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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