Hy, how would you make a buffer solution with KHCO3 to have a pH 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. Unfortunately, the pH drops drastically to a level of 4, which is not good for my plants…

So, in 1 L RO water, I added 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? What could be the problem?

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The solution is becoming more acidic due to absorption of carbon dioxide from the air. The more neutral the solution the more readily it will turn to carbonic acid. Air is of course an essential component for growth so we cannot exclude the air.

On the other hand adjusting pH upwards by adding a base such as potassium hydroxide or potassium bicarbonate solutions will put more potassium into the system which can be a problem since potassium interacts with other elements in the system. Some growers claim that for some crops you can allow the pH to drop to 4 without a problem. So the key is to watch the crop for signs of deficiencies and take that as a sign for action rather than aiming for a numeric pH level.

  • Thank you for your answer. The solution is not under plants. Only in a Jar, the plant is not affecting the solution. To make it clear, a previous solution was under plants before and neither of my plants was able to survive with pH 4 solution. This is the reason I started to find what is the problem. We need to make a solution that has a buffer capacity. – Krisztian Komjati Nov 5 '19 at 15:24
  • What makes you so sure that the pH was the problem? There are many possible links to making a mixture to raise pH but most of them are commercial so I won't add one here. – Colin Beckingham Nov 5 '19 at 15:40
  • I agree with you, this is true. But opinion is, whenever you have a hypothesis it is better to test it. You can't just turn your face whenever someone says something different. This would lead to where I am right now. So I have a hypothesis that it is caused by the low pH. Therefore I am looking for a way to keep the pH at the desired level. Colin, can you please explain to me how can I make a buffer with KHCO3? – Krisztian Komjati Nov 6 '19 at 7:39
  • Magnesium carbonate is soluble at only 1.4 mg/ml. That's another reason why carbonate buffers are tricky. Why not just stick wirh a nice phosphate buffer? You can get enough strentgh at 6.5. – Wayfaring Stranger Nov 29 '20 at 23:09

pKa of bicarbonate is 6.5 . Below that pH, the stuff fizzes off as CO2. Your pH of 4.1 sounds suspiciously like one of the buffering regions of phosphate. CO2 buffers are notoriously hard to maintain. I'd instead shoot for a phosphate buffer around pH 6.8. Being above the pKa of bicarbonate, that should give you pH stability. Add the phosphoric acid last, as your other components are clearly messing with pH I'd let the solution sit for a day, then check pH again to be sure. Here is what looks like a decent phosphate buffer calculator.

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