I thought I’d do a little experiment to check out how vinegar or a weak acid affects dying eggs with food colouring. The recipe I’ve seen most often is a teaspoon of vinegar in a cup of water so I used this recipe, but I added two other tests: just water and half water and half vinegar. All of them had 20 drops of food colouring and the eggs sat in the solution for the same amount of time.
As you can see there is a subtle difference between the just water and water with a tsp of vinegar. The colour was more consistent and slightly deeper for the small amount of vinegar. The third one had a much deeper colour but also had white patches and lots of pitting. The shell of this third egg actually felt rougher afterwards. So what’s happening, why is a little vinegar good but more is not?
Food colouring is an acid dye. An acid dye has a slight negative charge. When you mix the vinegar with the water the hydrogens separate and become hydrogen ions which have a positive charge. They will form a hydrogen bond (a bond between the positive hydrogen ion and the negative end of a polar molecule) with a protein coating that is on the egg. The protein coating isn’t visible but it surrounds the egg. This coating has amine groups which are a nitrogen with two hydrogens and two free electrons. This makes the nitrogen in the amine group seem negative which attracts the hydrogen ions from the acid but this actually ends up making the whole thing positive. So now, we have a positively charged protein coating on the egg which bonds with the negatively charged dye molecules.
So now, why is my tap water egg so similar to the one with vinegar? Tap water tends to be slightly acidic so it has the same reaction as the vinegar but not as strong. And why is my half and half egg darker but with white spots. The higher vinegar concentration actually breaks down the protein coating leaving some spots with nothing for the dye to bond to.