This is related to @anon32470837 's Adventures in Hydro #2 - LP Aero/NFT mash-up - or - switching to HPA?
From Larry’s experiments, there are some confounding results on how the PH is generally behaving. This has prompted me to start a series of tests to see if we can determine the buffering regions inherent in the Megacrop formula. Then, we’ll try the same with the addition of a variety of buffering formulations. These tests are ex-situ a hydro system.
Thinking of the following series or some subset of these:
1. Baseline
2. Titrate megacrop to determine inherent buffering.
3. MES buffer
4. Phosphate buffer
5. Citrate / Sodium Citrate buffer
6. Citrate / Phosphate buffer
7. Carbonate buffer (pk1)
8. CG-50 Ion exchange resion
9. Citrate + MES buffer
10. Jack’s 321 inherent buffering
11. Jack’s RO inherent buffering
12. Jack’s RO + MES buffer
= complete.
= cancelled.
= progress / planned.
(1) First up: Set-up and Baseline
Initial test is simply to baseline the initial PH, check for drift, and a PH up and PH down.
Made up ~3 gallons RO water:
Take ~450ml of the water.
Weight out ~790 milligrams Megacrop:
As indicated by the feeding chart, going high on the EC:
Nutrients into solution:
Stir, stir:
Calibrate the PH probe:
Turn off sitrring. Top off to 500ml. Verify the EC:
1.7EC. Yup, on target:
Insert the PH probe and let’s get going:
Results, baseline:
I lost some initial data and would like to run the intervals a little longer, so I’ll probably recollect this data. Minor drift on the order of 0.03PH or so over the measured intervals. This test is simply to evaluate the stability of our nutrient solution prior to running buffer tests.
Edit: extended data overnight:
Measured ~0.1PH drift up.
Note: simply used the GH Up/Down for the above while the subsequent titrations utilize 1M KOH.
Let’s start off by looking at some experimentally produced graphs produced by titrating just the nutrient formulation. Then we’ll do the same by combining it with some different buffering reagents.
A short aside. Titration means that we are adding either an acid or a base in small increments in order to “map” the effect on the PH. We are looking for how well we are buffering against the addition of OH- or H+. Recall, the plant (or microbes) as it feeds will be returning H+ or OH- into the solution. We are simulating that effect.
To illustrate how a buffer works, let’s take a moment to look at the following graph of this very slow buffer (using a resin):
On the Y axis we have PH. On the X axis we have time. What you’ll see in this graph is a small quantity of 1 Molar Potassium Hydroxide (KOH) being added (a titration) to a test solution near time zero. We start with a PH of 6.17 which then shoots up to a PH of 6.54 upon the addition the KOH. It then drift back back towards our starting PH due to the effect of buffering.
We’ve added a bunch of OH- ions into the solution which cause our PH to go more basic. The buffer, in this case a resin, will essentially “absorb” a portion of the excess OH- ions causing the PH to drift back towards a target PH (the pKa).
So, for the example the PH starts at 6.17, we add some OH- ions (plants eating) causing the PH to jump to 6.54, and the buffer pulls the PH back down to 6.25 or so by absorbing some of the OH- ions.
We’ve essentially compressed the action of adding OH- to the solution. We’ve slowed the change in PH. As we add more OH-, the buffer will pull the PH towards the pKa of the buffer.
This process is typically reversible. If we were to instead add an acid at some point (H+), the same effect will occur with the buffer pulling the PH towards the pKa.
It is a bit more complicated than what has been just described since for typical buffering chemistry, what I call “absorb” means the creation of a conjugate base or acid, but let’s not worry too much about that for the moment. Visualizing what a buffer does in use is most important. Also for illustrative purposes in this example, we’ve using a special buffer that is slow. A typical chemical buffer, on the other-hand, will be quite fast.
Different buffer formulations will have distinct pKa values. This is where the buffering effect is the strongest. As our solution PH moves away from the pKa, the buffering effect becomes weaker. We’d want a pKa value that is as close as possible to what we’d consider a good PH value for growing plants. We’ll look at some buffer formulas that have a pKa values near an optimal grow PH.
We can repeat this over and over again until the buffer “capacity” has been exceeded on either side of the pKa. At that point, the PH is outside of the buffering region and any additional OH- or H+ ions will cause relatively large PH changes. You’ll see this in the up-coming graphs where there is a region where the PH is much slower to change (we are in the buffering region) and regions where the PH changes rapidly (we have exceeded the buffer capacity).
Finally, we’ll discuss what the graphs actually mean.
(2) Titrate megacrop to determine inherent buffering
Using 1M KOH as a strong base to titrate two Megacrop (V1) samples.
Sample 1 : 791.5mg Megacrop in 500ml RO water. Initial PH = 5.033 (orange)
Sample 2 : 790.7mg Megacrop in 500ml RO water. Initial PH = 5.035 (blue)
(3) MES buffering
Sample 1 : 790.5mg Megacrop in 500ml RO water. 1953mg MES (0.020M). Initial PH = 4.114 (brown)
Sample 2 : 790.2mg Megacrop in 500ml RO water. 976.9mg MES (0.010M). Initial PH = 4.460 (blue)
Sample 3 : 790.2mg Megacrop in 500ml RO water. 487.8mg MES (0.005M). Initial PH = 4.850 (green)
Sample 4 : 790.2mg Megacrop in 500ml RO water. 82.9mg MES (0.000852M). Initial PH = 4.870 (brown)
Sample 5 : 790.2mg Megacrop in 500ml RO water. 194.4mg MES (0.002M). Initial PH = 4.712 (yellow)
(5) Citrate buffering
Sample 1 : 789.8mg Megacrop in 500ml RO water. 440.0mg Citric Acid (0.020M). Initial PH = 2.968 (blue)
Sample 2 : 790.7mg Megacrop in 500ml RO water. 220.5mg Citric Acid (0.010M). Initial PH = 3.172 (green)
Sample 3 : 790.7mg Megacrop in 500ml RO water. 110.0mg Citric Acid (0.005M). Initial PH = 3.293 (orange)
(8) CG-50 Ion exchange resin
Sample 1: 789.6 mg Megacrop in 500ml RO water. 500mg CG-50 (red).
Sample 2: 790.3 mg Megacrop in 500ml RO water. 132.3mg CG-50 (blue).
Combined Plots for Comparison
Zoomed into the region of buffering we’d normally be interested in:
(10) Jack’s 321
(11,12) Jack’s RO
Discussion
The effect of PH in a hydroponics system can have a particularly pronounced and rapid effect on the health of a plant. Wild swings outside of the safe PH regions can be detrimental by locking out nutrients and changing the osmotic pressure. A buffer acts as a safety to help slow the movement of PH.
We are looking at some of the chemical buffering options available as an adjunct to a nutrient formulation. The nutrient formulation chosen for these tests is the Megacrop (v1) all-in-one formula.
There are many reasons the PH in a solution change. Several example reasons include:
- Plants are eating and releasing OH- ions into the solution (PH increasing) or H+ (PH decreasing)
- Other biology are eating nutrients (or your plant) such as bacteria or algae
- Cross reactions within the solution are causing precipitation and changing the makeup of the solution.
Every system and situation differs. A buffer is not meant as the solution for problems that might occur within a hydroponics system but instead it is meant to slow the effect of natural processes and any problems on the PH. This allows for more time to address any issues.
The chemistry in these nutrients solutions are rather complex as there are many salts, potential cross-reactions, and buffering complexes. The easiest way to analyze how a buffer might work within such a solution is to start experimentally.
The above graphs detail the effect of the addition of different potential buffers that have a buffering region within a suitable PH range for hydroponics, are considered relatively safe to plants (I have not fully evaluated the safe concentrations, please review the scientific literature), and are relatively compatible with nutrient formulations.
Reading the graphs, the left scale (Y axis) is the measured PH. The bottom scale is the amount of titrant in milliliters added (X axis). The titrant, in the majority of the tests, is simply one Molarity potassium hydroxide (see discussion later in this thread that explains what Molarity means). Potassium hydroxide is a very strong base and represents the equivalent of a large amount of OH- being added into the solution. It is essentially a PH up.
Within the graphs, a straight line is estimated between PH5.6 and PH6.5. This is used as the buffering “power” metric of the solution. From the straight line, an equation is generated. From the equation you can calculate the PH of the solution versus the amount of titrant added. For instance,
y= 0.5x + 5.2
tells us that the PH will increase by 0.5 for each milliliter of 1M KOH added.
If we were to compare two solutions, we can look at the equations to determine which option has the greater buffering effect. The smaller the slope, the more buffering. For instance,
(1) y = 0.5x + 5.2
(2) y = 0.35x + 5.2
the second equation would be the stronger buffer since the slope of 0.35 is less than the slope of 0.5.
The following are the current results of the test buffer combinations:
MC
1.7EC Megacrop : y=2.9903x + 5.4262 (5.6<=PH<6.5)
MC + Citrate Buffer
1.7EC Megacrop + 5mM Citric Acid : y=2.676x + 3.0509 (5.6<=PH<6.5)
1.7EC Megacrop + 10mM Citric Acid : y=2.119x + 1.037 (5.6<=PH<6.5)
1.7EC Megacrop + 20mM Citric Acid : y=1.3839x + 1.2646 (5.6<=PH<6.5)
MC + MES Buffer
1.7EC Megacrop + 0.852mM MES free acid : y=2.2407x + 5.3674 (5.6<=PH<6.5)
1.7EC Megacrop + 2mM MES free acid : y=1.3363x + 5.2768 (5.6<=PH<6.5)
1.7EC Megacrop + 5mM MES free acid : y=0.6700x + 5.2229 (5.6<=PH<6.5)
1.7EC Megacrop + 10mM MES free acid : y=0.3353x + 5.1922 (5.6<=PH<6.5)
1.7EC Megacrop + 20mM MES free acid : y=0.2271x + 5.2296 (5.6<=PH<6.5)
MC + CG-50
1.7EC Megacrop + 132.2mg CG-50 : y=1.3049x + 4.8567 (5.6<=PH<6.5)
Jacks 321
1.7EC Jacks 321 : y=3.974x + 5.374 (5.8<=PH<6.5)
Jacks RO
1.7EC Jacks RO : y=6.64x + 5.1486 (5.6 <=PH<6.5)
1.7EC Jacks RO + 5mM MES : y=0.7353x + 5.1352 (5.6 <=PH<6.5)
Conclusions
Plants, particularly in a liquid media, are sensitive to PH. Swings outside of the nominal range can rapidly affect the health of the plant. Buffering is particularly useful for situations where the PH is encountering rapid and uncontrolled swings as a result of an infection or other issues. A buffer will slow the PH swing allowing for more time to address any deficiencies in the system or, at minimum, reduce serious PH related health effects on the plant.
For the less extreme cases, the use of a buffer at lower concentrations can also be a helpful tool for situations even when the swings are less pronounced. Particularly for systems where the media is a fluid and there is a significant amount of manipulation going on, RDWC, etc.
Currently, out of these buffer combinations, the use of MES has the most pronounced buffering result. This is not entirely unexpected as the pKa of MES is quite close to the optimal PH for nutrient solutions.
Many nutrient formulations, Megacrop as an example, will have an inherent phosphate buffer complex. This complex provides some buffering but it is relatively weak. An increase of the phosphate buffer would improve the buffering effect but it would also throw the nutrient ratios off kilter. As such, the addition of any of the alternate buffer options would improve the buffering of Megacrop to some extent where MES would be the best option out the evaluated buffers.
Cost-wise, MES is expensive relative to some of the other buffers. At 5 mMols, the cost would be around $20 per 40 gallons of nutrient solution. But, at 5mMol, it simply beats the pants off all the other buffers by a factor of 2 over the buffering strength of all of the other tested buffers.
Hmm, there’s something wrong with me (or, so I’ve been told) 
