Understanding pH

[d42zero]

Hi,

I'm a first time grower trying to understand pH. I'm growing in coco but I've posted here as I'm interested in the water itself and how pH works, and I'm talking about FRESH solution before it goes anywhere near the plants.

A few days ago I mixed up 20 litres with a pH right on 5.8, today when I was watering I re-tested it and the pH was right down at 4.6 Remember this solution is fresh so I was expecting it to still be the same but it's changed LOADS..

Then tonight I mix up a new 20l batch and I had the same problem I've been having for weeks.... it tests 7.6 after calmag+nutes so I add 0.8ml ph down (Hy-Gen brand) and it drops to 7.1... so I add another 0.8ml expecting it to drop another 0.5 or so, but this time it drops to 4.3. I add 0.8ml ph up and it goes to 5.0, so I add another 0.8ml and it goes to 6.3.

So why would the pH of my un-used nutes change so dramatically over a few days, and why doesn't it behave in a linear manner when using up/down?

Cheers

 

[Bush Doctor 77]

pH, I believe is a logarithmic scale. Does you pH meter hold on a standard and are you rinsing with RO/DI between readings?

 

[d42zero]

I'm rinsing with distilled water, then measuring two samples and retesting if they're markedly different.

The meter was calibrated fairly recently but I've ordered some more calibrating solution. The up/down acting crazy has been happening since it was calibrated though...

 

[Bush Doctor 77]

The mixture will drift some as some reactions take place, but that sounds like a lot. Are temps the same when you take the readings?

 

[d42zero]

I figured it'd vary a little since it was mixed but yeah going from 5.8 to 4.6 seems like a lot.. Temps should be similar, within a couple of degrees at least. I'll record temps when I test from now on though, just in case I notice something there..

Also if pH is logarithmic, why would the second shot of ph down make more of a difference than the first one? Shouldn't it be the other way around?

 

[Bush Doctor 77]

Also if pH is logarithmic, why would the second shot of ph down make more of a difference than the first one? Shouldn't it be the other way around?

You are correct. There is probably some chemistry going on. I do think most nute makers recommend fresh nutes.

 

[d42zero]

You are correct. There is probably some chemistry going on. I do think most nute makers recommend fresh nutes.

This was with the new fresh mix I made tonight.. calmag added and mixed, nutes added and mixed, left to sit for 30min or so then mixed again and tested.

I seem to always have trouble predicting results when I'm adjusting pH I wonder if there's a really dumb reason like I'm not mixing it well enough

 

[Chris Scorpio]

Are u using tap water?

Lots of buffers can be in it causing grief with yer ph

 

[Barney86]

Hey buddy there's a few obvious options that generally tend to be why this happens when people ask.
It shouldn't drift more than a point or 2 once it's mixed. I grow in reservoirs and even they don't go out by a full 1 per week and that's with 4 plants feeding from it.

So, without being cheeky, this is generally just down to user error.
You most likely didn't iether,
Mix it right before you checked the pH.
Never mixed it before you checked it again a few days later.
Have put entirely the wrong things into the mix.
Made it way too strong and keeping it way too hot so the evaporation effect is making the mix much stronger, or perhaps the most common reason, your pH meter sucks lol.
It almost has to be one of those reasons mate cos that shouldn't happen .
Grab a bottle of brand name water to check how accurate your meter is before changing anything though.
The pH will be on the label and by law has to be extremely accurate so it's a great way to check your ppm meter aswell. If that's not on the bottle a quick Google will find it

Oh and I just spotted the pH down question. I've noticed that aswell. The effects of pH down (I use phosphoric acid) multiply exponentially as the dose increases.
Takes 15 drops to get it from 7 down to 6 then only another 3 will take it down to 5.
It's annoying but that ones normal lol.
Just the way the stuff works.
Bit like cooling down your bath. Once the temp gets down it only takes a little more cold to make it baltic .

 

[d42zero]

Are u using tap water?

Lots of buffers can be in it causing grief with yer ph

I am using tap water, but I run it out into the container then let it sit for a couple of days before mixing nutes in, to hopefully let it stabilise and avoid stuff like this.

So, without being cheeky, this is generally just down to user error.

More than likely

You most likely didn't iether,
Mix it right before you checked the pH.

Possibly

Never mixed it before you checked it again a few days later.

I didn't, but I did think of this so I re-tested after a good mix and it was exactly the same

Have put entirely the wrong things into the mix.

Did that a while back haha but I've got my system sorted and I'm recording my mixes so that *shouldn't* happen

Made it way too strong and keeping it way too hot so the evaporation effect is making the mix much stronger

It's kept in the grow room at 20-25deg, but it's in a sealed container so it can't evaporate.

or perhaps the most common reason, your pH meter sucks lol.

Entirely possible as it's a cheap eBay one

That's a great list of ideas thanks @Barney86, if those are the common issues then that should help me to narrow down what's going on. I think the two most likely are poor mixing and a crappy meter.. I've got some calibration solutions on the way so I can re-check the meter.

Oh and I just spotted the pH down question. I've noticed that aswell. The effects of pH down (I use phosphoric acid) multiply exponentially as the dose increases.
Takes 15 drops to get it from 7 down to 6 then only another 3 will take it down to 5.

See this one I can't wrap my head around... if pH is a log scale and 7 is the centre point, I would have thought it would get harder to change it the further from 7 you got.... so if 1ml of ph down takes you from 7 to 6, another 1ml might take you from 6 to 5.5, and another 1ml might take you from 5.5 to 5.2, etc etc.... I can't understand why it's the other way around