Okay, you mix citric acid (H3C6H5O7) and baking soda (sodium bicarbonate, NaHCO3) together with some essential oils and a bit water to bind it together, pop it into a mold, let dry and you have a bath bomb. You dump it into the tub and it bubbles and gives off CO2 and…what? A precipitate maybe, some sort of salt?
Someone help! And please include a balanced equation because my mom and I haven’t had Chemistry in years and we’re going batshit trying to figure it out.
I saw this in the alt.aromatherapy newgroup and I think the equation balances. The OP asked “why do you need baking soda in bath bombs” and someone else answered: “If you leave out the Baking Soda your bath bomb will be a sour and silent little lump at the bottom of your tub.”
The exact stoichiometry of the reaction is a bit tricky because the position of equilibrium depends on pH. Sodium mono, di, or tri-citrate will form in varying proportions depending on the amount of sodium bicarbonate that’s present. pK’s for the three carboxylic acid groups in citrate are 3.0, 4.0, and 5.8, so in a typical (non-acidic) bath, you’ll have mostly tri-sodium citrate. The bicarb also has a pK, of ~6.5, which affects the extent of the reaction. In solution, you’ll have a mix of bicarbonate, carbonate, and CO[sub]2[/sub]. As the CO[sub]2[/sub] bubbles off, the pH of your bath will change.