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Determining Homemade KOH (LYE) Concentrations

Ok, so everyone has to buffer their pH back up with something, right? Slaked/hydrated lime (CaOH), Potassium Hydroxide (KOH) (usually reserved for the big leagues, UVI, Neslon Pade type systems, and rightly so. KOH can be dangerous to work with as it is highly caustic and a very powerful buffer). With most folks by now settling on alternating between CaOH and KHCO3 (known to most folks as Potassium Bi-Carbonate, here, in this country, it is referred to as Potassium Acid Carbonate as I've recently come to find out, and is gentler, kinder sort of caustic). KHCO3 also has the added benefit of being an organic fungicide (foliar), and in addition to buffering up your pH, is also a good source of Potassium for your system. But if you are hellbent on sustainability/self reliance, like doing or creating things yourself rather than buying/stealing them from others, can't buy/order Eco Rose in your part of the world, have a wood burning stove that you use, have a peeponic system and would like to bump up the K content of your humonia for fruiting cycles "natrually", then making your own KOH has certainly crossed your mind. We all know how it is done the old fashion way...Filtering water (low mineral content water, like rainwater, RO or distilled) through the ashes of hardwood trees. But how on earth are you able to tell what concentration you've achieved, or how many grams of KOH are in solution? This is a BIG problem/unknown. Sure the floating egg method as well as feather dipping method works alright for soaps, but you're not making soap, your potentially messing up your AP, Bio-ponic system, or organic garden, and end up with fish kills and possibly even messed up plants.

Ever since I've read the those Norwegian and Danish studies about the NPK value of aged human urine (K, not so much, but it's in there particularly when mixed with wood ash) I've been blown away by the potential implications and immediately went about utilizing the two in some of my hydro set-ups. (and have been stockpiling since summer since I plan on fertigating my gardens/fields this coming season. Problem is, I like to grow LOTS and LOTS of fruit bearing K hungry plants. 20 or 15 to 1 -water to humonia - seems good on the N and P side. Problem is I know I've been a real wimp with the ashes (K) side of things, and when dealing with this unknown, have erred on the side of caution

This has usually amounted to taking a given amount of water, running it through a hardwood ash filter until it reached an arbitrary 'high' pH (say 9-10 depending on how adventurous I'm feeling that day), then lowering the pH with HCl to something a bit more familiar to hydro (though this would be counter-beneficial in an AP buffering context) and using that.

The million dollar question to me is "how to know how many given grams of KOH you have in an known amount of water"?  

This has implications for AP buffering/Potassium dosing as well as peeponics, home-use organic hydro, (non-certifiable of course) organic gardening fertigation etc...short of precipitating it out and then measuring (which wouldn't be a bad idea for double checking the following equations I really have no idea, and neither have others I've touched on this topic with)...Here is the best that I could come up with. I hope that someone (one of you rocket scientist folk) can tell me if this is solid or not...

If we know that you can calculate:

pOH=1/[OH-] [OH-] = molraity of OH

10^2mL = 0.1L

and will say a liter of your solution has a concentration of 2.51g for the heck of it

the molar mass of KOH is 39+16+1 =56

molarity of OH- = 2.51/56 = 0.0448

pOH = log 1/0.0448 = 1.35

and we know pH+pOH = 14

so pH = 14-1.35=12.65   ...and we all have a decent pH meter right?

Is the above solid in assuming that if you have filtered one liter of water through ashes and arrived at a pH of 12.65, you would then have  2.51 grams KOH in solution? 

My hope is that someone else can confirm this, as this is pretty out of my league/comfort zone...

Bio-hydroponic greens saved from the low-temp sprouting 'experiment'  (another blog...actually it will be an update on the "using lights to heat  greenhouse space blog) growing under those PL-L lights I wrote about (a potentially better as well as cheap alternative to T-5HO lights... higher lumen out-put, longer lasting PAR, less expensive and tri-phosphor standardized). I've started putting them to the test so to speak. We'll see if they live up to my expectations (expectations created by industry blurbs/charts which is all "marketing" anyways :) Those PL-L might be another blog unto themselves if anyone is interested I might write it up... 

Well I hope someone can confirm/shoot down the KOH pH math. It would sure be nice to have a solid jumping off point. 


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Comment by Vlad Jovanovic on January 10, 2012 at 12:39pm

Thanks again Nate. I kind of figured that it wasn't that simple...and probably dependent on a bunch of different  factors (temps, whats in your particular water, DO maybe even... et al), but having a rough idea of a 'threshold level' would in advance would be good if not exactly necessary...I've installed, or rather placed the 4,850 liter fish tank in the GH and once the rafts are completed will have over 16,000 liters of water to play with. My water has quite a bit of buffering capacity on its own at first, but was wondering about a ballpark-type figure to start with...

Comment by Nate Storey on January 10, 2012 at 11:49am

I use KOH regularly.  You have to know the molar strength/purity of the KOH to determine.  Unless you shell out for lab quality stuff (I don't) this is an unknown.  So, instead of calculating every time I add KOH, I just add a threshold level and then test pH a day later and adjust with a smaller adjustment dose.  This is actually safer than calculating because it allows other variables to impact pH over the course of 24 hrs.

Comment by Vlad Jovanovic on January 10, 2012 at 11:03am

Thanks for confirming Nate. Have you ever buffered using KOH? Do you have any pointers as to how many given grams of KOH in a given amount of water (100 liters, 10,000 liters, whatever...) will bump pH up 0.2 points? 

Comment by Nate Storey on January 10, 2012 at 9:20am

That's the right calculation Vlad.

Comment by Vlad Jovanovic on January 10, 2012 at 8:01am

It seems you've misunderstood...Oh they've heard of it all right, they just can't figure out why it is so unavailable here. For years it was commonly used here, and then poof...gone, not available for sale..? 

Comment by RupertofOZ on January 10, 2012 at 7:55am

Don't know why the "wine makers" haven't heard of it...

Comment by Vlad Jovanovic on January 10, 2012 at 7:32am

Thanks Rupe, great advice but the wine shops as well as lab supply stores here are a no go on KHCO3. The wine making forum folks here are bewildered as to why as well. Thanks again for pointing out the snafu. 

Comment by RupertofOZ on January 9, 2012 at 9:10pm

By the way... Potassium BiCarbonate can usually be found in wine making supply shops...

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