# How does ph work

#### SouthAmericanCichlids

I was recently wondering how exactly ph works, I hear it has to do with Co2. Do you guys have an article explaining it? And I'm fine with scientific words. And maybe also one describing how it works in conjunction with ammonia and beneficial bacteria.

#### Quiche

I'm in AP Chem right now and we're actually learning this! I can't say anything about beneficial bacteria but I can give a quick overview. I don't know how familiar you are with chemistry already so let me know if you want me to elaborate on anything! Or you might already know a lot of this, but perhaps someone else who reads this thread will find it interesting.

pH is a measure of the concentration of H+ ions in solution, which is expressed as [H+] with the units of mols/liter. This is a dissociation equation for water.

H2O <=> H+ + OH-

A bit of the H2O always dissociates into H+ and OH- ions. The equilibrium constant for this equation is 10^-14, meaning that when you multiply [H+] with [OH-] (these are both concentrations in moles/liter, or M), you will always get 10^-14. The + and - represent the charges on the ions; H has a positive charge and OH has a negative charge.

[H+][OH-] = 10^-14

In pure water, because 1 H2O atom dissociates into 1 H+ and 1 OH-, [H+] and [OH-] are equal. Therefore, in order for [H+] times [OH-] to equal 10^-14, both must be in concentrations of 10^-7 M in pure water.

The equation for pH is pH = -log([H+]). As you can see, this makes sense; -log(10^-7) is equal to 7, which is the neutral pH. Distilled water is neutral.

Here's where the tricky part is, if you want to know about CO2 and ammonia. When you add CO2, the equilibrium equation shifts. See, this reaction occurs:
CO2 + H2O <=> HCO3- + H+
But not all of the CO2 reacts with H2O to form the HCO3 ion with a negative charge and an H+ ion. Some does though, and the more CO2 in the water the more H+ ions form, decreasing the pH and making it more acidic. This might seem counterintuitive — why does pH go down when H+ ions go up? The answer lies in the
pH=-log([H+]) equation. Because it's taking the negative log base 10 of [H+], when [H+] increases from 10^-7 M to 10^-6 M, the pH goes down from 7 to 6. It's not a linear relationship because of some equilibrium stuff that happens, which might be too long to explain in a forum post, but if you're interested I can work out the exact equations of CO2 concentration to pH change. There might be a calculator out there already.

Ammonia is NH3. It does this reaction with water:
H2O + NH3 <=> NH4+ + OH-
Similiar to the deal with CO2, not all ammonia reacts. In fact, very little actually does! But it's enough to contribute OH- ions to the solution. If you add OH- ions, the H2O <=> H+ + OH- equation actually reacts backward to maintain equilibrium (L'Chatlier's Principle, if you've ever heard of it!), decreasing the concentration of the H+ ion, which increases the pH and makes the solution more basic.

I can't speak for beneficial bacteria, but one thing you might've heard is that ammonia becomes less toxic at lower/acidic pHs, or in other words, when there's more H+ ions in the water. This is because of good ol' L'Chatlier's Principle again; basically, what happens is that those excess H+ ions react with the OH- ions being produced from the
H2O + NH3 <=> NH4+ + OH-
equation to form water. Then, the equation shifts to the right; more products, NH4+ and OH-, are formed, to make up for the decrease in OH-. A lot of the OH- continues to be "eaten up" by the H+ ions, making water, but the NH4+ remains. Essentially, NH3 is being converted to NH4+, which is the nontoxic form of ammonia. The reverse is true in more basic solutions with high pH's; an excess of OH- ions means that the forward reaction of {H2O + NH3 <=> NH4+ + OH-} doesn't occur as much. Instead, the backwards reaction occurs, consuming the excess OH- ions but also the NH4+ with it, making a lot of NH3 in comparison, which is toxic.

Come to think of it, this would be a cool Fishlore article idea if I actually went into depth. I fear this is a really confusing explanation for someone who's not super well versed on chemistry yet, but I hope you were able to glean at least a bit of information! If you really want to know the nitty gritty math behind everything, I recommend watching Khan Academy videos about pH and equilibrium.

#### GlennO

Crikey. I’m glad you didn’t go into depth.

#### MacZ

In short and reduced to the essentials for aquarists. In my opinion everything else is only of importance for people keeping extremophile animals or add a lot of different buffers, stabilizers and last but not least fertilizers to their tanks:

pH is the measurement of H+ ions. The lower the pH, the more free H+ ions are present.
KH is the measurement of carbonates (CO3 2-) and hydrogencarbonates (HCO3-), though what we measure with our tests is the acid neutralizing capacity, i.e. the amount of H+ the water can take until reaching a pH of 7.
CO2 binds with carbonates and neutralizes them. After the equilibrium is reached it reacts with water to form carbonic acid, which releases free H+ again.

All three are dependend on each other, thus making it possible to calculate the reading for one of them from the other two.

Beneficial bacteria (Nitrobacter and Nitrosomonas) need carbonates and hydrogencarbonates while metabolising Nitrite (NO2) from Ammonia (NH3) and in turn Nitrates (NO3) from Nitrites. At a pH of about 6.0 the BB activity goes substantially down. At a pH of 5.5 they stop working completely and go into a kind of hibernation state.

While the pH goes down and the BB gradually slow down their metabolism, other microorganisms like Archeans, Yeasts and Fungi gradually take over the cycle, and, sometimes with detours, metabolise the same chain from NH3 to NO3. They also take ages to develope, so cycling in low pH will not just take weeks but months.

Why Ammonia is nontoxic in acidic environments is explained perfectly in Quiche 's post. All we as hobbyists have to know is: If the pH is under 7 Ammonia is nontoxic. The higher the pH goes from there, the higher the toxicity of ammonia.

I think those are the relevant basics.

#### CaptainSupport

This is informative, thank you all. I always left my understanding of ph at "is my ph at levels ok for my fish". It is interesting to hear the scientific side of it.

#### MacZ

I always left my understanding of ph at "is my ph at levels ok for my fish".

Maybe an interesting short answer to this:
The pH levels many fish profiles give are either the range of pH in nature OR the range they are known to be able to live in under aquarium conditions.

You can keep "softwater fish" in hard water and high pH if correctly acclimated, thanks to osmoregulation being set to keeping a higher level of retention of minerals in their bodies. This may have influence on their kidneys though, which tend to be affected after some time. Thus, while they can survive and even thrive for quite some time in hard water, they are likelier to die earlier as if kept in soft water conditions.
While captive mass breeding has led to a higher adaptability, this has only worked for soft water species to a considerable degree.
"Hardwater fish" on the other hand are hard to acclimate to soft water and low pH environments, because their osmoregulation is by nature set to excrete minerals and the like as much as possible to even out the probably higher concentrations outside.

In both groups there are always some extremophiles, like blackwater fish or the cichlids of Lake Natron. Those can only be acclimated to a certain degree and take generations to be possibly kept in standard aquarium conditions. Not all have reached that stage yet.

#### SouthAmericanCichlids

I'm in AP Chem right now and we're actually learning this! I can't say anything about beneficial bacteria but I can give a quick overview. I don't know how familiar you are with chemistry already so let me know if you want me to elaborate on anything! Or you might already know a lot of this, but perhaps someone else who reads this thread will find it interesting.

pH is a measure of the concentration of H+ ions in solution, which is expressed as [H+] with the units of mols/liter. This is a dissociation equation for water.

H2O <=> H+ + OH-

A bit of the H2O always dissociates into H+ and OH- ions. The equilibrium constant for this equation is 10^-14, meaning that when you multiply [H+] with [OH-] (these are both concentrations in moles/liter, or M), you will always get 10^-14. The + and - represent the charges on the ions; H has a positive charge and OH has a negative charge.

[H+][OH-] = 10^-14

In pure water, because 1 H2O atom dissociates into 1 H+ and 1 OH-, [H+] and [OH-] are equal. Therefore, in order for [H+] times [OH-] to equal 10^-14, both must be in concentrations of 10^-7 M in pure water.

The equation for pH is pH = -log([H+]). As you can see, this makes sense; -log(10^-7) is equal to 7, which is the neutral pH. Distilled water is neutral.

Here's where the tricky part is, if you want to know about CO2 and ammonia. When you add CO2, the equilibrium equation shifts. See, this reaction occurs:
CO2 + H2O <=> HCO3- + H+
But not all of the CO2 reacts with H2O to form the HCO3 ion with a negative charge and an H+ ion. Some does though, and the more CO2 in the water the more H+ ions form, decreasing the pH and making it more acidic. This might seem counterintuitive — why does pH go down when H+ ions go up? The answer lies in the
pH=-log([H+]) equation. Because it's taking the negative log base 10 of [H+], when [H+] increases from 10^-7 M to 10^-6 M, the pH goes down from 7 to 6. It's not a linear relationship because of some equilibrium stuff that happens, which might be too long to explain in a forum post, but if you're interested I can work out the exact equations of CO2 concentration to pH change. There might be a calculator out there already.

Ammonia is NH3. It does this reaction with water:
H2O + NH3 <=> NH4+ + OH-
Similiar to the deal with CO2, not all ammonia reacts. In fact, very little actually does! But it's enough to contribute OH- ions to the solution. If you add OH- ions, the H2O <=> H+ + OH- equation actually reacts backward to maintain equilibrium (L'Chatlier's Principle, if you've ever heard of it!), decreasing the concentration of the H+ ion, which increases the pH and makes the solution more basic.

I can't speak for beneficial bacteria, but one thing you might've heard is that ammonia becomes less toxic at lower/acidic pHs, or in other words, when there's more H+ ions in the water. This is because of good ol' L'Chatlier's Principle again; basically, what happens is that those excess H+ ions react with the OH- ions being produced from the
H2O + NH3 <=> NH4+ + OH-
equation to form water. Then, the equation shifts to the right; more products, NH4+ and OH-, are formed, to make up for the decrease in OH-. A lot of the OH- continues to be "eaten up" by the H+ ions, making water, but the NH4+ remains. Essentially, NH3 is being converted to NH4+, which is the nontoxic form of ammonia. The reverse is true in more basic solutions with high pH's; an excess of OH- ions means that the forward reaction of {H2O + NH3 <=> NH4+ + OH-} doesn't occur as much. Instead, the backwards reaction occurs, consuming the excess OH- ions but also the NH4+ with it, making a lot of NH3 in comparison, which is toxic.

Come to think of it, this would be a cool Fishlore article idea if I actually went into depth. I fear this is a really confusing explanation for someone who's not super well versed on chemistry yet, but I hope you were able to glean at least a bit of information! If you really want to know the nitty gritty math behind everything, I recommend watching Khan Academy videos about pH and equilibrium.
Maybe an interesting short answer to this:
The pH levels many fish profiles give are either the range of pH in nature OR the range they are known to be able to live in under aquarium conditions.

You can keep "softwater fish" in hard water and high pH if correctly acclimated, thanks to osmoregulation being set to keeping a higher level of retention of minerals in their bodies. This may have influence on their kidneys though, which tend to be affected after some time. Thus, while they can survive and even thrive for quite some time in hard water, they are likelier to die earlier as if kept in soft water conditions.
While captive mass breeding has led to a higher adaptability, this has only worked for soft water species to a considerable degree.
"Hardwater fish" on the other hand are hard to acclimate to soft water and low pH environments, because their osmoregulation is by nature set to excrete minerals and the like as much as possible to even out the probably higher concentrations outside.

In both groups there are always some extremophiles, like blackwater fish or the cichlids of Lake Natron. Those can only be acclimated to a certain degree and take generations to be possibly kept in standard aquarium conditions. Not all have reached that stage yet.
This is very interresting.

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