Non-linear changes in PH?

Indoor Growing Hydroponics
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Ive been meaning to ask this question since my first soil grow, but never got around to it. Now that Im on my second hydro grow, its become more of an everyday issue.

That title may not make much sense at first. I am having a hard time coming up with a succinct description of whats going on.

My tap water (community well) starts out near 7. Some days its over 7 some days a little under 7. The EC also varies from around 80 to near 200. Im sure it has to do with rain amounts.

When I go to add PH down to lower the PH, I add say 10 ml of acid to the rez and the PH will drop from 6.8 to 6.75. Add another 10 ML and it drops to 6.6. Add another 10 ML and it drops to 6.4. But when I add another 10 ML it drops to 4.2! Add a little tap water back in and it jumps back to 6.4.

In other words, the PH does not drop linearly when adding acid to lower the PH. It takes a huge amount of acid at first to barely move the PH, but once it gets down to the low 6’s, even a small amount of additional acid drops the PH a bunch. Plus once its down low, even small amounts of tap water can make it jump right back up into the 6’s.

Anyone else notice this or is it just me?

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The water may lack sufficient minerals to stabilize.

Dolomite lime does come in a more soluble powder form. A small amount might stabilize the H2O.

Good luck!

99

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Two things come to mind. First, the scale is logarithmic, not linear to begin with. Second, the acid does take a bit of time to work IME.

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As @99PerCent alludes to, buffering capacity. More specifically, likely has to do with the carbonate capacity ( aka: KH, alkalinity, carbonate hardness ) of the water.

Buffers have a strength and capacity to them. And, the buffer may or may not act symmetrical depending on the buffer chemistry. The ratios of the weak acid to the conjugate base for a buffer dictates a specific buffering PH. A combination of a weak acid and it’s conjugate will have a specific PH based on the pKa for which the buffering strength is maximum. Changing the ratios will allow you to change the buffer PH to +/- one unit or so but you’d end-up on either side of a bell-shaped curve. That is, the buffering capacity changes depending on which direction the PH is moving. Exceed the buffering capacity and you’ll see an increase in the rate of change of the PH.

Didn’t read this fully, but might be of interest: Beginner: Water Chemistry

Visualizing this, here is an example of some random buffer chemistry:

This is the capacity of the buffer. At a PH of 4.8 or so, the capacity is maximal.

“Above plot shows how the buffer capacity changes for the 0.1M solution of acetic buffer. As expected buffer exhibits the highest resistance to acid and base addition for the equimolar solution (when pH=pKa). From the plot it is also obvious that buffer capacity has reasonably high values only for pH close to pKa value. The further from the optimal value, the lower buffer capacity of the solution. Solution containing only conjugate base (pH 8-10) has buffer capacity of zero, for the higher pH presence of the strong base starts to play an important role. In the case of pure acetic acid solution (pH below 3) pH is already low enough to be resistant to changes due to the high concentration of H+ cations.” [1]

Some additional thoughts and discussion into the PH of buffers. In this case, let’s look at a sodium phosphate buffer:

image.png1482×975 41 KB

This buffer consists of two chemicals, Monosodium Phosphate (a weak acid) and Di-Sodium Phosphate (the conjugate base, a salt). Notice the ratios. At a ratio of 1:1, the PH is equal to the pKa of the buffer combination.

As the ratios change, so does the buffering PH. At a ratio of 7:3, the buffering PH is approximately 6.5.

What is buffer capacity? The buffering capacity is defined as the amount of strong acid or strong base that needs to be added to cause a change of 1 PH per liter. The capacity is is related to the amounts of the buffering component added to solution along with the slope at the solution PH. Let’s ignore the quantity for a moment and just look at the slope.

The buffering capacity is greatest where the slope is minimal on that graph (first derivative). For this buffer, this is at the pKa of 7.21 with the ratio of 1:1.

If we add a strong acid, intuitively we know that this will have an effect on the PH. From a chemistry standpoint, the addition of an acid titrates the conjugate base converting it into an acid. This, in essence, changes the buffer ratio and will move us to the left on the graph.

As we continue to add an acid, we continue to change these ratios and continue moving to the left. Eventually, we will deplete the conjugate base and fall off the edge of the earth. At the left side of the graph, there is little to no buffering capacity left and we experience rapid PH change as we continue to add acid. The buffering capacity has been depleted.

Adding more buffering components or just the conjugate base will move us back onto the graph, increasing the strength of the buffer, placing us back into the buffering region.

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Yeah give it time to buffer, the problem is most likely you adding too much too fast.
Do a test with a few gal, and messure it every 30 min or hour and messure how many ml pr gal u will need.

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What kind of ph meter are you using? Also as said already use a buffer get your ph to 5ish n bring up with baking soda.

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Add more tap water to raise the pH!
Baking soda will raise the pH, but also your Ec with useless ions.
Your tab water will contain usefull buffers and nutrients, that your plants can use.

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Yeah, I know its a log scale, but I assumed that would make the interaction “feel” more linear to us humans. Thats one of the things log scales do in general - they make interpreting non-linear ‘stuff’ feel more linear.

The time thing I am aware of too. I mix the acid very thoroughly and then wait at least 30-60 minutes between measurements - with the rez circulating at max rate plus the fluming and waterfalls.

Ok, this I did not know. Those graphs help explain a lot of whats going on. It seems the buffering curve starts to change slope radically once I get down to the low 6’s while adding acid.

Yeah - been doing exactly that.

I started with a cheapo $13 meter and tossed it immediately. Then I got a $150 fancy job but it had issues with the EC readings, so it got returned for repairs - three times. Then I went back to the cheapo meters - two more of them before I got fed up with the sloppy readings and not staying calibrated. Now I have a $50 Apera meter and its working well - finally!

The point is - they all showed the same results as far as it taking a huge amount of acid at first, and then the PH dropping fast with almost no acid added.

As far as adding baking soda or using PH UP, I prefer to just add more tap water if it gets too low.

Yup :slight_smile:

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Thank you all for the replies and help, but you all also kind of missed the point of my question, so I guess I worded it poorly :smiley:

Im not worried about how to handle this, and Im not that curious about the details of the chemistry beyond my current vague understanding :wink:

I wanted to know if YOU GUYS also see this same thing when you are adjusting PH with YOUR water/nute mixing?

I was mainly curious to see if my well water was ‘normal’ or if the buffers in my well were outside the norm or what :smiley:

By the way - things are more linear when I am running a higher nute concentration than I am now. When Im running an EC up near 2 (PPM = 1000) then the rate of change is much more linear, but still drops off more radically as the PH gets lower.

Right now Im running clones in a 15 gallon system with an EC of .5 (PPM 250), and the reactions are more exaggerated as the PH drops.

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Yes, I see this effect. Depending on what I’m doing to the solution, it may be more or less pronounced in either direction. But, with everything the same, the effect is predictably consistent batch to batch.

For straight tap water, not obvious in my case (not on well water). But, I’m rarely adjusting plain water so it may be that I haven’t noticed.

Hard to know. The non-linear effect is not surprising to me. The quantity of PH adjustment that you are adding may better hint at whether something is going on with your water. Your EC is reasonable. If you are adding a bottle of PH adjustment, then something is amiss.

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Ah! Good to know Im not the only one :slight_smile:

Im currently using homemade PH down, but I saw the same thing using the store bought stuff. One cup of battery acid (sulfuric acid) added to one gallon of tap water. Its roughly the same strength as PH down - but seems to be more effective.

In my first hydro15 gallon setup, when I did a rez change it took aprox 120 ml of my mix to drop the PH from the 6.8 - 6.9 range down to the 6.2 - 6.4 range. Adding another 10 ML would drop it to under 5. That varied from time to time as my well water changes a lot depending on how much rain we are getting. The starting PH has been as hi as 7.6 and as low as 6.5. EC varies from around 250 or so down to 80. It also varies some with the amount of nutes I added - which you would expect.

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Yes. Just the other day, I had a 5L pail of water to add to my res. I had it at 6.0 but I was shooting for 5.5. I added a few drops of pH down & the pH dropped into the 3s. And it was literally just a few mls. I tested it again 5 minutes later & it was the same.

tbd

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I would imagine most people are taking discrete samples every now and then with a PH pen that can measure in 0.1 increments?

I measure every 3 seconds using a sensor than can measure in 0.001 increments (stable accuracy ± 0.005, 3 readings averaged to reduce noise giving a more accurate reading every 10 seconds). As such a world of new information has come to light regarding a lot of things that happen in the tank.

When I use a doser and graph my PH when I manually add some acid to a 100l tank of tap water, (PH 8.1, fairly hard) at the point the acid is added the PH drops to the lowest point it will in less than a minute or two as the acid mixes into the nute solution. It then bounces back, over up to 3 hours, to a maximum (sometimes very) close to the original value. It then slowly settles down to the actual value the acid has adjusted it to, this whole process can take up to 12 hours. This final value is usually quite close to (but normally slightly higher than) the value reached in the initial PH drop, just after dosing.

I find that the exact time between dosing and taking a reading can put your reading out from the actual ‘settle value’ by almost the amount you have dosed. I find that the larger the dose you use, the larger your margin of error can be. As a result of seeing this, I now micro dose my nutrient tank. The vastly smaller dosing (around 1-2ml at 1-20 dilution) reduces the variance in PH and often the slower rate of change is one that can be overcome by the effect the plants, irrigation etc have on the PH. I find this reduces dosing errors to so low they can be ignored.

Even with just a 100l tank, when the solution has to interact with roots and pipes etc it can take 7-12 hours to settle down from a dose that would take the PH from 7 down to 6.

So, as an example, with my system you could dose an amount of acid that would take your tank from PH7 to PH6 at midday, measure 3 minutes later when the solution has mixed and you would read around 6 (usually just slightly lower than 6 but you would not see that with a pen). If you continued to take readings over the day you would see the PH rise until around 3pm, when you would see the PH be around 6.7. If you continued to take readings you would then see the PH drop down to around 6.2 by 6pm, and it would then settle at 6 by midnight. As you can see, if you take these readings as true and adjusted your PH at 3pm down to 6 again, your final PH would be closer to 5.3 and it would reach this value while you were asleep. As you can see, this would lead to ‘weird’ PH fluctuations (in my system at least) caused by not taking readings at the right time and dosing at the wrong point. After seeing and graphing the interactions between acid and tap water to a high accuracy my advice is to measure PH 3 minutes after dosing (or at the lowest point it gets to immediately just after dosing), or 12 hours after dosing, when everything has had time to settle, not in-between. Then you will accurately measure the actual PH your tank will end up at.

I find when my nutrient strength is higher this effect is lower as the nutrients often contain a PH buffer which tries to keep the PH closer to 6 than over 7. This effect is also very much reduced when dosing pure RO water with no nutes. That reacts almost instantly (<5 mins) and does not take hours to settle down.

I also find that with tap water lowering the PH from >PH7 to <PH7 takes a lot of acid compared to changes under or over PH7. I suspect this is due to the buffering going on.

This is with my system, the local water, and the acid I get from my local hydro shop.

Your mileage may vary.

EDIT :

I always dilute my PH down at least 1:10 usually 1:20 so this type of problem is less of a problem.

If, like my system, it takes a relatively large amount of acid to move from 7.1 to 6.9 but far far less to move lower then you would have the effect you experience. I add that sort of amount to my 100l tank to shift the PH down 0.1-0.2. A 5l bucket would show 20 times the change, being a drop of 2-3 PH points so a drop from 6 to around 3-4 is the change I would expect.

A few ml to drop 0.5PH in a 5l bucket would be at a dilution of 1:10.

I would say give that a go or to give you more of a margin 1:20. You can always add a bit more, you can’t add a bit less


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Very interesting. I had no clue it could/would take that long to stabilize the PH.

I wonder though, how much of that is due to the roots/plant interactions? I know if I dont keep my EC close to 1.3, then I get PH drifting up or down depending on how much the plants are using and how much change Im getting in the EC as the water level goes down.

You almost have me wanting to do an experiment with some of my tap water to see if the same thing is going on with my water. Im feeling pretty lazy at the moment though
 :smiley:
Thanks for the detailed notes!!! Makes me for sure wish I had the $$ to do up a system like yours or @Northern_Loki’s :smiley:

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Neither did I until I saw the graphs. I thought you put the acid in, and when it was mixed that was it. I was very surprised to see the drop, rebound, and settle.

If you see your PH wander more when your EC is low, then you are probably seeing a reduced buffering effect from the buffer in the nutes. I ALWAYS have some sort of auto water level device, even running a single bubbler bucket I would fit one of those autopot valves. Helps with keeping your readings more stable.

I find the effect of the roots on the solution is to lower PH as they slightly acidify the nutrient solution when feeding. With plain tap water topping up your tank, the high PH of the tap water more than offsets this so you only need PH down. With RO water though the roots are the main effect on PH so you only need PH up.

I would imagine there is a mid point where the right ratio of RO water and tap water would mean you did not need to adjust your PH.

I may make a device to test this which has two valves, one which is open when the PH is too low, the other which is open when the PH is too high connected respectively to tap water and RO water.

I suspect it would eliminate the need for Calmag and also PH up and PH down


(then I could give them more food at a particular EC, hehe)

For a cheap, manual version : If you had two buckets, one with RO water, one with tap water, two tank connectors, two valves that fed an autopot valve you could do it manually. PH too high? Open valve one, close valve two. And vice versa.

After a while of experimentation you could open valve one this much, valve two this much and your PH would be remarkably stable I suspect. In theory anyway.

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More interesting stuff!

I dont have a source for RO water, so I cant try that trick.

I found that my PH usually went up in the rez when I was under feeding (EC dropping) and PH went down when I was over feeding (EC climbing) as the water level in the rez went down. Thats without topping off the rez or making any other additions. It also dropped - like crazy - when I had root rot.

I wonder if thats water/buffer dependent as well or more of a plant specific thing?

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I always have some top-up device fitted so I am not too familiar with the changes that happen in a fixed tank any more. The least I would do is an autopot valve and a water butt to maintain a stable tank.

As the feed normally contains something to lower the PH from tap water, having more of it relative to the water might be the thing making your PH drop. I would take a PH reading each time you top up with water and graph those as it would let you know the overall trend separate from the effect of your plants feeding.

In my system, with tap water, that trend would be upwards and with RO water it would be downwards.

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I decided to do a simple test of my tap water to see if I had the same PH bounce, and long time to stabilize, that you get with your water. I havent figured that out yet as I ran into an unexpected, and so far, mysterious issue.

It seems that when I aerate my untreated tap water the PH goes up fast, and dramatically! The last several days, my tap water has been at a slightly elevated PH of 7.1 to 7.2. I took 4 different samples over the last two days and they all react the same.

I take a sample in a 2 quart jar and test the PH. Its been 7.1 to 7.2. Then I have been letting it just sit for several hours and testing again and its always been almost the same ± .1.

I was preparing to add some acid to one sample, so I set up an air line to agitate/mix the acid in the water. I got side tracked and left the air running for about 45 minutes and decided to re-test the water right before I put the acid in and I was shocked to see the PH at 8.2!!

I have repeated that twice with the same results. The PH goes from 7.1 to 8.1 or 8.3 in about minutes of aeration.

I dumped that water out thinking I had contaminated the sample in some way. I also checked the calibration of the meter and it was off about .1, on the low end, so I re-calibrated the meter and then double checked it. Drew a new sample and it initially read 7.1 again, so I just let it sit. Three hours later no change. So I put the air line back in and waited 1/2 hour and the PH is back up to 8.1.

Now the question is why?

Im using brand new black airline tubing with nothing on the end. Its being weighed down by a marine grade stainless bolt. The air pump is a typical diaphragm type with no oil and a filter on the intake.

I suspect that what ever is going on with these samples is why my rez PH keeps creeping up over night, but the rez change is slow and very mild compared to this. The rez goes from 5.8 to 6.3 in about 24 hours.

Same airline tubing, same stainless weights and same air pump on everything.

Now I need to do some more testing to see if I can narrow down whats doing this - tubing, weights, pump or just aerating the water. I need to figure out a way to agitate/aerate the water without using my air pump - its the only pump I have at the moment.

Edit: its been about 5 hours since I removed the air line from the latest sample and the PH has gone up slightly. It was 8.2 and its now 8.3. My meter is only ± .1, so that may mean nothing.

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:astonished: :scream: IMO that’s for batteries & sociopoathic foreigners, not plants


Have you considered/tried simple household vingear?

:evergreen_tree: ay caramba

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Maybe, take a look http://www.anotherperfectpoolnews.com/wp-content/uploads/Tech_Talk/onbalance_ks_co2inwater.pdf

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