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[APD] How to get around tannins and weird KH's for pH measurements

Are you sure you want to know how organic acids influence pH?
I'd suggest a method that side steps all that mucky stuff.
If you are truly interested, there is a definitive book on the topic of the limnology of humic waters. Check the local College library system(or book vendors etc). Applying it to a planted tank will be a challenge (I've read the book when I was at UCSB). 
To side step pH influences and to get a rough estimation from very low even absent KH's:
Measure the pH/KH in the tap water you use after it sits for 24 hours.
Next, estimate the target for CO2 you want(say pH 6.5 and a KH of 4).
You can add a stable amount of CO2 and _then_ add the tannins. As long as you are adding the CO2 in a stable manner(eg , no pH controllers!!!), the plant uptake of CO2 will be relatively similar.
So if the pH is 6.5 without the tannis influence and is now 6.0, you have the fudge factor of 0.5 pH units for this amount of tannins. We do not measure tannins in our tanks, this is a variable and one we cannot get at easily(they do sell tannins test kits).
Thuis is improtant to note that the tannins levels do not stay the same in our tank, many make this mistake in assuming so (thus a bad idea to use a pH controller in these situations).
The CO2 being added for the plants is the SAME in both cases(ph 6.5 and the pH 6.0).
We will assume that plant demand for CO2 is also the same as well as non limiting light/nutrients.
KH can be addressed in a similar manner. By taking a CO2 measurement, as well as a gauge for plant health at a KH of say 4, you can maintain the the same CO2 dosing rate, then remove all the KH. KH = 0.
You are not able to measure the CO2 but you are still adding the _same amount of CO2_ from the measurement at KH = 4, and the plant uptake will also be the same or very close(we will assume it is). The only variable is the KH that you are changing. 
So having KH is not needed even if you use this back door method. 
Plants care about CO2, not the KH *(it would seem).
They do make ultra pure pH testing methods for very low alk waters. 
For our application, I think this is the easiest solution.
If you suspect the tap water and other organic or inorganic issues influencing the KH, do this also very simple backdoor test:
Take reading of the tap, see what the KH is. Next make a reference solution using pure DI water+ baking soda to get the same KH. Test this reference solution with the same test kit.
Any difference?
If so, then the difference is likely from the non bicarbonate alk.
You should be able to make a ref solution for KH with about 1-2ppm of accuracy using a scale/teaspoon x 20 measurements with an accurate scale.
Simply add the known volume of puire DI water in liters/grams.
Then you can check those pesky KH test kits for accuracy.
This is seldom ever done yet folks rely heavily on KH test ktis while spending $$$ for accurate pH measurements.
Tannis and other acids change over time(and can be added to the tap water peroidically to perform various functions for the Utility company), stray current changes in many cases depending on the device being on, and few folks ever calibrate their KH test kits. These issues can cause problems for controller users if they are complacent and do not also eyeball the plants as their "test kit". I saw plenty of evidence for this recently with user error.
Doing so may resolve many of the seemingly excess CO2 ppm levels that folks are seeing.
Given that 95% of algae issues are CO2 related, you'd figure this is a good place to really focus on improvements in measurement, accuracy and methods for practical use.
Tom Barr 

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