Salt and Vinegar explored

 kw: experiments, vinegar, salt, acidity, ph, photo essays

Kitchen chemistry: putting vinegar on an oxidized penny doesn't seem to do anything. Adding table salt makes the penny shine right up. Also, I get lime deposits in the ceramic cup holder in the bathroom. I've tried the same method. Adding salt to vinegar on the lime makes it much easier to clean off.

Question: Does the salt make the vinegar more acidic?

I have some pH paper that I bought when I was a chemistry major many, many moons ago. Here are a couple of pieces on a salad plate, initially dry, near the paper dispenser with its scale. The pieces have a pH near 5 because of carbon dioxide in the air, which makes any moisture in the air shift from a neutral pH of 7 to about 5.6, the natural pH of rainfall.

The color bars on the scale are 1, 3, 5, 7, 9, and 11.

I next added a couple of drops of vinegar to each piece of paper. They became a little more red, indicating a pH near 4. Then I put some salt on the lower one. These pictures show what happened (not much!):

The salt is visible in the pic on the right, on the lower paper. If I try hard, that piece of pH paper may look slightly redder than the other. But really, there is no measurable change in the pH after adding salt.

So why does vinegar with salt added clean pennies, and help remove lime scale more rapidly?

I think the effect is due to kinetics. Adding table salt, sodium chloride, causes an equilibrium reaction such that some of the acetate ions in the vinegar solution shift their "allegiance" to sodium ions, and some of the hydrogen ions are then free to "work with" chloride ions. Hydrogen chloride, or hydrochloric acid, is a stronger acid than the acetic acid in vinegar. "Stronger" doesn't mean it has a different pH necessarily. It refers to how strong the reaction is when it encounters a material it can attack, such as copper patina or lime scale.

Without doing experiments for which I don't have the equipment, I can't go further. The hypothesis, "Hydrogen chloride attacks susceptible materials more effectively than hydrogen acetate, in low-pH solution" will do for now.