pH of water I'd previously adjusted seems to be changing over time.

[N1ghtL1ght]

Fertilizer isn't a pH buffer (Bugbee et al). Also acids will react very quickly with bicarbonates or CaCO3. I mean you stir it and the water isn't freezingly cold....
What happens is that some of these reactions release CO2 in water which will slow dissipate from the water, then pH rises again. It also depends on your acid used.

 

[radagast97]

Whats the ppm? You can't really ph distilled water-and you shouldn't even try.

It's about 550ppm. Being I buffered it, it's no longer distilled water. The base was about 1500, but I dilute it before using. I test the pH of the diluted solution prior to use.

The problem is the pH is *changing". Originally it jumped from 6.5 to over 8 in a couple of days. Now, every day, it rises from 6.5 to 7. While pure water at that pH is extraordinarily sensitive to acids and bases, this is a buffered solution.

**Buffered solutions resist change in pH yet I'm still getting the rise.

 

[radagast97]

Fertilizer isn't a pH buffer (Bugbee et al). Also acids will react very quickly with bicarbonates or CaCO3. I mean you stir it and the water isn't freezingly cold....
What happens is that some of these reactions release CO2 in water which will slow dissipate from the water, then pH rises again. It also depends on your acid used.

That depends on the fertilizer. Ammonium and phosphates can act as buffers when used properly. I also use some acetates.

I know it is still buffered because pushing the pH back down from 8 to 6.5 (after it rose on its own accord), took a surprising amount of a strong acid - an amount that would have dropped the pH to below 4 in unbuffered water.

 

[N1ghtL1ght]

That depends on the fertilizer. Ammonium and phosphates can act as buffers when used properly. I also use some acetates.

I know it is still buffered because pushing the pH back down from 8 to 6.5 (after it rose on its own accord), took a surprising amount of a strong acid - an amount that would have dropped the pH to below 4 in unbuffered water.

A buffer (in this context) is a substrate that can be a reactionpartner when an acid gives of a proton and neutralize the pH drop instead. The general problem with fertilizer is that once the plant has taken it in, it's not going to be able to do this for the solution.

It's true that NH4+ etc can also have an effect on pH, it's mostly due to dissociation of molecules accompanyied by release of either protium or oxonium, or due to exchange ions, but it's not a buffer per se.

In the right pic you see the difference between 1,5% and 6% Calciumcarbonate, it works different than e.g. CalMag.

 

[Cirroji]

You can't really ph distilled water-and you shouldn't even try.

? I think I'm missing something

 

[radagast97]

A buffer (in this context) is a substrate that can be a reactionpartner when an acid gives of a proton and neutralize the pH drop instead. The general problem with fertilizer is that once the plant has taken it in, it's not going to be able to do this for the solution.

It's true that NH4+ etc can also have an effect on pH, it's mostly due to dissociation of molecules accompanyied by release of either protium or oxonium, or due to exchange ions, but it's not a buffer per se.

In the right pic you see the difference between 1,5% and 6% Calciumcarbonate, it works different than e.g. CalMag.

Excess NH4OH plus HCl will form an alkaline buffer (or NH4Cl plus HCl). The equations for how much of each is needed to target the pH desired are basic first-semester chem. The NH4Cl + NH4OH form the buffer. The salt's ammonium ions will limit the dissociation of the ammonium hydroxide. Adding an acid will neutralize a little of the ammonium hydroxide by the basic system remaining intact, with the pH virtually unchanged. Adding a base will introduce hydroxyl ions which bind to the ammonium ions, again keeping the pH virtually unchanged. This is an example of Le Chatelier's principle.

Mixing excess weak acid with a strong base will form an acidic buffer. Mixing excess weak base (such as ammonium hydroxide) with a strong acid forms an alkaline buffer.

You are correct that a plant's absorption of fertilizer will change the pH, but that's not what you said. Plants always change pH when taking in nutrients, pushing the pH down. They trade hydrogen (oxonium) ions for cations of the fertilizer.

 

[radagast97]

? I think I'm missing something

pH meters have a very hard time measuring pH with virtually no ions in the solution.

That said, an indicator will give a correct reading.

 

[radagast97]

When i worked in the greenhouse this was always the case. We would use sulfuric acid to lower the ph and most importantly the alkalinity. The next day the ph would always go from 5.8 to 6.8 but if i tested the solution for alkalinity it was always what it was adjusted to the day before. We always paid more attention to alkalinity vs ph

pH is a measure of alkalinity or acidity. Not sure what you mean when you say the alkalinity was the same.

 

[Cirroji]

pH meters have a very hard time measuring pH with virtually no ions in the solution.

That said, an indicator will give a correct reading.

i think you mean deionized water.?

 

[radagast97]

No, I actually did read your post. Your water is behaving exactly how I would expect it to react if it were "hard water."

Alkalinity means that over time, there's more alkaline buffers in the water source than the pH down that you used to reach 6.5 pH. This is normal behavior when it comes to water with "high alkalinity." Once the buffering neutralizes the acid, the pH begins to rise again. You might have to adjust your water source several times to actually see it stabilize.

Once a buffer is used up, you are correct, but I had a lot of buffers in that solution and the amount of CaCO3 in water is in the mg/L range. My buffers would have reacted with the carbonates in the process of mixing the solution, overwhelming the calcium carbonate buffering effect. If you've ever mixed chalk with an acid, the reaction is immediate and obvious.

Pure water has a pH of 7. 1 millionth of a mole of strong acid added to a liter of pure water will change that pH to 6. As an example that would be 0.0000036 grams of HCl gas, a vanishingly small amount.

Buffers are designed to fix the pH in one place, resisting any change. My issue is it's changing AFTER it stabilizes at a given pH. I first posted this because that didn't make sense. Since that time I realized that my use of urea as a nitrogen source is the likely culprit given it's much less basic than ammonium hydroxide, which the "compost tea" bacteria are likely producing as they break down the urea. This ammonium hydroxide is what's overwhelming my buffer.

 

[radagast97]

i think you mean deionized water.?

I mean pure water. You can get an approximation from distillation or deionization. You can get fairly pure water by the IKEA method, burning hydrogen and collecting the condensate.

 

[Edinburg]

A bit of white vinegar waterd down will lower your ph, ALLSO a bit of baking soda waterd down will raise it naturally just go light bc you can repeat, or you can blow cash on ph up/ down at your hydro store or online.

 

[radagast97]

A bit of white vinegar waterd down will lower your ph, ALLSO a bit of baking soda waterd down will raise it naturally just go light bc you can repeat, or you can blow cash on ph up/ down at your hydro store or online.

Or I can use the lab-grade nitric, sulfuric, or phosphoric acid I keep in my lab. Diluted of course.

That said, altering the pH isn't the question. I learned how to do that 55 years ago. The question is "Why is my pH changing over time?".

I've gone with the idea that bacteria, acting on the urea I added initially, is converting it to ammonium hydroxide, slowly but continually raising the pH.

 

[Edinburg]

Or I can use the lab-grade nitric, sulfuric, or phosphoric acid I keep in my lab. Diluted of course.

That said, altering the pH isn't the question. I learned how to do that 55 years ago. The question is "Why is my pH changing over time?".

I've gone with the idea that bacteria, acting on the urea I added initially, is converting it to ammonium hydroxide, slowly but continually raising the pH.

Mostly my pH is in check, however sometimes in flower it starts to creep up a bit, but the white vinegar baking soda trick works well and I'm not blowing cash on ph up / down. Once I get it back in check it pretty much stays there. Just throwing it out.