Technically, the pH scale measures how many free hydrogen atoms you'll find in a given substance. The term "pH" is actually short for the power of hydrogen. For household purposes, however, you probably don't need to understand all the scientific stuff unless you're into that. It's enough to know that the pH scale runs from 0 to 14, with 7 being neutral, anything below 7 being acidic, and anything above 7 being basic.
Don't let pH designation fool you. We tend to think of acid as a nasty, poisonous substance, and it can be, but basic substances can be as well. The pH measurement of a substance tells you only how acidic it is on the pH scale; it does not indicate toxicity or safety. Vinegar, for instance, has an acidic pH of 2 but is often found in recipes. Ammonia has a basic pH of 11, but you don't want to drink it.
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When it comes to household cleaning, acids like vinegar and lemon juice are powerful cleaners that can attack mineral buildup, hard water stains, and rust. They also do a good job of cleaning stains off masonry and can take care of discoloration on metals, like aluminum, bronze, and copper. More basic substances, like baking soda and bleach, are generally recommended for more mild cleaning on items like fine china and plastic dishes.
A pH of 0 to 1
The most acidic substances in your household are those with a pH of around 0 to 2. With a pH of 0, battery acid is perhaps the most acidic substance you'll encounter at home. Hopefully, you won't encounter it at all, as contact with the acid from a leaking battery can cause chemical burns. If you come in contact with it, flush the area with warm water for 30 minutes.
The next most common acidic household item may be muriatic acid. Muriatic acid is used to lower the pH of swimming pool water if it climbs too high. It's also excellent for tough jobs, like cleaning masonry work. Wear goggles and gloves when handling muriatic acid and always dilute it by adding acid to the water. Never add water to the acid.
Depending on their ingredients, toilet bowl cleaners can also have a pH of 1 to 3. Make sure you read the directions before using these products. Some may recommend wearing rubber gloves or goggles just in case you splash.
A pH of 2 to 3
As you climb the pH scale, the next most acidic items are lemon juice and vinegar, each with a pH of around 2. Vinegar's acidic nature makes it an excellent choice for removing hard water stains from glass and rust stains from sinks. It can also remove tarnish from brass and copper. With a similar acidity level, lemon juice will do the same but adds antibacterial and antiseptic properties to the mix.
Apples, grapefruits, orange juice, and soda all have a pH of around 3. They don't tend to clean much, but they sure are tasty. If you're surprised that food is acidic, remember that acids can (but don't always) taste much better than basics. Soap, for instance, is fairly basic with a pH of 11 or 12, but if you ever cursed in front of your mother as a child, you know you don't want to eat it.
A pH of 4 to 6
Hanging around a 4 on the pH scale are tomato juice and beer. Acid rain also has a pH of around 4, although this can vary depending on where you live. Black coffee and the stomach medicine Pepto-Bismol both have a pH of around 5. It may seem odd that a medication that eases heartburn is somewhat acidic, but remember that your stomach's acid naturally has a pH of around 1. Compared to 1, a 5 is pretty tame.
Around the house, you'll find that milk has a pH of 6. There are a few other common household items with a pH of 6, but you won't want to eat them or clean with them. These include healthy human hair and nails, saliva, and urine.