The Science Behind a Perfectly Dyed Easter Egg
http://www.wired.com/2015/04/science-behind-perfectly-dyed-easter-egg/
The vinegar part has always bugged me. Hard boiled eggs have a pungent enough aroma on their own; why do we need to add another acrid smell to the dying process? It’s not just to keep the kids dunking instead of drinking, it turns out. Most food dyes are acid dyes, so called because they only work in acidic conditions. The vinegar—a solution of 5 percent acetic acid in water—is there to bring the pH low enough that the dye will actually bind. But is there an ideal pH for perfect egg-dying saturation? A normal box of food dye says to add 1 teaspoon of vinegar for every half-cup of water—but would tweaking that acidity by adding more or less vinegar get you better results? WIRED decided to find out.
First, some explanation: Why does acid make the dyes dye better? The colored molecules themselves are sodium salts of a phenolic acid. Once those dyes get thrown into water, the sodium ions fall off, leaving behind the negatively-charged part of the molecule. Add vinegar, and you’re adding lots of free protons—positively charged hydrogen ions—which fly in to take the place of those missing sodiums. The hydrogens, now associated with the dye molecules, are important because they allow hydrogen bonding. Their slightly positive charge acts like a magnet, attracting it (and the dye, in tow) to slightly negative atoms in the protein molecules and calcium carbonate (CaCO3) in the eggshell.
The color you see on the egg—red, yellow, blue, green–depends on how each particular dye molecule absorbs and reflects different wavelengths of light. But the saturation of that color depends on how strong a bond you can get between the egg’s calcium-filled surface and the dye molecules. So you gotta add vinegar. But how much? We tested the effects of different levels of normal white vinegar, 5 percent acetic acid, on the color of a hard-boiled egg, while tracking its pH. We tested six different conditions: Pure water (pH 7), a cup of water with 1/8 teaspoon of vinegar (pH 6), a cup of water with 1/2 teaspoon of vinegar (pH 5), a cup of water with 2 teaspoons of vinegar in it (pH 4), half and half water and vinegar (pH 3), and pure vinegar (also pH 3). The alkalinity of your water will have a very slight effect on the pH of those mixtures—but definitely not enough to notice in the appearance of your eggs, or enough that you’ll notice it on a universal pH test strip.
We boiled all the eggs for the same amount of time and put the same amount of food coloring (6 drops) in each of our cups (controlling for the total volume of each cup, of course). After a perfectly-timed 5-minute dunk for each egg, here’s what we saw:
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