How to make a reference KH solution for CO2 measurement

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Aquarium Advice Freak
Jun 11, 2006
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.
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 reduces errors(less 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 solution!!
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 10ml sample of the 40KH reference and add it to a 90mls 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 solution.

This is extremely accurate.
DO membrane material is fairly cheap per unit.

Tom Barr
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