If you can smell Ammonia in the air, what would the equilibrium total ammonia concentration be in the tank water?

[taricha]

The pKa of ammonia will be different (higher, easier to make NH4+) in seawater than in fresh.

I don't follow the freshwater.
I smell (we'll assume) 50ppb in the air in the room.
If the skimmer has equalized my saltwater tank (at pH 8.2, 77F) with that room air concentration, then I don't see the role for freshwater equilibrium.

 

[KrisReef]

The pKa of ammonia will be different (higher, easier to make NH4+) in seawater than in fresh. That's a complication, then pH is on top of that.

Very nice. I'm going with RHF's answer for the initial question.

 

[KrisReef]

So the follow-up question would be, could the ammonia smell be from other sources, like a damp Depends?

On the original question, would the injection of fresh air into a skimmer change the equation(s) that RHF was discussing?

And no, I don't want another answer, "That Depends!"

Were all adults here.

 

[Dan_P]

I don't follow the freshwater.
I smell (we'll assume) 50ppb in the air in the room.
If the skimmer has equalized my saltwater tank (at pH 8.2, 77F) with that room air concentration, then I don't see the role for freshwater equilibrium.

The freshwater equilibrium was a demonstration and starting point only for developing the saltwater solution.

 

[Randy Holmes-Farley]

50ppb NH3 in air being in equilibrium with ~50ppb NH3 in water is the sort of chemistry relationship that feels pretty wrong, but sometimes isn't.
(not sure if I believe you yet or not)

Go with your gut. Its wrong.

Think of it this way, if it were true for ethanol water mixtures, then 100% (1 million ppm) ethanol would have 100% of the vapor (1 million ppm) above it be ethanol, but it’s not even close. That is only true at the boiling point. At lower temps there is air present.

 

[Randy Holmes-Farley]

The freshwater equilibrium was a demonstration and starting point only for developing the saltwater solution.

I think experimenting will beat calculating.

 

[Randy Holmes-Farley]

FYI, Raoults law is only an ideal situation, and it relates to the fraction of the vapor pressure at that temp, not to the fraction of 1 million ppm.

Raoult's Law

Raoult's law states that the vapor pressure of a solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present.…

chem.libretexts.org

 

[Dan_P]

FYI, Raoults law is only an ideal situation, and it relates to the fraction of the vapor pressure at that temp, not to the fraction of 1 million ppm.

Raoult's Law

Raoult's law states that the vapor pressure of a solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present.…

chem.libretexts.org

Ammonia is a gas, though one that reacts with water. Are we sure Raoult’s law applies here?

 

[Randy Holmes-Farley]

Ammonia is a gas, though one that reacts with water. Are we sure Raoult’s law applies here?

All of these sorts of “Laws” are approximations and assume no interaction between the two chemicals. Water does interact well with ammonia (better than ammonia does interact with itself), so water will help hold it in the liquid phase (reducing vapor pressure) by hydrogen bonding and formation of NH4+.

 

[taricha]

let's back up...

I would start with the detection limit of ammonia by humans of 5-53 ppb. Let’s pick 50 ppb and report our answers.

NIH source is
"Ammonia is a colorless, corrosive, alkaline gas that has a very pungent odor. The odor detection level ranges from 5 to 53 ppm."
....
"The odor of ammonia can be detected by humans at concentrations >5 ppm; the odor is highly penetrating at 50 ppm (10 min). Human volunteers exposed to ammonia showed slight irritation at 30 ppm (10 min)..."
That NIH page defines it in terms of 1 ppm = 0.7 mg/m^3 so the "ppm" is the typical v/v used in gases which is the same as moles/moles or atmospheres

So let's go with 10ppm = 10*10^-6 moles NH3/mole of air

Gonna do the Henry's law for pure water, then see if we can estimate how much correction might be needed for saltwater.

Henry's law wants things in atmospheres
If I take Henry' law constant for NH3 = 57 L*atm/moles at 25C (77F)



57 L*atm/moles = 10*10^-6 atm / Concentration
we want the concentration, so....
Concentration = 10*10^-6 atm / 57 L*atm/moles = 0.175 *10^-6 moles/L
so our water concentration in equilibrium with the 10 ppm (by volume) NH3 that we can smell in the air is 0.175 micromoles/L
molar mass of NH3 = 17.03 so
0.175 micromoles/L *17.03g/mole = 2.98 micrograms/L or 2.98 ppb NH3.
This is only the NH3 portion in the water so the total ammonia can be calculated from that (using 35ppt, 8.2pH, 77F)....



0.042 ppm total ammonia in the water.

edit: that's my best attempt, I'll poke around and see if I can figure out if doing Henry's law for pure water instead of seawater makes a big enough difference to care about.

 

[Dan_P]

Let's make some (not good) assumptions to simplify things.

So I walked into the building where my tank is and my nose inspired the question...

If, in a room there is enough ammonia gas to smell, the aquarium skimmer has been running constantly, and the ammonia consumption by the system is small (we'll pretend zero) - what is the total ammonia in the tank water (pH 8.2, 77F) that would be in equilibrium with that smell-able concentration?

(I haven't tried to look up/work out the answer yet. )

You kinda did this experiment when you looked at ammonia consumption with Prime. You examined the headspace of an ammonia+ Prime solution with a Seachem ammonia sensing disk. Any wild approximations from that experiment might help steady our feet going forwards.

 

[taricha]

Any wild approximations from that experiment might help steady our feet going forwards.

Yeah. The disks reacted to ammonia but I never tried to correlate disk color to smell.

The above calculation suggests that I should be able to smell ammonia in the equalized headspace above an 0.05ppm total ammonia saltwater solution at pH 8.2
That feels pretty wrong (like decimals wrong by a place or two), but I don't know if I've ever checked before.

 

[BeanAnimal]

Hi - I don't currently have access to a handheld PID, but wouldn't that be the idea device for such experiments? Maybe in combination with a device like a Seneye or other water based sensor? I honestly have no idea how VOCs are measured in a lab setting, but always assumed expensive PIDs or PID type sensors were used.

 

[Randy Holmes-Farley]

If one needs >= 5 ppm ammonia in air to smell it, I don’t think there is any reefing scenario, even during cycling, where one will be able to smell it.

I understand my assertion is not convincing, but I’ll just repeat that it is easy to make a say, 1 or 2 or 5 ppm total ammonia in seawater solution, and smell for ammonia. Way more convincing than complicated calculations or Randy assertions.

One could even make a series from high to low, and see where smell is detected. Then where you think the limit is near, do A/B smell comparisons blind and see if it is detectable vs the same seawater without added ammonia.

 

[taricha]

I understand my assertion is not convincing, but I’ll just repeat that it is easy to make a say, 1 or 2 or 5 ppm total ammonia in seawater solution, and smell for ammonia. Way more convincing than complicated calculations or Randy assertions.

Having attempted the calculation, I agree. I'll believe it when I smell it.

 

[taricha]

If one needs >= 5 ppm ammonia in air to smell it, I don’t think there is any reefing scenario, even during cycling, where one will be able to smell it.

Just a comment on the question inspiration.
In the building with the tank, they refinish the floors in the rooms every year. They do a few rooms a day, and one of the chemicals reeks of ammonia (found "ammonium hydroxide" high in the ingredient list on one of the buckets).
So the entire building has an ammonia smell essentially all day for ~ a week.
So I was curious what equilibrium concentration the skimmer was trying to push the tank toward during that time.

 

[KrisReef]

Yeah. The disks reacted to ammonia but I never tried to correlate disk color to smell.

The above calculation suggests that I should be able to smell ammonia in the equalized headspace above an 0.05ppm total ammonia saltwater solution at pH 8.2
That feels pretty wrong (like decimals wrong by a place or two), but I don't know if I've ever checked before.

I would add that “the smell of the ocean “ might also complicate ammonia detection at lower levels.

As a kid I lived 25 miles inland and on nice days we could detect the smell of the ocean. I always loved those days.

 

[Dan_P]

Just a comment on the question inspiration.
In the building with the tank, they refinish the floors in the rooms every year. They do a few rooms a day, and one of the chemicals reeks of ammonia (found "ammonium hydroxide" high in the ingredient list on one of the buckets).
So the entire building has an ammonia smell essentially all day for ~ a week.
So I was curious what equilibrium concentration the skimmer was trying to push the tank toward during that time.

Another way we might look at this is ask what is the maximum that could get into the aquarium. As a first approximation, assume that aquarium air exchange occurs 100% through the skimmer. Also assume the ammonia absorption is 100% efficient, every mg in the air goes into the aquarium water. As a reference, my skimmer uses 6 L of air per minute and that is a wet skim (1-2 liters/day of condensed foam for a 100 gallon system). Assume an ammonia concentration. The EPA’s formula for ppm NH3 in the air is

24.45 x mg of NH3 per m^3 divided by 17 = ppm NH3 in air

Plug in the 50 ppb to figure out how many mg of ammonia per liter that is. Now it is a matter of calculating the overly optimistic amount of NH3 that would enter the aquarium per day.

 

[Tavero]

Considering the buffer capability of saltwater I think that any produced ammonia will instantly react to ammonium.
I could be wrong, but in my opinion the only way to smell ammonia in saltwater would be to increase pH over 9.3 (no matter how high the concentration is, there will be no smell below it, unless the buffer is spent).