Well you really need to look at the fact that the product primarily derives it's Nitrogen from ammonia, so I'm fundamentally against products with such high Amoniacal-N. All it takes is one stupid blunder and the party's over for your fish/shrimp. This all depends on pH and temperature of course, and we really don't know the rate of NH3=>HN4+ equilibrium.
Also, it's complicated trying to figure out what the concentrations are. The straight ammoniacal-N value is easy enough. But the ammonia derived from urea has to be done stoichiometrically, because urea is converted by the enzyme urease in the aquarium sediment according to the equation:
(NH2)2CO + H2O ? CO2 + 2NH3
So, for every mole of urea, 2 moles of ammonia is generated.
Well, let's see what we can figure out...
50g of product evidently has 3.5% N-Ammonia
50g x 0.035 = 1.75g N-Ammonia
N is 82% by weight of NH3 so total ammonia from this source is 1.75g/0.82 = 2.13g NH3
In 500ml of water the concentration will be 2.13g x (1000) / 0.5L = 4260 mg/L
Each ml delivers 4.26mg NH3 per liter
Each 1/2 ml delivers 2.13mg NH3 per liter or 2.13ppm NH3
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50g of product reportedly contains 11.5% N from urea
50g x 0.115 = 5.75g N-Ureic
N is 47% by weight of (NH2)2CO so total urea from this source is 5.75/0.47 = 12.2g Urea
In 500ml of water the urea concentration will be 12.2g (1000) / 0.5L = 24,400 mg/L
Each ml will deliver 24.4mg urea per liter
Each 1/2 ml will deliver 12.2mg urea per liter
The molar weight of urea is 60,000 mg per mole
The number of moles per dose will be 12.2mg / 60,000 mg/mole = 0.000203 moles
Therefore:
0.000203 moles (NH2)2CO + 0.000203 moles H2O ? 0.000203 moles CO2 + 0.000406 moles NH3
Molar weight of NH3 is 17,000mg per mole
Weight of NH3 per liter produced by adding 1 dose of urea is 17,000 mg/mole x 0.000406 mole = 6.91mg NH3 per L
Total NH3 per dose = 2.13ppm + 6.91ppm = 9 ppm
I looked at the numbers, not very closely, but they look huge, so much so that I reckon I must have missed a decimal place somewhere. I'm sure I messed up somewhere.
On the face of it, it looks like you are adding a huge amount of ammonia to the tank. This is something like 100X above the lethal dosages for fauna. I assume that the NH4 equilibrium is achieved quickly so maybe this isn't a big deal. I just find this all very messy. For years folks have been winging about the evils of NO3, and here we have people happy to play with fire. Go figure...
Cheers,