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Ammonia Calculations

 

By

Ian Guisseppi Millichip

 

 

A brief bit about Nitrogen.

 

The Nitrogen Cycle is a complex series of reactions where Nitrogen atoms are transformed from one form to another.

The Cycle is not linear and is not a one-direction cycle: it is reversible; has many side reactions; and has a number of loops (smaller cycles) within it.

 

Animals, Plants, Micro-organisms, and non-biological chemical reactions all play their part within the Nitrogen Cycle. Those reactions are modified by a number of physical parameters such as pH, Temperature and Pressure.

Also, and fundamental to many reactions that are reversible, the relative concentrations of chemicals within the Nitrogen Cycle will determine the direction of flow.

 

Nitrogen exists within one of three basic forms:

Elemental Nitrogen…..eg the molecular dinitrogen gas (N2);

Organic Nitrogen…….eg DNA, Proteins, and many other bio-molecules; and

Mineral Nitrogen…….eg Ammonium, Nitrites, and Nitrates.

 

This article, however, is not about the Nitrogen Cycle itself but about Ammoniacal-Nitrogen.

 

 

Ammoniacal-Nitrogen

 

Ammoniacal-Nitrogen (NH3-N) and its measurement is of extreme importance to fish keeping in view of the extreme toxicity of ammonia to aquatic animals.

 

In focusing in on ammonia within the Nitrogen Cycle, we see that the exact nature of ammonia in water is a mix of ‘localised’ reactions.

 

Ammonia (as NH3) is often called Unionised Ammonia (UIA) in fish-keeping circles; ammonium is often called Ionised Ammonia in the same circles.

 

It is a weak base when dissolved in water, but can act as a very weak acid in the presence of some compounds to form amides.

 

Ammonia reacts with itself (very much like water reacts with itself) to form ammonium (NH4+) and amide (NH2-) ions.

This is a very weak molecular autoionisation and the product concentrations would be negligible within a fish-keeping consideration.

 

Ammonia in the fish tank

 

The major reaction of ammonia in water, that is of interest in fish-keeping, is in reacting with the water to produce ammonium (NH4+) and hydroxide (OH-) ions.

 

This reaction is reversible (see side note on Equilibrium).

 

 

At standard temperature and pressure, the pK for this reaction as shown (ie going from left to right) is ~9.2 (as this is an acid dissociation, then the pK is more often called the pKa of ammonia).

 

The pKa means that this dissociation would produce an alkaline condition if the added water were pure.

It also indicates that if the pH of the water were then changed to a pH = to the pKa then we would have 50% ammonia (NH3) and 50% ammonium (NH4+)…and at that pH the ammonia/ammonium mix would act as a buffer of some sort.

 

The reaction of ammonia to ammonium could be written in reverse (ie if we add ammonium and hydroxide ions to pure water, we will get ammonia and water produced).

 

Similarly, the reaction of adding ammonium ions to water gives:

 

 

Ammonium (being a salt of a weak base) is a weak acid…..as can be seen by the production of aqueous hydrogen ions (H+) on addition to water.

 

That is also a reversible reaction.

 

 

Why does pH affect Ammoniacal-Nitrogen?

 

Going by the principles in the side-notes on Equilibrium, we can see that anything that upsets these equations will case the system to react to oppose upsetting the equilibrium constant (K).

 

Hence, if we add more hydroxide (OH-) to equation 1 then we will drive the system towards unionised ammonia (NH3) ie back to the left. This can be done by simply increasing the pH.

 

Similarly, if we add more acid (H+) then that would drive equation 2 to the left towards more ammonium (NH4+). Or, addition of hydroxide in equation 2 would force the reaction towards the right (ie produce more ammonia….which is exactly the same as in equation 1).

 

As unionised ammonia is very highly toxic to fish, and ammonium very much less so, then we can see that pH has an effect on the effective of ammoniacal-nitrogen from NH3-N in the aquarium.

 

HIGH pH INCREASES THE RELATIVE CONCENTRATION OF UNIONISED AMMONIA WITHIN THE WATER.

 

But it is not just pH that affects the ratio of ammonia to ammonium; temperature and pressure will also affect the ratios.

 

 

Do You Know Your Test Kit?

 

Measuring Ammoniacal-Nitrogen (“the Ammonia Test”) is of importance to fish-keepers.

There are, however, a number of different methods and principles to measure ammoniacal-nitrogen.

 

Some kits use a Total Ammoniacal Nitrogen (TAN) principle, and some use a principle of measuring Unionised Ammonia (UIA).

 

The Total Ammonia kits measure both ammonia (UnIonised Ammonia) and ammonium (Ionised Ammonia).

 

I will not go into the details of each type here, but it is suffice to say that if using a TAN kit that measures both then a zero reading is the safe bet.

If, however, a UIA kit is used then a zero reading is not necessarily a safe bet as the zero reading may not necessarily indicate the ammonia within the actual water at the pH and temperature of the fish tank (this being particularly important if having water tested off-site).

 

The graphs and calculations below indicate the approximate ratios of ammonia and ammonium within the fish tank at given temperatures and pH in freshwater aquaria.

 


 

Ammonia Calculations and Graphs

 

{this is a copy of a thread I submitted on various internet fish forums}

 

Part 1. The Calculation Formula.

In gathering that most people use test kits that measure Total Ammonia (ie Ionised Ammonia + Un-Ionised Ammonia), here is an equation to plonk into an XL spreadsheet.

The equation calculates a very good approximation of Un-Ionised Ammonia (ie Ammonia itself) from a measure of Total Ammonia at a given Temperature and measured pH.

{Now, ionic strength has an effect on the ratio of ionised (ammonium) to un-ionised ammonia. However, the effect of taking ionic strength into consideration can be somewhat neglected here if we are considering freshwater.}

1. Open a new XL (or other spreadsheet that will have similar syntax)
2. In the box A1, write 'pH'.
3. In the box B1, write 'Temp/in Celsius'
4. In box A2 (below A1)....enter the pH measured at a given temperature (that is important).
5. In box B2 (below B1)....enter the temperature in degree Celsius.

Then select a suitable box below and paste the following equation into it...

=1/(10^((0.0901821+2729.92/(273.15+B2))-A2)+1)

(noting that B2 and A2 refer to the temp and pH.

So.... you get a value.

What do you do with it?

Having measured the Total ammonia (ionised plus unionised), you multiply that value with the answer given in spreadsheet to obtain the very close approximation to the concentration of Un-Ionised ammonia at that temp and pH.

(noting that it is the un-ionised ammonia that is especially highly toxic).

example.

Temp = 25 C; pH = 8.5 (at that temp).
The fraction of un-ionised Ammonia in Total ammonia is ~0.082. Multiply your Total ammonia by that to obtain unionised ammonia.

If you play around with entering differing values of pH and temp, you will see that there can be some pretty surprising changes in unionised ammonia.

Have fun.

Part 2. Reading the Graphs


To add to this, here is a graph I did to show the type of changes you will see with a change in pH at 25 Celsius.




Note the line where 50% of the total ammonia exists as un-ionised ammonia.
If you follow the line to the pH axis, the value is just above pH 9.
This represents the pKa of ammonia at 25 celsius.

And if you notice there are sharp rises and falls in fraction of un-ionised ammonia around that pH with small changes in pH.
What is happening here is the ammonia/ammonium equilibrium attempting to buffer the system and keep the pH near to its pKa with small additions of acid or base.

Part 3. Reading the Graphs…..a more detailed look.


Change of Un-Ionised Ammonia (UIA) with change in pH for different temperatures.




This is assuming that ionic-strength and pH are not affected by temperature.




Now, if we look at a smaller detail….we see….




So, what might this indicate?

Total Ammonia = Un-Ionised Ammonia + Ionised Ammonia

In aqueous solution, ammonia is a weak base, and ammonium is a weak acid.
In an aqueous solution, they exist in equilibrium as:

Ammonia (UIA) < = > Ammonium (Ionised ammonia).

(as the concentration of water doesn’t change substantial in this reaction, then there is no point in showing water).

Without going into the details of thermodynamics etc, decreasing pH will push the reaction to the right, increasing pH will push the reaction to the left.

Now, if we have a close look at, say, a fish tank at pH 8.5 and at 30 celsius.

Imagine that the water is tested at 10 celsius (assuming that the change in temperature doesn’t vastly change the pH…even though pH is temperature dependent).

It can be seen that if the test at 10 celsius is done using a test system that measures un-ionised ammonia then what it will measure is only one quarter the actual un-ionised ammonia in the tank at 30 celsius (ie the tank is 4 times higher than a test for UIA at 10 celsius).

Hence, from this a few important points should be noted for fish keepers:
1. Always measure pH and ammonia (and any other parameter) at the temperature of the fish tank.
2. Increasing temperature increases the proportion of un-ionised ammonia (NH3 or free ammonia).
3. Increasing pH increases the proportion of un-ionised ammonia (NH3 or free ammonia).
4. The ammonia/ammonium system is a pH buffer to some extent.
5. Ionic strength is not included in these calculations, but it should be noted that ionic strength has an effect on pH and on the proportion of UIA.
6. What is not, and cannot be, shown is the effect of pH and water ammonia concentration on the ability of a fish to rid ammonia from its gills. But that is a vital additional point to consider. Ie a fish can die of ammonia poisoning even if you cannot detect a high level of ammonia.
7. If cross-comparing ammonia readings from different test systems, then be clear about exactly what each if measuring.
8. Any sign of ammonia (ionised or un-ionised) is a sign of concern.

Now, in this the exact science has been tamed down. A complete story is quite involved.

ian guisseppi millichip

 

Endnote:

 

I have added an active spreadsheet at https://docs.google.com/file/d/0B07wAC7V5eLqQ1JmajllQnAyT1U/edit?pli=1

 

For download and use.

Equilibrium.

If we take a reaction that is reversible…

A+B <> C+D

The reaction will proceed towards equilibrium whereby the ratio of the products to the reactants remains ‘constant’. We give that constant a name, and represent it by a simple mathematical equation.

The Constant, K, is called the Equilibrium Constant and mathematically it approximates to

ie the concentration of C multiplied by the concentration of D, then divided by the concentration of A multiplied by the concentration of B.

The smaller the value of K, the weaker the reaction in producing C and D.

But, if K is very small for the reaction as written, then that means the system prefers to remain profoundly as A and B: and that means that if we start with C and D then the reaction to produce A and B would be very strong.

The reaction is dynamic and will always try to maintain that ratio.

If we then add more of, say, D then the reversible reaction will change to re-establish that equilibrium by driving the reaction backwards towards to the left (ie to make more of either A or B).

This a simplified version of Le Chatelier's principle….which in essence says that if something is stable then anything that opposes that stability will be opposed.

In the above example, other things might affect the equilibrium, eg:

Removal of D by some other reaction (that would drive the reaction to the right);

Removal of A by some other reaction (that would drive the reaction to the left to replace the A);

Temperature, Pressure, or pH may also alter the equilibrium.

The reaction of water with water is like the example equation and with a low K value. The concentration of the water will always remain very high compared to the products, but the products (aqueous hydrogen ions and hydroxide ions) are present at very low and equal concentrations and give pure water its pH of 7.

The range of values of K may be very large. For convenience, chemists use a scale similar to pH (and for the same reason) to denote the value of K. This is called the pK value and is the negative base 10 logarithm of K. (thus a higher pK means a lower K value, a 1 point change in pK means a 10-fold change in K).