Having done quite a lot of chemistry and having a pH meter that reads to .001 with a temperature probe....

I would suggest that quite a lot of the bouncing is not only the good suggestion of plant waste but also the sheer volume of the solution at hand and the minute changes you're making.

Even with a run to waste system, I find the tank pH will change quite a bit over a few days. Particularly near the start of a newly mixed tank.

When you add the pH down (acid) you are altering the ionic properties of the salts in the nutrient mix. It takes a long, long, long time for all of it to mix and homogenize 100%.

What tends to happen with mine is I'll start with water at 6.8. Adding some nutrient might bring it down a few .1's to somewhere closer to 6. I then add acid to bring it down further to the 5.7 / 5.8 mark.

Over the next 24h+, it'll gradually go back towards 6.

When the acid first goes in, you can imagine it like dropping food dye into a bath, even if you're swirling it with a pump or airstone. It might mix well, but it needs to mix to a molecular level before it will finally sit still. The pH bounces up as the acid becomes 100% homogenized and changes the ionic makeup of the solution.

I'm interested in the idea of really carefully setting your pH to optimize growth, but the level of information available for specifically tuning cannabis is low from what I've seen so far. And anyway, I doubt it really has to be that precise, I'm sure there are a lot of other areas that could be optimized first and produce a greater result for less effort.

If pH bounce is pissing you off, a buffered solution might be something to look at.

In the same way that you can make you own pH up and down solutions, you can also make your own buffers. Buying it will probably be a far easier option for most. The chemicals aren't expensive or difficult, but it requires some chemistry to work out the amounts needed for a specific solution and pH.

Dissolving (even if it's a liquid into a liquid) is something you can't trust your eyes and senses with when it comes to fine detail - like honing an engine out and saying "yea... that seems close enough" will leave a lot of room for it being shit. When I was doing higher level chemistry, we'd often make up solutions of things and then test our skill at working at concentrations of other solutions by using one as a standard. To make this standard, we'd precisely measure out a known amount and concentration of one chemical and then dissolve it in water or alcohol. Even working with 100ml volumes, we'd have to shake and turn the flasks for a few minutes at least to make sure there weren't patches of uneven concentrations in it.

With tens of liters, tiny pH changes and more things going on than I can count in the solution, it's natural that it can take a lot of time.

You can expect bounces to last considerably longer if you add anything as a solid, even if it looks dissolved. Your concentrated nutrient mix probably looks liquid to you, but mine tends to produce some very fine silt if I leave it to stand for a while. Those same tiny specks will need to dissolve over time before their affect on the pH will become entirely clear. If you like my reply, pls vote in my bulb poll (sig).

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That's interesting, peach +rep.

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Temp is a factor that can cause pH to fluctuate. Pure water only has a pH of 7.0 @ 25°C, as temperature increases the water becomes more acidic. At 30°C the pH drops to 6.9.

While the water in the reservoir generally isn't going to swing by 5°C, cheaper pH pens don't compensate for this and can aid in this swing. Also these apply to pure water, after adding stuff it may actually make the swing more drastic but I am honestly not sure.

I think a lot of the swing has to do with the purity of the water you use as well as the nutrients you use. I'm using Advanced Nutrients 3-part ATM and have noted a few things:

When mixing the nutes with Distilled/RO water, since the nutes are buffered pH is automatically "adjusted" to 5.5-5.7 depending on how concentrated I make the solution. The pH swings when using this setup are minimal for me, I only have to add 0.2-0.5mL a day to keep pH within range. pH never climbs above 5.7 when monitored daily.

When mixing the nutes with tap water (my PPM is 350 using 0.7 conversion factor, pH is around 8.0 ) the pH only drops to 6.8 or so. I then have to add 7.0-8.0mL of pH down to get it within range. After the initial setup, daily maintenace starts by requiring 4.0-5.0mL of pH down and gradually gets down to only requiring 0.5-1.0mL. It takes about a week to get to this point and thats when I change the water in my buckets.

You've touched on two phenomenon that plagues me daily as a hydro grower.

1 PH is directly related to temperature for 2 reasons. One because the ph probe will deal with h ions differently at different temps, and second because the dissolved CO2 in a solution is determined by its temp and also effects the PH.

2 The EC or TDS or PPM of the solution effects the way that the PH reacts when acids or alkalines are added to the solution. If the EC is high, there are more "buffers" with more(or less) ions. As acids or alkalines are added, they will give(or take) ions to effect the total PH of the solution. The more of these buffers there are, the more acid(or alkaline) will be needed to offset the amount of H ions to effect the overall PH of the solution.

I'd also note that keeping a dead-set steady PH is generally counterproductive. Certain nutrients will become easier or harder for the plant to use depending on the PH of the solution(or soil). Unfortunately, there is no magic number that all the nutrients become available. PH swing(within reason) is, IMO, natural, unavoidable and necessary.

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Agreed. I lower my pH to 5.2 and let it climb to 5.8 or so until I adjust it back down to 5.2. Looking at uptake charts easily confirms this idea.

dam thats a nice booty

I would say that unless you consider growing to be your full-time job you've got to use a buffered solution. But then any decent commercial solution is buffered these days. It's just a question of how well.

A properly buffered solution will strongly resist a deviation outside the "safe" growing range and mildly resists variations within that range.

What's really cool is this new line Advanced Nutrients is releasing. They say it will actually react to changes in pH. From what I've read it's buffered but it also has some kind of chemically-reactive ability that works to keep the pH in range.

They also say they've done something to make nutrients available to plants at a wider pH range than normal.

If all that's true the pH woes could be a thing of the past.

pH is a measure of the acidity or basicity of a solution.

Thanks man, I needed a laugh.

He asked whether lower pH was more acidic or more basic - essentially which end of the range is which. He didn't need the unit of measure defined.

ste147 - The lower numbers are more acidic and the higher numbers are more basic (alkali).