When you buy hydrogen peroxide, you are buying peroxide of a particular grade and at a particular concentration. I’ll discuss grades of peroxide elsewhere. (What grade you buy will affect what sort of stabilizers and additives it has in it. Where you go to buy peroxide will also depend on what grade you are looking for.)
But what about concentration? What are the concentrations we need to know about, and what dilutions will you need to make?
All the peroxide you and I are likely to see is “low” concentrations, like 3% or 10%, or 35% at the most. The brown bottles at the drugstore are usually 3%. It will say on the label what PERCENTAGE it is.
So, bear with me here: If you buy a bottle of 3% peroxide, you do NOT add water to it to make 3%. It is already 3%, without diluting it. So, don’t go adding 97% water to it to try to get it to be 3%, okay?
Higher concentrations, like 50% and 70% and 90%, are used for some industrial purposes, and as rocket fuel. The higher the percentage of peroxide, the more dangerous it is, and the more special handling it takes.
I refer to the concentration “3%” a lot when I’m talking about peroxide. Even when I’m saying to dilute it further, I often refer to an amount of “3% hydrogen peroxide” to add to water. Why 3%? Three percent is a familiar concentration, since the brown bottles at the drugstore are generally 3%. Maybe that is why I’ve gotten used to using 3% for examples, amounts and measurements.
Three percent also seems to be a “standard” of sorts. Because hydrogen peroxide is generally sold as 3%, many books, websites, and charts use 3% in examples, amounts, and formulas. I’ve just gotten used to 3% as a concentration.
Three percent is also what I mix and use in my home for most general everyday household purposes. There is no reason I have to do it this way – I could mix and use 5% instead. But I don’t. I mix and use 3%.
I keep a gallon bottle of 3% hydrogen peroxide in my kitchen. I use the 3% for a number of things without dilution, and, of course, I also often further dilute it by mixing it into water. I also keep 3% in spray bottles, and containers, in my kitchen and bathroom.
But, there is no rule. If you use a lot of 1.5% peroxide as a mouthwash, you could keep a bottle of 1.5% peroxide in your bathroom. Please BE SURE to clearly label all containers of peroxide!
I also refer to “35%” a lot. Again, this is a common concentration, as well as a concentration that I personally use. Most food grade peroxide is sold as 35% concentration. (It is also possible to buy food grade that is 10%, 8%, or 6%, but this is less common.)
I use 35% for only a few things: I use it in baths, I use it in my garden sprayer, and I use it to make 3%. You may notice that ALL of these involve diluting it. I’m never actually applying 35% peroxide. That’s because 35% peroxide is simply way too concentrated to use directly for normal household purposes. Even 3% often gets diluted further. 35% always gets diluted.
35% peroxide is a hazardous material, for shipping purposes. Don’t try sending some to a friend through the mail. Special handling procedures (and additional fees) are required to send it anywhere. 35% peroxide is quite caustic, and needs to be treated with real care and respect.
Sometimes I refer to ”1% hydrogen peroxide”. To make 1% hydrogen peroxide, use 2 parts water and 1 part 3% peroxide. For example, ½ cup of water and ¼ cup of 3% hydrogen peroxide = ¾ cup of 1% hydrogen peroxide.
There are also several places where I say to use “1% to 3% hydrogen peroxide”. This means that anything in the range between 1% and 3% would be okay.
There are some types of peroxide that are sold in other concentrations, besides 3% and 35%. I suppose this is so that we can all practice our math and keep up on fractions? Seriously, it can get a little confusing, and sometimes I have to stop and sit down and think it all over.
Cosmetics grade peroxide usually comes in 3% to 12% concentrations. (I know I said I wasn't going to talk about grades, but it is all related.)
There are also some applications where hydrogen peroxide is measured in PPM (parts per million), for instance in water treatment.
Gardening: attach sprayer to garden hose; use to dilute peroxide and spray on the garden
Spray bottles for peroxide