip of the erlenmeyer flask containing the nicotine/indicator solution. Also, you should place a sheet of white paper under the erlenmeyer flask to help see the color in the flask clearly.
6. If the e-liquid vendor is correct, it will take 25 mL of the 0.1N acid to complete the titration. Slowly open the stopcock on the burette and establish a flow of approximately 5 mL/minute.
7. As the acid solution drops into the nicotine solution, swirl the erlenmeyer flask to ensure complete mixing. THIS IS VERY IMPORTANT. SWIRLING MUST BE MAINTAINED. You are looking for the point where you start to see the solution turn a greenish tint from blue. (Wikipedia has a picture of the color change under their bromothymol blue article, refer to it). As soon as the color of the solution has gone to greenish, close the burette stopcock.
8. Re-open the burette stopcock, and allow the 0.1N HCl to drop at a rate of 1 drop every 3 seconds. Continue to swirl the flask until the solution turns from green to yellow and remains yellow. If you overshoot by a few drops, don't worry. The titration is done.
9. Record the volume of 0.1N HCl solution dispensed.
---------------------------------
Calculation:
mg/mL nicotine in e-liquid:
= supplier's claimed mg/mL X (volume 0.1N HCl dispensed in mL / 25 mL)
---------------------------------
Good luck!
