It's easy to get confused about whether milk is an acid or a base, especially when you consider that some people drink milk or take calcium to treat an acidic stomach. Actually, milk has a pH of around 6.5 to 6.7, which makes it slightly acidic. Some sources cite milk as being neutral since it is so close to the neutral pH of 7.0. Milk contains lactic acid, which is a hydrogen donor or proton donor. If you test milk with litmus paper, you'll get a neutral to slightly acidic response.