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No, you can't successfully do that. What is required is water with 4 dKH made up entirely of carbonates, nothing else that would affect either the KH or the pH. Tap water doesn't ever qualify for that.
 

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As Hoppy said, the 4 dkH reference solution that is to be used in drop checkers must only consist of carbonates, and nothing else. Tap water has other buffers that may give a "false" 4 dkH reading.

This is why you cannot use the pH/kH/CO2 relationship unless you are using a fixed 4 dkH reference solution, where only carbonate are contributing to the carbonate hardness.
 

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I saw ANS brand of drop checker and the instruction say : Add 4 drops of CO2 reagent and add aquarium water to fill the ball level to half. Is this gonna work?
As I mentioned above, using aquarium water in a drop checker will not work. A 4 dkH reference solution must be used.

Disregard all instructions that tell you to use tap water, aquarium water, deionized water, distilled water, etc.
 

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I can understand not using aquarium water, but why not distilled or deionized?
If you were to add bromothymol blue (the indicator reagent) to deionized/distilled water, the solution would turn yellow very quickly (if not immediately). With no buffers in the water, any carbon dioxide that diffuses into the water inside the drop checker would cause the indicator to change colour quickly, giving a false CO2 concentration reading.
 

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Discussion Starter · #9 ·
If you were to add bromothymol blue (the indicator reagent) to deionized/distilled water, the solution would turn yellow very quickly (if not immediately). With no buffers in the water, any carbon dioxide that diffuses into the water inside the drop checker would cause the indicator to change colour quickly, giving a false CO2 concentration reading.
Oops.. I just used distilled water to set up my solution. What water are you supposed to use?
 

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Oops.. I just used distilled water to set up my solution. What water are you supposed to use?
4dkh Solution.
You must use distilled or deionized water to make your 4 dkH reference solution (i.e. when you mix the sodium bicarbonate with the water, it must be free of other ionic species).

However, you cannot use just distilled or deionized water in place of the 4 dkH reference solution inside the drop checker.

Edit: Here is a set of instructions written by billionzz.

http://www.plantedtank.net/forums/general-planted-tank-discussion/42429-kh-standard-how.html
 

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Discussion Starter · #13 ·
You must use distilled or deionized water to make your 4 dkH reference solution (i.e. when you mix the sodium bicarbonate with the water, it must be free of other ionic species).

However, you cannot use just distilled or deionized water in place of the 4 dkH reference solution inside the drop checker.

Edit: Here is a set of instructions written by billionzz.

http://www.plantedtank.net/forums/general-planted-tank-discussion/42429-kh-standard-how.html
Looks like I can't really do it without having a degree in chemistry.. I'll just buy my own :)
 

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You can get a new KH kit, cross your fingers and trust it completely, and make a 4 dKH standard by the trial and error method. For a drop checker to be useful it doesn't require that the KH be exactly 4 dKH. Even 3 or 5 dKH will make it useful, making the green color ppm of CO2 be 20 or 35 instead of 30 ppm. Since we have a hard time judging what color is exactly the right green, the drop checker, at best, only tells you the CO2 concentration is between about 20 and 40 ppm.
 

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Could you please explain more on how to make use of a KH test kit to make 4 dKH solution?
You essentially take a large volume of distilled water (say 1 litre) and then add a tiny bit of amount of sodium bicarbonate to it. Take a small aliquot that is double the standard testing volume (i.e. if the test kit asks you to use 5 mL, use 10 mL instead) and then test the kH of the water. Count the number of drops required to change the colour of the test sample. Divide this number by 2. This is the kH of the solution you have just made.

Continue to add sodium bicarbonate and/or water and keep testing until you achieve ~4 dkH reference solution.

If you have a scale, then the easiest method is to follow the instructions written by billionzz (link was posted above).
 

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You essentially take a large volume of distilled water (say 1 litre) and then add a tiny bit of amount of sodium bicarbonate to it. Take a small aliquot that is double the standard testing volume (i.e. if the test kit asks you to use 5 mL, use 10 mL instead) and then test the kH of the water. Count the number of drops required to change the colour of the test sample. Divide this number by 2. This is the kH of the solution you have just made.
Sorry, I'm still a little confused by this. Why do you double to amount of water and then divide the result by two? Can't you just mix the baking soda with water until it tests at 4dKH?
 

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With the standard KH test you will either have 3, 4, or 5, and seeing as how titration is a bit not exact in terms of color change judged by the person. Its not going to be an exact number, so if it is actually 4.65123etc. some people will see some change some at 4 and call it good and others might go to 5 before calling it changed completely.
So if you double the sample size then each drop will be 0.5KH instead of 1, so then you could see if you are 3.5, 4, or 4.5 so you can adjust it a bit closer.
 
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