Ammonia calculation

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 ammmonia 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.

Ian

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.

ian

I'll be adding some posts to this showing the effect of temperature (graphically).

Now, the importance of that is that we can see that if, say, water from a warm tank is taken to a LFS and by that stage the temp has dropped from 30 C to 10 C (and we assume that the pH of the water has not changed) then if the LFS measure the un-ionised ammonia (UIA) and state it at a safe level (umm..whatever 'safe' really is?)....the problem could be that at 10 C the fraction of UIA is only 0.1 of the total YET back in the tank it is a whopping 30% of the total (if we are in alkaline pH region). ie the fish will actually be sensing the 30% of UIA (depending upon pH of the water of course).

I just have to compile the graphs.

ian

Here we go, the final part (for now anyway)

Part 3.

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

Interesting stuff Ian,if not a little bit hard to follow initially.
If Ammonia is zero then I assume those calculations will be all zero also.
In other words the plan would be that ammonia should be zero at all times ideally.

Gavin

Fishowner wrote:

Interesting stuff Ian,if not a little bit hard to follow initially.
If Ammonia is zero then I assume those calculations will be all zero also.
In other words the plan would be that ammonia should be zero at all times ideally.

Gavin

If TOTAL AMMONIA measured (eg ammonium plus free ammonia where zero then we would always get zero.

However, if one is measuring JUST free ammonia (un-ionised ammonia, NH3) a zero reading does not necessarily mean it will be zero at all temperatures and all pHs.

if we have 10 mg/l of TOTAL ammonia, then at pH 4.5 and at 10 celsius we will have 0.0000059 * 10 mg/l of unionised ammonia (=0.000059 mg/l). That concentration is so low that it would take a pretty good test system to detect that if we're testing for un-ionised ammonia. (on the graphs this is in a region that looks like everything is on the ground at zero)

However, if the temperature of that same water rose to 30 celsius and the pH increased to 8.5, the concentration of un-ionised ammonia would be 0.2418 * 10 mg/l (=2.418 mg/l). (reading from the xl spreadsheet that powers the graphs).

Hence, a warning here is know exactly what type of ammonia is being measured. If it is TOTAL ammonia, and we have zero reading then there should be little to worry about.

If, however, the test kit is reading un-ionised ammonia and gets zero, then it is imperative that the test is done at the exact temp and pH of the fish tank.

Irrespective of that, the graphs do show the importance of pH.

ian

I've added the spreadsheet for doing the calculations at

docs.google.com/open?id=0B07wAC7V5eLqQ1JmajllQnAyT1U

To download the XL spreadsheet, then select File and Download.

ian

Excellent post Ian, thanks for sharing this detail! I know you have explained this subject several times particurlarly the impact of temp and pH on the amount on amonia present in the total reading. The charts you posted speak volumes to that point! but then they do say a picture paints a thousand words

Pictures are great, but they take a lot longer to do on computer than simply typing.

Fortunately, though, when doing chemistry exams it is much quicker to draw pictures than to write. That is why I would have told my students to get use to sketching graphs and drawing cartoon to explain the chemistry/biochemistry.

Equations are much quicker than writing loads of text....that is why chemists use equations = because they are lazy. !!

I chuckle when people have asked how I passed my biochemistry/toxicology, and I reply by say "spending years drawing good cartoon animations in the exam hall".

Possibly, I could do some YouTube animations of the processes (when I get some time free).

ian