How to re-calculate my solutions?

kizwan

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Aug 17, 2019
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Apparently I made a mistake when making my solutions (DIY fertilizer). What I did:-
1) weight salt
2) prepared desired volume of RO water
3) put 1) & 2) in a clean container
4) mix until all salt is dissolved in the water

The correct way that I just learned:-
1) weight salt
2) add salt to a clean container
3) add RO water to the container to desired volume
4) mix until all salt is dissolved in the water

I did not wrote down the final volume of the solutions. So, does anyone know how to re-calculate, so that I would be able to know the correct amount of fertilizer I add in the tank every week?

KH2PO4 : 30g, 250ml of RO water
KNO3 : 40g, 400ml of RO water
MgSO4.7H2O : 200g, 500ml of RO water
DTPA Fe 11% : 8 tsp, 400ml of RO water
 

Allwissend

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option 1 : do another batch of solutions and measure the final volume
option 2: add water to the existing solution to a fixed volume. eg. add water to the MgSO4 solution so that the final volume will be 1L , then you have 200g in one L of water
option 3: estimate 1 g ~ 1 mL, thus the
MgSO4.7H2O : 200g, 500ml of RO water
would be ~200 mL + 500 mL = 700 mL. From this you can recalculate the conc. of Mg added by your dose. The margin of error is good enough for our purposes.

As you may notice this becomes a bigger issue the more concentrated the solutions are, as was the issue in the original post.
For the sake of simplicity I omitted an aspect in the original post. It works as is without issues with the chemicals we commonly have and with the precision we target. But to elaborate (*) on that original comment, it is generally good practice to add some water (50-80% of the final volume) , add the salts, then mix, add water to the desired volume and mix. This is done to avoid high local concentrations when you first pour water over, effect of trapped gasses on the final volume, etc. This complication will likely result in some users adding too much water to begin with thus getting the solutions too dilute or adding too little water and complaining that it didn't dissolve.
 
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kizwan

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Aug 17, 2019
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I actually did the same way; add salts into the water when making my (diluted) solutions. I will definitely going to do as you suggested when mixing new solutions.

I may go with option 1. Do another batch & measure the final volume. Then adjust the dosage from there.

Thanks!
 

area66

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Jun 10, 2021
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would be ~200 mL + 500 mL = 700 mL. From this you can recalculate the conc. of Mg added by your dose. The margin of error is good enough for our purposes.
I think it's wrong, adding 200g of Mg to 500ml will not make 700 ml, since the water weight 500g adding 200g of Mg to 500ml of water will weight 700g but the volume will be nearly the same if not the same, this is because the Mg particles occupy the spaces between the water particles. If only the empty spaces are occupied, the level of water will not increase
 
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Allwissend

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It is true that the Mg and SO4 ions will be surrounded by water molecules once dissolved, however the ions will still occupy space thus increasing the volume. This is exactly what caused kizwan's issue.

Try it! It's simple. Take a transparent container, add some water to it. Mark the water level. Add salt or sugar to it. You will see the water level rising. Make a second mark. Fully dissolve the salt or sugar you added and notice that the water level stays by the second mark.
 

Lmuhlen

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Jan 20, 2021
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You added more water than was intended, so your solution is more diluted than it should be. The easier way to go around this would be to increase the volume you dose in your aquarium until this batch ends.

Compare your final solution volume and what was the original final volume intended and increase your dosage proportionally. If your originally intended volume was 500ml and you ended up with 600ml, your dosage should be increased 20% (600/500-1).

You said that you don't know the final volume, is that because you already used part of your solutions? If you know how much you already used, it is a matter of estimating how much you used and adding it to what is left.