[KrisKom]

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. 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?

[chenbeier]

The Problem is you dont have a buffer. A buffer you would have with a mixture of potassium hydrogen carbonate and potassium carbonate, you add only one component. You add phosphoric acid as well Mixtures of hydrogen phosphate and dihydrogenphospate working as buffer as well.
But if I see your numbers, did you calibrate the pH meter. The first 3 days it's a slowly increased then the pH drops at 5th. day. Make no sense.
How much bicarbonate did you add.

[KrisKom]

Yes pH meter is calibrated daily with pH 7.01 and pH 4.01 buffer solution. Made the experiment multiple time to avoid errors. Got the same results. Mixed 90 mg from KHCO3 in 1 Liter of RO water.

[chenbeier]

90 mg is very less. The moles are 0.9 mmol. But what I said add 124 mg potassium carbonate as well to get a real buffer.

[KrisKom]

Understood.

What should be in this case the total amount of KHCO3 in 1 Liter of RO water which I should add? You are telling that 90 mg is too low.

Is this not a buffer cause of the solution can't hold the pH against pH raising? Like not able to keep pH against alkaline elements?

[Yggdrasil]

Part of the problem with a bicarbonate buffer system is that it is sensitive to the amount of CO₂ in the air, so ensuring a stable pH with a bicarbonate buffer requires regulation of the amount of CO2 in the air (e.g se https://en.wikipedia.org/wiki/Bicarbonate_buffer_system).

Also, while buffers can resist pH change, there is a buffering capacity related to the total concentration of buffer present.  Higher concentrations of the buffering agent will resist larger amounts of acid/base, but if acid/base is continually added (without adding additional buffer), then eventually the acid/base will win out.

[KrisKom]

I see, but how can I calculate the amount of KHCO3 that I should add to get a good buffer in my case?
Now it is 90 mg/L.

[chenbeier]

Do potassium harm your plants? If not increase on 10 times means 900 mg KHCO₃ and 1240 mg K₂CO₃.

[KrisKom]

I can have an overall number of 1,2 mS/cm of conductivity. This is 840 ppm, which is 840 mg/L of dissolved solids.

So if I add 900 mg of KHCO3 there would be no place for other elements, like the nutrient solution. I know it is hard to calculate it as we don't know what elements would the plant uptake or what elements would the root system put inside the solution, but can we set a minimum amount to put inside from KHCO3 in order to keep the solution stable for at least 2 weeks? If I can achieve it by holding the solution in a jar without touching the plant it would be OK!!!

[chenbeier]

Probably you have to change to Di-Potassium hydrogenphosphate . Carbonate is to instable.