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El_Cadejo's avatar

Why is the pH scale always thought of as 0-14?

Asked by El_Cadejo (34472points) September 23rd, 2010

So I have biology class right now and my professor was talking about a lake that had a pH of -7. And then it hit me that I’ve always known pH to be 0–14. Mainly because thats always what I’ve seen and been taught in school. Now thinking about it, it makes sense pH wouldn’t be confined to those numbers as its a negative logarithmic scale of dissolved hydronium ions, so why couldn’t a substance have a very high concentration of them? Fluoroantimonic acid has a pH of -25 (i could only imagine what that would do if spilled on something lol)

So the question is, if these super low pH’s are possible, why are we always taught the 0–14 scale?

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15 Answers

talljasperman's avatar

0 means a hydrogen atom… 14 means none…its impossible to have less than zero or higher than 14

cockswain's avatar

It’s always measured in water, and you can’t get greater than 1M Hydrogen ions in water.

EDIT: Actually, I’m not certain I stated that correctly, don’t accept that as the truth. I have to think about that more.

FireMadeFlesh's avatar

As far as I can remember, 0 and 14 are the maximum concentrations of hydronium and hydroxide ions respectively. pHs outside this scale occur when you have ions with a stronger acidic effect than hydronium, or a stronger basic effect than hydroxide.

El_Cadejo's avatar

@cockswain then what about the lake my professor taught us about….

Seek's avatar

According to Wikipedia, the “negative” is actually a base ten.

so a pH of .1 = -1, .0001 = -4
and .0000000000000000000000001 = -25.

Since I’m math illiterate, here‘s the article so you can make sense of it.

cockswain's avatar

@uberbatman I’m still thinking, and I don’t know how such a lake could happen either. Do you have more info about it?

El_Cadejo's avatar

@Seek_Kolinahr yeaa i understand all that, what i dont understand is why it took all the way till college to learn about it. like why arent we taught these things earlier?

@cockswain its my understanding that it got that way from factory pollution during the industrial revolution. My professor does research of bio-remediation and this lake was something he was studying. I unfortunately dont have more info on it though as I was fighting sleep pretty bad that class so I kept passing out only catching bits and pieces of it.

cazzie's avatar

@uberbatman I second that comment you made to @Seek_Kolinahr. Science is soooo dumbed down in schools to be almost useless. I had a year of chemistry in middle school and then a year in high school and I learned WAAAY more in the middle school class (we worked in a lab and this was in the US…) My high school chemistry class was almost useless. No LAB!!... I barely remember a thing about it, except having to memorize the periodic table.

I’ve become a lay chemist because of my hobby/business and I’ve had to teach myself all the stuff about pH and avoiding bacteria and spore growth in products as well as the mumbo jumbo busting of some of the stupid things people come up with…. and debunking the crap ads they see on tv. MORE science in schools, please.

From what I’ve read, @FireMadeFlesh has the best answer here so far.

(I want a lab quality pH meter for Christmas and a new soap cutter, please, Santa.)

LostInParadise's avatar

@uberbatman , I agree that more explanations should be given for things taught in school. I asked my sixth grade teacher why the area of a circle is pr r^2, and she was completely dumbfounded. It turns out there is a simple intuitive explanation, by taking the limit of the areas of regular polygons and assuming, as I was willing to do, that the limit of the perimeters is the circumference of a circle and that the circumference is given by 2 pi r.

JLeslie's avatar

Although I did not know the info @Seek_Kolinahr gave us about being able to have negative numbers for pH, my science education was not that bad. In fact, if you have seen me around fluther, I tend to be very science and medically oriented, and I did not take any science in college, it is all high school science, and information I have read on my own of course.

@cazzie Did you go to public school? In what city did you not have a lab for chemistry class? This is why I think our federal government should be more involved with standards for education.

Seek's avatar

I went to school in Florida, Pasco County.

I took a semester of Chemistry before I dropped it and went to Anatomy and Physiology.

Chemistry was required one lab per quarter. In an advanced Science class, this actually consisted of dipping a glass stick in a chemical, putting it in front of a torch, and recording what colour the flame turned.


Then, I switched to Anatomy and got to dissect a fetal pig. Best academic decision I made in high school. ^_^

JLeslie's avatar

@Seek_Kolinahr I too went the Anatomy and Physiology route. My chemistry class was a half a semester I think. Not sure if it was in Jr high or high school? Maybe Augustlan would remember (we grew up in the same place). I could have taken more, but that was the requirement. Still, I made a few things in science lab, like acetone, and I don’t remmber what else. We had to balance equations, learn the periodic table, learn a little about protons and neutrons. Enough to get the basics, and get a feel for chemistry. I don’t feel like we all need to be experts in everything.

gggritso's avatar

Whoa now. This thread’s getting out of control. Let’s break it down.

Water spontaneously separates into [H3O] and [OH] ions on its own, therefore a certain concentration of each ion is always present, even in pure water.

The definition of pH is derived from the “equilibrium constant” – a relationship which is the product of the concentrations of the results in an equilibrium reaction divided by the product of the concentrations of the initial chemicals.

For the dissociation of water this would be

Ka (the equilibrium constant) = [H3O][OH]/[H2O]^2, so
Ka * [H2O]^2 = [H3O][OH]

Through experimentation we found that the product [H3O][OH] is a constant equal to 1.01E-14 in pure water; this is called the ion product constant. Additionally, we know that in pure water, the concentrations of [H3O] and [OH] ions have to be equal (they are mutually dependent); long story short we found that in pure water, [H3O] and [OH] are equal to each other, and are always equal to 1E-7 (at room temperature).

Now, we define pH = -log[H30]

Applying that to the value for [H3O] in pure water above, we find that the pH of pure water is 7. Life is good. By extension, at pH = 0 [H3O] = 1, at pH = 14 [H3O] = 10E-14. So far so good. So, most acids lie in that range. Not all of them, I presume.

So then what is going on with fluoroantimonic acid? A pH of -25 would mean that everything is completely fucked up, and the math is broken and oh my God everything is horrible ‘cause there are more atoms than makes sense.

Turns out, pH isn’t really a good way of measuring strength of acids, only the strength of solutions. To describe acidity we use a value called the pKa. This value is defined as

pKa = -log[Ka] (the value described above). So, fluoroantimonic acid does not in fact have a pH of -25 at all, it has a pKa of -25, which is a whole different beast.

Hope that helps?

There’s a whole bunch of math and background there that isn’t necessary, so to sum up: it’s from 0–14 ‘cause that’s way the mathematical definition of pH works. Fluoroantimonic acid does not have a pH of -25, that’s something else.

gulab's avatar

there may not be any base having ph value more than 14.

28lorelei's avatar

The thing is, pH+pOH=14, and most of the time, the pH is larger than 0. If the pH of a lake was -7, the pOH would be 14+7, which is 21. pH= -log[H30] and pOH= -log[OH].

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