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[APD] How to make a good KH reference stock solution for CO2 measurement

I'm going to detail out some issues for folks with making stock
solutions. I've heard some odd stuff folks are doing and getting
way off in their measurements and methods.
This post will hopefully address some of this before more issues

Given the wide ranges of KH related issues and suspiciously high
CO2 ppm readings in some folk's tap water/tank water, this KH
reference method is a very useful thing. 

The hardest thing is getting folks to actually make one and
calibrate things. 

So I did some digging and have a couple of useful links that
make it clear so folks do not have do chemistry(I know you will
all avoid it like the plague,<grin>).

Here is a good KH Calculator using baking soda to make your KH
reference sample. 


Note: make larger volumes(5 liters) and high concentrations, and
then dilute (this allows you to make 5KH, 10 KH, 4KH, 1 KH etc
from the same higher concentrated stock solution). 

This reduces errors(less error is better). 

So adding 5l of DI H2O (1.321 gallons) and 4.992 grams of baking
soda = 40KH. 

Play around with the calculator, try 50 liters and 4 KH, the
higher volumes and higher the concentration, the less error you
have, well up to a point[:p] 

50 liters of DI water and 40 KH will give you 49.923 grams. 
Now you have pretty high accuracy but you have a lot of KH

5 liters is not bad and you can toss the rest down the drain or
trade to friends etc in the hobby. 

Then you may take a 100ml sample of the 40KH reference and add
it to a 900mls of DI water. 

This will give you a reference of 4 KH to within a very close
tolernace depending on how careful you are weighing the baking
soda, measuring the water volume and cooking the baking soda for
30-45 min at 400F before weighing to remove water and CO2 in the
baking soda. 

For diutions: 


This should help you get going on making the KH reference
solutions and putting them to good use. 

You may use the drop checker method, it's slow, cheap, simple,
or you can DIY a DO membrane on the tip of a pH probe, and put
the KH ref solution inside instead of the dissolved O2 KCL

This is extremely accurate. 
DO membrane material is fairly cheap per unit. 
Using a hose on the tip of pH probe and making a melted groove
or a cut for the O ring can make your own DIY version if you buy
the DO membrane material.  

Now some folks have taken to using their KH test kits to make a
reference solution!!!

Do NOT use a KH test kit which is not calibrated to make a
reference solution. You compare the reference solution to the KH
test kit, not the other way around!!!!!

I really really hope folks understand that last part.

Play around with this.
Try different volumes and different KH's and see how accurate
the baking soda weight needs to be to achieve good accuracy.

The water volume measure is also reduced when you increase the
volume. So when you make a batch of KH=4, use 100mls, not 1 ml
unless you have a very accurate pipette(1ml)or micro pipetter.
That willr educe the rror but waste some ref solution which is
cheap and now you have 5 liters:-) May as well use it.

Note, for basic practical purposes(oh my, I do think about such
stuff), scales are cheap that are in the 0.01 range, while they
shoot up in cost when you get to the 0.001, 0.0001 range.

So making 5 liters DI water with 4.99 grams of cooked baking
soda ought to be fine. That will be pretty accurate and not
require much in the way of expensive scales nor volumetric flask

I know and realize some will round things off another digit, but
the cost of a scale is pretty cheap, about 20$.

Tom Barr


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