[countycraig]

Ok so i have used BATE pH calculator and have tried this so many times with no change the pH of my tank does not drop. Here is what i have been doing.

Sulphuric acid : H2SO4    pKa1= -3, pKa2= 1.99
Im using 95%-98% sulphruic acid so im assuming I have a concentration of 18 mol/l
 
adding this to water
H2O  pKa = -15.7  55.5 mol/l

Using the pH calculator Ca+Cb+Va+Vb  to get a pH of 4 in 70L of water i only add 0.2ml of acid.

Ive done this and the pH in my tank does not drop at all.

what am i over looking please help

thanks
Craig

[Borek]

What is in your tank? You are assuming it is pure water - it is most likely not, I suppose it already contains some dissolved substances that are buffering the solution.

[countycraig]

dechlorinated municpial city tap water. i am in an aquatron fish research facility. essentially it is lake water that is treated by the city for drinking than dechlorinated by us.

How would this effect my calculation. is there another method i can use to get an estimate of how much H2S04 to add besides trial and error

[Borek]

Most likely it contains carbonates, as long as you don't know exact composition trial and error is the only way to go. However, I would try with smaller volume of your water (say 1000 mL) and diluted acid (perhaps even 1:1000) first, to get the idea about how much acid to add.

Note that 1:1000 dilution is back-of-envelope estimate, it can be way too high.