[salicylsre]

Hi,

I'm working in a biological laboratory and as a hobby I am growing chillies in hydroponic systems at home. Those plants prefer a pH of ~6 for growing. however, after adding the fertilizer to the water, the pH of the solution is 7.4
Now I want to adjust the pH of the solution to ~6-6.2 using Nitric Acid. In the lab we have 65% Nitric Acid. I learned at some point how to do pH-calculations, but my teacher was so bad, that I never really understood it and now I don't manage to calculate it myself anymore Maybe someone here could help me with the following?

Have 8 liters of watery solution with pH of 7.4 and want to adjust the pH of this solution to 6-6.2 using 65% Nitric Acid (I guess it might be easiest to dilute the acid to 1% before use, so it wont be too strong as I don't have a pipette at home to handle microliter-quantitites

How much (possibly in milliliters) of 1% Nitric Acid do I have to add to 8 liters of watery solution in order to get a pH of 6-6.2?

HNO3 pKa~-2  M: 63.01g/mol

Anybody here able to help me out?

[Borek]

Forget about trying to calculate. Amount of acid required depends on the exact composition of the solution (buffering effect of the dissolved salts, most likely phosphates), which you probably don't know. Just adjust against pH meter.

[salicylsre]

could I simply adjust the pH of 80 ml of my solution in the lab to a pH of 6-6.2 and then add the 100 time the necessary amount of nitric acid to the 8 liter-solution or does that not work due to the logarithmic pH-scale? (can't really bring my 10 liter bucket to the lab and start adjusting the pH...)

[salicylsre]

otherwise, could you make some rough estimated guess of how much I would need to add? then I could use these kind of paper pH testers to roughly adjust the ph...

[Borek]

could I simply adjust the pH of 80 ml of my solution in the lab to a pH of 6-6.2 and then add the 100 time the necessary amount of nitric acid to the 8 liter-solution

Good idea if you can't use pH meter where the mixing will happen, and it will work.

Checking pH afterwards won't hurt, just in case, but in general procedure is correct.