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Hy, how would you make a buffer solution with KHCO3 (Potassium bicarbonate) to have a pH of around 5,5–6. It would be used in a hydroponics system to make a buffer solution in RO water (demineralized) that can resist the acidic nutrient fertilizer. Tried with 60 mg/L and 90 mg/L of KHCO3, but unfortunately, the pH drops drastically to a level of pH 4 after 4-5 days, which is not good for my plants… IMPORTANT: the system is a passive system not bubbling CO2 in the solution!!!

So, in 1 L RO water, I added 90 mg of Potassium Bicarbonate (KHCO3) as a buffer and make the solution to 90 ppm (128,6 µS/cm). This brought the pH to 7,86. Then, after half an hour I added the Nutrient Solution. 935 µL Nutri Forte A (specification attached to the comment) + 935 µL Nutri Forte B (specification attached to the comment). I stirred it. At the moment the EC showed 1203 µS/cm (1,203 mS/cm) and pH was 7,13. I added 80 µL Phosphoric acid (H3PO4) to adjust the pH. I stirred it. The EC at the moment showed 1205 µS/cm (1,205 mS/cm) and pH was 5,56 on 23,8 ºC.

The solution is in a Jar, not touching any plant only under pure air. The values changes by days:

Day0 - EC 1205 µS/cm | pH 5,56 | 23,8 ºC

Day1 - EC 1220 µS/cm | pH 5,81 | 22,0 ºC

Day2 - EC 1238 µS/cm | pH 6,01 | 21,5 ºC

Day5 - EC 1360 µS/cm | pH 4,02 | 21,9 ºC

I am asking your help figuring out how can I solve this. Should I raise the KHCO3? If yes, what should be the overall amount? What could be the problem?

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    You got below the pKa of carbonate when you added phosphoric acid. Over the next few days, buffer equilibrated with CO2 in the air, about 300 ppm. I'd forget the bicarb, and just go with a phosphate buffer pH 7.2 or so (monobasic plus dibasoc). However, you'll need to find a replacement source of potassium. I'd use K2SO4. Carbonate buffers are untrustworthy over long periods. They always come into equilibrium with atmospheric CO2, usually at a pH you do not want. – Wayfaring Stranger Nov 7 at 17:31
  • @WayfaringStranger Thank you. So, based on that you recommend them, I assume phosphates are not phytotoxic, are they? – Krisztian Komjati Nov 8 at 7:24
  • Phosphates are not phytotoxic at reasonable concentrations. They're the P in the NPK of fertilizer mixes. If you use too much you'll get osmotic stress on your plants. Here's a typical mix: wikihow.com/Mix-Hydroponics-Nutrients It's best to stay away from carbonates as 1) You'll get that from the air anyway 2) Mg ions form a rather insoluble salt with carbonate. 3) CO2 evaporates. Magnesium phosphate is also only poorly soluble, which is why most premix fertilizers don't come with Mg. Still, your plants need some Mg 2+ too, so recipes come with Epsom salts or dolomite. – Wayfaring Stranger Nov 8 at 15:04

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