How to make a stock solution of known concentration.

Question

I'm looking to make a stock solution to dose an aquarium, any assistance would be appreciated. 

 

I would like to make a 150l (litre) solution of 0.02ppm K2HPO4. How much K2HPO4 would I require to make a 2L stock solution, so that the addition of 1mL of stock solution to the 150L gives me 0.02ppm?

 

 

Answer 1

ppm is the same as mg per liter.  Thus, 0.02 ppm = 0.02 mg/liter

0.02 mg/L x 150 L = 3 mg needed to be added to the 150 liter 

Since you want this to be in 1 ml, then you need a stock solution that is 3 mg/ml.

You want to make 2 L of this stock, which is 2000 ml.

So, 2000 ml x 3 mg/ml = 6000 mg = 6 g of K2HPO4

 

In summary, if you dissolve 6 g of K2HPO4 in 2 liters of water, and then add 1 ml of this to 150 liters, the final concentration in the tank will be 0.02 ppm