[algaeman]

Hi guys,

I am working on an algae farm and we are trying to reformulate our fertilizer blend to increase ammonia concentration in our ponds in an attempt to kill some of the zooplankton (such as rotifers) that are causing us problems with production.

We have initially tried using 0.0015M ammonium dihydrogen phosphate (NH₄H₂PO₄) as the base of our fertilizer mix but we have had a lot of trouble getting the pH above 7 (ideally we want it >9) in order to have a higher percentage of free ammonia (NH₃) in the system. In addition this puts waaaaayyy to much phosphate into the system which will never be used up.

I am a biologist by training with some chemistry experience from university so I have been trying to learn pH calculations for weak acids and bases to better understand how we can achieve our goals.

I have determined that ammonium dihydrogen phosphate will dissociate into ammonium and dihydrogen phosphate ions. The Ka for dihydrogen phosphate (Ka=6.2*10^-8) is around two orders of magnitude above the Ka for ammonium (Ka=5.6*10^-10). So when looking at the pH calculation for this salt should I;

1. Just determine the resulting pH from the dissociation of dihydrogen phosphate?
2. Determine equilibrium [H⁺] from dissociation 1. THEN input that into a second calculation involving the dissociation of ammonium?
3. Take the resulting equilibrium [H⁺] from 2. and further determine the result of the dissociation of hydrogen phosphate to hydrogen and phosphate ions?

Obviously 2. and 3. are going to have a much smaller effect on pH, so basically what I'm getting at is when you are looking at a complex mix of multiple salts and trying to determine the resulting pH how do you approach the problem? Do you just look at the strongest acid/base and assume that will dictate the outcome of the entire process? Or do you work through from strongest to weakest and determine every influence on the pH? How do you decide the order you solve the problem in, or which components of the system matter?

Assuming I just go with point number one to simplify the problem, would the following calculation be correct?

        H₂PO₄^- <>    H⁺     +     HPO₄^2-
I   0.00015           0                   0
C   -x                 +x                  +x
E   0.00015-x        x                   x

Ka=6.2*10^-8 = [HPO₄^2-] [H⁺] / [H₂PO₄^-]
6.2*10^-8 = x² / 0.00015-x

assume x<<0.00015

x = 3.05*10^-6

[H⁺] = 3.05*10^-6

pH = -log [3.05*10^-6]

pH = 5.52







At this point I am thinking we are better off using 0.00015M diammonium phosphate ((NH₄)₂HPO₄)) to reach our desired phosphate concentration. This has the added benefit of dissociating into NH₄⁺ (Ka=5.6*10^-10) and HPO₄2^- (Ka=4.8*10^-13) which have lower Ka values than dihydrogen phosphate.

Using this the ammonium dissociation should dominate leading to less reduction in pH than dihydrogen phosphate.

The plan is to then add ammonium hydroxide until we reach our desired N:P ratio (10:1) which I believe works out at around 0.0012M. This should also significantly increase our pH leading to more free ammonium.

The thought of throwing this into the mix and trying to calculate the resulting pH with this mixture of ammonium hydroxide and diammonium phosphate does my head in. I have tried doing it a couple of different ways but I get different answers each time. Again I am really struggling with how to tackle the problem... strongest to weakest effect? Just the strongest effect (ammonium hydroxide) and discount the rest?

As a side note I have found multiple references stating that the addition of diammonium phosphate should raise pH above 7 but I fail to see how this could happen given it dissociates into two weak acids?

Any assistance anyone can give me in tackling this problem would be MUCH appreciated. I am really enjoying learning all this chemistry but frustration is beginning to mount and I'd love to be able to fully understand what is happening chemically in our system rather than just adding chemicals till it works out alright  .

Thanks for your time, sorry for the wall of text...  this took a long time to write.

[Borek]

Don't bother with calculating pH of these systems by hand. Doable, but never exact, always time consuming, and too slow for "what if" games.

Try to download Buffer Maker from ChemBuddy site - there is a free 30-days trial. And built in pH calculator will do everything you need (plus some).

<a href="http://www.youtube.com/v/ReKHRo7I9x0" target="_blank" rel="noopener noreferrer" class="bbc_link bbc_flash_disabled new_win">http://www.youtube.com/v/ReKHRo7I9x0</a>

[algaeman]

That's fantastic! Cheers for that.

Out of curiosity though what is the general technique for tackling a problem like that by hand?

[Borek]

Out of curiosity though what is the general technique for tackling a problem like that by hand?

There is no general technique, other than trial and error.

For more complicated systems it is always a matter of trying simplifying assumptions and checking which give consistent results. And sometimes nothing works.

[Arkcon]

Furthermore, you have a tank full of algae and you have pH sensors.  You want it more alkaline, then add more base.  Can you inject a fairly concentrated ammonium hydroxide solution?   In small amounts, periodically to not kill algae too close to the nozzle.  Under high sun, phototrophs will consume ammonia rapidly, so you may have to add more frequently.  Or you can do it in the dark, to kill off the protists, and let the algae handle it when there's light.