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Discussion Starter · #1 ·
What does NH4+ + Cl- <=> NH3 + H+ + Cl- mean?

I am trying to cycle a twenty gallon planted tank because the only inhabitant died last Summer and the filter stopped working. This morning before the lights went on the tank had 0.7 ppm NH3. Last weekend I made a NH4Cl dosing solution. I used the estimated volume of my aquarium, molar masses to determine ratios, an accurate Ohaus SPX-223 scale and a volumetric cylinder to compound the dosing solution. After the 0.7 ppm NH3 test result this morning, I dosed my solution to add 2 ppm NH3. The aquarium ran an hour before the lights to mix in the NH4Cl. I retested the tank at 1.7 ppm NH3 using an accurate LaMotte colorimeter. My result was exactly half the increase in ppm NH3 I expected. Why the big underdose?

The following is a excerpted quote from a post starting a thread on reef2reef.com:

"...MW of NH4Cl = 53.49 g/mol
in solution the NH4+ + Cl- <=> NH3 + H+ + Cl- (essentially a 1:1 molar ratio of NH4 to NH3 since the reaction is strongly favored to the right)
MW of NH3 = 17.03 g/mol; that means it's 31.8% by mass of the NH4Cl molecule..."

The values in the calculations I used to make my dosing solution all match. The 53.49 g/mol, the 17.03 g/mol and the 31.8% all match. I think my math is correct and that it's the chemistry I don't understand. When the poster on reef2reef.com says, "(essentially a 1:1 molar ratio of NH4 to NH3 since the reaction is strongly favored to the right)", does that mean I will get 50% of my dose?

For future reference, I plan on doubling my dose of existing dosing solution. The next batch of dosing solution I compound will have twice as much NH4Cl in it. Also, I will be feeding the filter NH4Cl until we get a new fish.

Can anyone here explain the chemistry of the dissolution of NH4Cl. How much NH3 do you get out of it? What does NH4+ + Cl- <=> NH3 + H+ + Cl- mean?
 

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Let's simplify to this since the chloride ion is unchanged on both side:
NH4+ <=> NH3 + H+

On the left side is an ammonium ion, on the right is an ammonia ion and a hydrogen ion. The symbol in the middle just means that it is reversible, so depending on conditions it might favor one way or the other and will reach an equilibrium depending on conditions. Below a pH of 8, you'd expect mostly ammonium:


I'm not sure the context of the bit you quoted; maybe they were working at a high pH?

Plants can use either ammonium or ammonia - they can directly uptake ammonium, but they can change ammonia into ammonium easily. What is your pH? I'm not sure how your colorimeter is supposed to work, but most of the test kits used in the hobby lump both NH4+ and NH3 together and reports the sum. This is fine for the purpose of feeding bacteria and plants, but less informative if you want to know how toxic your water is to fish as ammonia is way more harmful than ammonium.
 

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Discussion Starter · #3 ·
@ElleDee I just checked the test kit box and it says "Ammonia Nitrogen." My pH is 6.7 with CO2 injection, down from 7.5 when the CO2 is off at night. The colorimeter is a dedicated computer, a device, that uses light to read test results with lab grade reagents. There are no fish, so toxicity is not an issue. It sounds like you're saying I'm measuring Ammonium and Ammonia together. I thought I was just measuring NH3. That begins to explain why my dosing solution is off. In fact, if what you're saying is true, I'm way off because my test result only accounts for half of what I expected to find. I wasn't counting on Ammonium. Which means my dosing solution would be off by a factor of four if anybody follows that. I appreciate your response. But I still don't understand. I will reflect on your thoughts.
 

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@ElleDee I just checked the test kit box and it says "Ammonia Nitrogen." My pH is 6.7 with CO2 injection, down from 7.5 when the CO2 is off at night. The colorimeter is a dedicated computer, a device, that uses light to read test results with lab grade reagents. There are no fish, so toxicity is not an issue. It sounds like you're saying I'm measuring Ammonium and Ammonia together. I thought I was just measuring NH3. That begins to explain why my dosing solution is off. In fact, if what you're saying is true, I'm way off because my test result only accounts for half of what I expected to find. I wasn't counting on Ammonium. Which means my dosing solution would be off by a factor of four if anybody follows that. I appreciate your response. But I still don't understand. I will reflect on your thoughts.
Ah, ok - you're just using the colorimeter to interpret the color of the tube?

I don't know about your test kit specifically, but if it's salicylate or Nessler based then it's measuring both together. I will say that "ammonia" is sometimes used to mean both ions and sometimes just NH3. It can be confusing. I use API, it clearly states it's a salicylate test and also that it looks at both.

I have to ask though - does it really matter for cycling? There's no magic amount that's going to make your tank cycle much faster. I sort of aim for more than 1 ppm and less than 4, but it's not that big a deal.
 

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Discussion Starter · #5 ·
Thank you @ElleDee. You're right it doesn't matter. Part of this is OCD. It's just that when you are cycling and you go below 1 ppm NH3 then you're supposed to add another 2 ppm NH4Cl until you get a significant level of Nitrite and then you dose 1/4 the amount NH4Cl until you get Nitrate. The 1/4 Amount of NH4Cl is the amount I assume I would feed my filter until I get a new fish. I would dose this on Macro dosing days instead of dosing KNO3. My dosing is half off. There is no magic amount as long as you don't go over 5 ppm. That would inhibit the aerobic bacteria. The test kit is Salicylate because I use Amquel as a dechlorinator which throws off Nessler Ammonia tests. It is so good to know that I am measuring Ammonia and Ammonium together. That's a big clue. But there is something still not quite right about this. The dosing is off. I can't put my finger on it because I don't understand the chemistry.
 

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Discussion Starter · #7 ·
Wow there is a lot of big words in here that I don't understand haha.
You're right @Hania41806: cycle, molar masses, volumetric cylinder, NH4Cl, NH3, compounding, ppm, colorimeter, dissolution, Chloride ion, uptake, CO2 injection, toxicity, g/mol, equilibrium, dechlorinator, Nessler, Salicylate, macro, aerobic bacteria, KNO3...

Point well taken. It is kind of silly. I am obsessed. But why is my Ammonia dosing so low? No matter, it's half as much as I expected, so I'll double the dose without understanding why the dose is wrong.
 

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You're right @Hania41806: cycle, molar masses, volumetric cylinder, NH4Cl, NH3, compounding, ppm, colorimeter, dissolution, Chloride ion, uptake, CO2 injection, toxicity, g/mol, equilibrium, dechlorinator, Nessler, Salicylate, macro, aerobic bacteria, KNO3...

Point well taken. It is kind of silly. I am obsessed. But why is my Ammonia dosing so low? No matter, it's half as much as I expected, so I'll double the dose without understanding why the dose is wrong.
😂👍 that sounds like the story of my life, if you see my posts a lot of them will say "I do (whatever my answer is to the question), I don't know why it works, but it does". Sad thing is I know exactly where you're coming from but spending years working with animals and children has helped me to let go and just accept that sometimes things don't have to make sense, they just have to work. Personally I would've loved to have seen an explanation of this because I do actually 100% understand your question but I have no clue what's up with that. This is why I use the table shrimp method lol I'm not patient enough to be a chemist
 

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Discussion Starter · #10 ·
@Savetheplants could you please give us the amount of NH4Cl and water that you used to make the solution, the amount you dosed and the estimated volume of your aquarium? Maybe there is some miscalculation that was not noticed.
@Ordinary Joe, Thanks to my shoddy lab work I did not log the recipe I followed. If I tried to redo it, it would be 5.179 grams of NH4Cl topped off with distilled water to the 125 ml line on the cylinder. It's 25 5 ml doses for 1 ppm Ammonia each dose. The estimated amount of water in the aquarium is 66 liters. 66 liters/0.3186% Ammonia x 25 doses = 5,179 mg NH4Cl. I looked for a dosing calculator to check the math but Rotala Butterfly | Planted Aquarium Nutrient Dosing Calculator doesn't have NH4Cl.

I found some calculations in the test book:

"To express results as Unionized Ammonia [NH3}:
ppm Unionized Ammonia [NH3] = ppm Ammonia - Nitrogen [NH3-N] x 1.2

To express results as ionized Ammonia [NH4]:
ppm ionized Ammonia [NH4+] = ppm Ammonia - Nitrogen [NH3-N] x 1.3

To determine the percentage of Unionized and ionized Ammonia-Nitrogen, consult the Appendix."
 

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I've just checked the numbers and they seem correct to me.

It is a big stretch, but maybe your tank is off gassing some NH3 after the 1 hour period. The problem with this hypothesis is that this usually happens at high pH values, so the equilibrium is shifted towards NH3 production. Adding base and heating can be used to remove or detect ammonia in a sample of unknown material. However, these conditions are far from what you have in your tank.

So, all I can say is that I also don't understand what is happening LOL. Maybe you could try calibrating your colorimeter again?
 

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Discussion Starter · #12 ·
I've just checked the numbers and they seem correct to me.

It is a big stretch, but maybe your tank is off gassing some NH3 after the 1 hour period. The problem with this hypothesis is that this usually happens at high pH values, so the equilibrium is shifted towards NH3 production. Adding base and heating can be used to remove or detect ammonia in a sample of unknown material. However, these conditions are far from what you have in your tank.

So, all I can say is that I also don't understand what is happening LOL. Maybe you could try calibrating your colorimeter again?
That graph @ElleDee posted seems to have something to do with the underestimation. I checked the appendix in the test book and it cites Trussel [1972] with a table showing the same data as the graph shows. It is 50% at my pH like ElleDee said. I always thought the pH only had to do with toxicity of Ammonia to fish but it also somehow affects the plant's Ammonia nutrient. The colorimeter is brand new. I called technichal support and they said it's calibrated at the factory and does not need recalibrating. I'm actually off by a factor of four instead of two according to the table. Things have only gotten more inaccurate since I started this thread. The dose is only half off though.
 
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