Although I’m providing these instructions now because I promised to do so in my recent discussion of lemon juice (“Real Lemon versus ReaLemon,” April 19, 2011), I took most of the pictures you see here more than a year ago, after someone asked me for advice in using the strong cider vinegar from her boyfriend’s orchard. The vinegar had tested at 10-percent acid. I checked with an Extension agent I know: “To use 10-percent vinegar in a pickle recipe calling for 5-percent vinegar, you cut the vinegar with an equal amount of water, right?” No, said the agent. She would never tell anyone that it was okay to use any vinegar not commercially labeled as 5-percent acid. How could the woman know her boyfriend’s vinegar was 10-percent acid? I pressed, but the agent was firm. People should always get their pickling vinegar from a store. You just can’t trust regular people to know how to titrate vinegar. Well, my husband does titration, as do a lot of home winemakers. The process is simple, and the equipment and supplies—a graduated 100- or 250-milliliter cylinder, a graduated 10-millilter pipette, a 250-millimeter buret and stand, a 250-millimeter flask, distilled water, phenolphthalein indicator, and .2N or .1N sodium hydroxide—together cost only about $120, or less if you choose plastic instead of glassware. The chemicals are available at brewing- and winemaking-supply shops, and the glassware from science suppliers.
Here are the steps in titration:
1. Bring some distilled water to a boil to drive off any carbon dioxide. You’ll need a little less than ½ cup water per test. Measure 100 milliliters water in a graduated cylinder. Then pour the water into a small flask.
2. Draw 5 milliliters wine, vinegar, or juice into a pipette—a glass tube with a very narrow opening at the bottom and a wider one at the top. You can draw up the fluid either by putting the top of the tube in your mouth and sucking or by using a rubber bulb made for the purpose. Then put your finger firmly over the top opening, and check the fluid level. Do you have a little more than 5 millimeters? If so, lift your finger to drain a bit out. Because the pipette is so skinny, this is a very precise way of measuring.
3. Hold the pipette over the flask of water, and lift your finger to let the wine, vinegar, or juice drain out. Add three drops of phenolphthalein indicator solution. Phenolphathalein is the ingredient that made Ex-Lax useful for acid-base experiments when you were a child.
4. Now you’re going to use the buret. It’s a graduated glass tube, on a stand, with a small lower aperture and a stopcock. Pour .2N sodium hydroxide into the buret to near the top of the numbered scale. (Scientists read the N as “normal.” If you’re using .1N sodium hydroxide instead of .2N, see the paragraph following this. Also, keep in mind that sodium hydroxide, however normal, is very corrosive. You don’t want to suck it up with a pipette.)
5. See how the surface of the fluid in the buret curves, like a contact lens? This curve is called a meniscus. Record the number at the bottom of the meniscus.
6. Now turn the stopcock so the base solution in the buret slowly drips into the indicator solution while, with your other hand, you swirl the flask. As each drop of base solution falls into the flask, a spot of pink may briefly appear. As you continue adding the base solution, the pinkness will take a little longer to dissipate. Add the drops slowly, and keep swirling. As soon as the liquid in the flask turns a uniform pale pink, stop adding drops. If you wait for the fluid to turn hot pink you’ll have gone too far, and your results won’t be accurate.
7. Record the level of the fluid remaining in the buret. Then record the difference between this number and the one you recorded in step 5.
8. If you’re measuring acetic acid (in vinegar), divide the difference by 4.16. If you’re measuring citric acid (in lemon or other citrus juice), divide the difference by 3.90. If you’re measuring tartaric acid (in wine or verjuice), divide the difference by 3.33. The result is the percentage of acid in your sample.
I could give you formulas for figuring out the percentage of acid regardless of the size of your sample or the normality of your sodium hydroxide, but the formulas might confuse you as much as they confuse me. If you can’t find .2N sodium hydroxide, you’re likely to find .1N instead. In this case, just double the divisor in step 8. If you start with a 10-milliliter sample instead of a 5-millimeter sample, do the same: Double the divisor. If you use .1N sodium hydroxide and a 10-millimeter sample, multiply the divisor by 4.
See, that wasn’t so hard, was it? Now, for practice and to ensure accuracy, repeat the titration, preferably twice. If you have any trouble, watch the very detailed video on titration technique at http://www.youtube.com/watch?v=9DkB82xLvNE.
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This is awesome. I was just reading your post about fresh lemon juice for canning and then had to read this one based on the title. I don’t have any immediate plans to titrate anything, but it’s cool knowing that all those titrations that I did in my high school chemistry class could actually be put to use in my real life! I still remember that quest for the perfect pale pink….lol.
Thanks for the info on fresh vs. bottled lemon juice, by the way. I was wanting to use fresh lemon juice when I can tomatoes tomorrow, but nearly every other website expressly prohibits it. Your post has put me at ease that I won’t be poisoning everyone.
From where did you get those numbers in last step ??
Swati, here’s your answer, from my scientific advisor: The different correction factors for acetic, citric, and tartaric acid result from their different molecular weights and different numbers of titratable acid groups that are neutralized by the NaOH in the titrating solution.
Acetic acid: 60.1g/mole, 1 titratable acid group
Citric acid: 192 g/mole, 3 titratable acid groups
Tartaric acid: 150.1g/ mole, 2 titratable acid groups
Thanks so much for this info! You’ve confirmed something I’ve suspected for quite a while regarding the bottled vs fresh lemon juice, and having recently read an article about where powdered citric acid comes from (black mold grown on corn syrup- yuck!), I am thrilled to know that fresh squeezed is a viable option!!
I have some infused balsamic vinegar that is not marked with percent acid, and I’ve been wondering how I could find the info myself. I’ve been eying pH meters, but those babies are expensive!
I am a little confused by your instructions, though, and am hoping you can clarify Step 4. You state, “Pour .2N sodium hydroxide into the buret to hear the top of the numbered scale.” I do not understand what you mean by “hear”… Are we listening for something? Or is this just a way of saying we’re filling it up a bit beyond the ‘full’ mark? I have done titrations before, but it’s been a few years, LOL.
Zoe, thanks for pointing out that error, which somehow happened in the transition from Word to WordPress. The word should be not hear but near. I’ve just corrected the error.
Okay, that makes *much* more sense! LOL, I’m normally pretty fluent in typo but could not figure that one out! Thanks!
Is there a step 6 somewhere???
Sheila, thanks for alerting me to the fact that step 6 disappeared. I’ve just reinserted it.